I'm new to this list, but am an experienced tandem, cyclist, and mechanic. I'm preparing to order a custom tandem frame to build up as a road tandem with disc brakes. I've built all my own bikes, including a mountain bike & cyclocross bikes with disc brakes. I've been doing a lot of research on tandem lists and websites to make sure I have all the technical issues worked out. I've noticed a few of you posting consistently excellent and informative messages about putting disc brakes on tandems. I plan to use Avid mechanicals, and I seriously doubt if my team weight would be over 300 lbs. So here's what I want to know.
1. Spoke lacing: can I get away with 32 spoke wheels front and back. Both my mtn bike and cx bike use 32 spokes wheels, but these are singles.
2. What fork? I noticed the recent thread regarding the Winwood/Nashbar cx disc fork. Lost Coyote thinks it's OK. I don't want to make serious compromises with safety, yet I think that some of the "not rated for tandem" responses by component manufacturers has more to do with liability and warranty, on products that would work reasonably well. My other affordable alternative is a steel fork made by Surly (this one gives me disc and canti options) and a disc only fork made by dimension. Would a steel fork give me a bigger safety margin than the winwood, which has an alu steerer? Has anyone put a larger rotor (203 or 185) on the Winwood? Will the carbon disc tabs on the winwood take the forces of such a large rotor? I've seen one technical piece where someone took a fork made for a single and had a machine shop put a re-inforcing sleeve inside the crown area of the steer tube to reinforce it against failure. Has anyone on the list tried that?
3. Rear spacing. I know there are tandem hubs out there with a shell designed to fit wider tandem spacing, so that the chainline and the disc have no spacing compatibilities. Most of these are really expensive. So I was contemplating using a Shimano XT disc hub, which is a good basic hub, and using a longer axle and spacers to make it fit a 145 or 160 spaced frame. But if I do that, either the chainline won't be great on the right side, or the caliper and disc won't line up on the left side. If others have used a standard mtn disc hub, how have you solved this problem? Here is how I think the problem could be solved:
-Order the tandem with 135 or 140 spacing to eliminate or minimize the problem of not having the disc & caliper line up. What are the pros & cons of doing that? I've heard that narrower spacing for a tandem makes the wheel's weaker.
-Find an adapter / spacer kit to make my avids work with the XT disc hub in a 145mm dropout spacing. Anyone know of a source of such a kit?
Finally: Lost Coyote says he has used 160 mm on his Winwood and seems to be comfortable with the amount of stopping power he has. Is 160mm on both wheels going to get the job done? Most everybody, including the disc tandem manufacturers and most folks on the tandem lists seem to be using 203.
thanks for any advice you can give.
Richbiker
The BikeForums Team
-adv-
This is an archived thread, you can find the full version of this thread, with images, links and more content here.
just a couple of quick thoughts. Disc brakes work very effectively, so if there are any doubts as to whether the forks/ disc mounting will be man enough, then definitely go for strength. Wheels will once again take some strain, so I personally would go higher than 32 spokes. XT hubs would be suitable, but there are better ones out there, and there is a Tandem form of this hub that may be more appropriate than the standard Hub
DocF
I would also suggest using a purpose built tandem hub. Since I'm a belt and suspenders man, I personally would feel much better with 40 spoke wheels.
Doc
TandemGeek
Don't shave grams on disc brake systems if they will be used as your primary brakes and you expect to encounter challenging terrain where brake use will be demanding.
Steel fork from one of the builders who are producing tandem-specific disc compatible forks, e.g., Steve Rex of Rex Cycles, Glenn Erickson of Erickson Cycles, Dennis Bushnell of Bushnell Cycles, or Todd Shusterman of daVinci tandems.
203mm Avid BB ordered from Mark Johnson at Precision Tandems, Alex Nutt at MTBTandems.com, or another tandem-specialty dealer who can sell you the right parts to mount the 203mm rotors; they aren't sold as bolt-on kits. Also, be sure to add helper springs and to consider compressionless housing to optimize the performance of the rear disc as outlined in previous posts to this forum (search on "boldt and spring" posted by "livngood")
32h - 36h spoking w/145mm tandem-specific disc hubset; why screw around. On spoking, if you've built your own bikes and wheels then you'll understand that the quality of the wheel components, the wheel builder's experience, and his attention to detail in the building of your wheel are what will make it reliable. Double Butted spokes would be strongly recommended vs. straight gauge or even DT's Alpine IIIs. As for hubs, yes, tandem-specific hubs may cost a bit more but they "should" be more durable and most of them are completely serviceable for long-term care. Models to consider: White Ind. Racer-X, Chris King, DT-Hugi. All three, and particularly the last two, should be taken apart, cleaned, and re-lubed using the manufacturer's recommended lubricant on a regular basis to mitigate potential performance issues associated with contaminated or gummy lubes in the ratchet and/or pawl mechanisms.
Just my .02.
Brian
Alex at MTBTandems.com probably won't sell you a 203mm to use up front. The standard 9mm front axle can't handle the extra torque. I'll agree with Mark that you can't skimp on spoke count, or quality. With discs, you shouldn't be using straight gauge spokes, they'll break under the stress, while butted spokes will give a little. We went with 135mm spacing on our tandem becuase parts availability in Australia is something of a joke. But what do I know? Alex will answer all your questions better than most on the forum, but I think Mark knows just as much when it comes to road tandems.
TandemGeek
Alex at MTBTandems.com probably won't sell you a 203mm to use up front. The standard 9mm front axle can't handle the extra torque.
Not sure what Alex will sell or not predicated on how the product will be used or on what fork they will be installed. You can certainly send him an Email and ask (alex@mtbtandems.com), but I'll make a point to ask him when I see him tomorrow. However, as noted in other threads on discs and tandems, anyone contemplating the use of Avid's discs should bear in that Avid (now part of SRAM) has in the past only supported the use of the 203mm version of it's discs on road or off-road tandems, e.g.., OEM Avid discs on Cannondale's US MT1000 and RT1000 & Euro RT1000 are 203mm models.
With regard to the 9mm skewer, the suspected weakness with forks/axles/skewers is related to drop-out alignment on conventional fork designs which is the basis for suggesting that our friend contact one of the builders who -- thus far -- has been building road tandems with 203mm front & rear Avid BB disc installations. Todd Shusterman at daVinci has been using the Avids as primary brakes on both off-road and road daVinci tandems longer than anyone else and worked closely with Avid's folks (conveniently located in Colorado Springs right down the street from daVinci). Erickson & Bushnell have been fitting (and more recently retrofitting) their tandems with dual 203mm Avid discs for two seasons. One of the first-year Erickson dual disc tandems is owned by one of teams we routinely ride with here in Atlanta who weigh in at about 350lbs and they used it without nary a concern during their Erickson Tour in Austria this past summer. Regardless, it's not simply the 9mm QR skewer that needs to be addressed, it is the entire front axle interface with the fork. Now, with all of that said, the number of reported pull-out incidents involving tandems with conventionally designed forks, 9mm skewers, and left fork leg mounted calipers remains low. I am very familiar with James Annan's incident as we have been correspondents for 6 years, primarily for the past 4.5 years on my off-road tandem enthusiast's discussion forum, Double_Forte: http://home.att.net/~double_forte/. I'm also aware of another involved a tandem team in Seattle and the root cause appeared to be a failure to appreciate how tight the skewer needed to be, coupled with the sub-optimal drop-out alignment of the fork to which the disc brake was attached and the lack of any drop-out tabs (aka., bicycle slang = lawyer lips). There may be others, but information regarding those incidents has not been forthcoming so one can only work with what is available. There is a wealth of information regarding MTB QR issues and potential wheel extractions on Jame's chronology regarding the QR skewer issues: http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/diary.html
Alex will answer all your questions better than most on the forum....
Alex is truly the guy to talk to regarding off-road tandem parts compatibility and manufacturer endorsement for use on tandems. Since deciding to jump into the somewhat obscure off-road tandem niche market a couple years back, he has established contacts and driven a variety of tandem-specific parts improvements with the major fork, shock, brake, and other tandem-specific product manufacturers. Moreover, Alex is what you'd call a "big old southern boy" who has enough power, weight, and a riding style that allows him to break just about anything he gets his hands on. Thus, he has a pretty good idea of what works and what doesn't for teams of differing weights. Coupled with the information that gets shared on our off-road tandem enthusiast discussion forum by those of us who have been riding (and breaking) off-road tandems for several years, Alex is pretty much in-the-know on many of the obscure "systems" that have been successfully used by a variety of different teams in different conditions.
Brian
I meant no disrespect to you Mark, as you seem to know your stuff quite well, but I've never dealt with you personally. Alex advised me to go with the 20mm thru-axle, as he basically stated we could generate enough force to twist the skewer off, or something to that effect. But our tandem is set up for off-road use, and I'm no lightweight myself. If you're going to see him tomorrow, tell him I'm (im)patiently waiting to hear about a replacement crank arm.
Cheers,
Brian
SDS
Could we have some clarification on the characteristics of the parts being used? Are we talking about a normal-sized 9mm axle (?) with a normal-sized skewer, and thus the skewer on the QR, which fits inside the axle, has a diameter of less than 9mm?
I have always been told NOT to overtighten the QR on a skewer because it will cause fatigue and then failure. If this is true, overtightening a normal-sized skewer to keep the axle of a hub with a disk brake in the dropouts, would expose the riders to additional risk. This seems foolish when it would be unnecessary with a proper disk-compatible fork, designed so that the torque of disk brake application only pushes the axle even deeper into the dropouts?
I am not opposed to disk brake use on a tandem as long as the system performs at least as well under all likely conditions as a rim brake system. In either case, the owner's manual and stickers on the bike should note that changes will need to be made (addition of a "drag" brake) before descents that exceed the safe capacity of the system can be done. I note, for example, that one poster on the "Cantilevers v. V-brakes v. Disk" thread says he has already melted plastic parts on the Avid ball-bearing dual-disk system on his 2004 Cannondale, doing a 500 ft descent. I think I remember reading reports of very short pad life as well, not suitable for touring distances (?). It seems to me that a rim brake system with an Arai drum brake (and that's the way Trek is selling its tandems?) will safely do common descents (Colorado, Oregon, Washington, Nevada, California, Arizona, perhaps even back east as well?) that the Cannondale would not descend safely?
It is one thing to be a relatively intelligent (we hope) owner of a custom tandem intended for specific kind(s) of use, that has a braking system of relatively limited continuous capacity, and another to be a large-scale manufacturer of tandems (Santana, Trek, Cannondale, Burley) who must take reasonable precautions to prevent legal exposure, and yet may not require an intelligence/knowledge test of purchasers. Therefore manufacturers must use components of sufficient capacity and lifetime for likely use, which are easy to inspect and maintain, by most purchasers. I am concerned that Cannondale's 2004 tandem, as sold, does not meet this standard for capacity and lifetime and maintenance, and less intelligent purchasers may expose themselves to health risks (high speed crash after brake system failure due to fade or pad consumption on steep, long hills) during operation.
Please consider this post to be composed of questions intended to encourage enlightening discussion, and not bald and rigidly stated facts.
Brian
I worked with Alex at MTBTandems and Dean at ATC Racing to determine the best setup for our use, as we wanted to go off road, and do a bit of light touring. Per Alex's suggestions, we had Dean set us up with a 20mm thru-axle fork. This eliminates any concerns about the strength of the axle/skewer assembly. Sadly, I didn't consider that this being an S&S coupled tandem, the beefy dual crown DH fork is simply too big to make the bike "packable". Brakes work great, but we'll need to swap to a single crown Marzocchi Dirt Jumper in order to fit the bike into luggage. Personally, I would not be comfortable with a standard axle and skewer on a road-going tandem with the larger rotors, regardless of what the manufacturer states. The drag brake is a separate issue, as that's for slowing, not stopping. We have no use for one. Just goes to show, you can never ask too many questions.
TandemGeek
I meant no disrespect to you Mark,
No worries; none taken or assumed.
Alex advised me to go with the 20mm thru-axle, as he basically stated we could generate enough force to twist the skewer off, or something to that effect. But our tandem is set up for off-road use, and I'm no lightweight myself.
If, as you note, (1) you're riding off-road and (2) you're not a lightweight team, then a 20mm bolt-through axle on a robust off-road suspension fork is the right call. No question about it. However, a couple of data points relative to this thread...
1. Our friend Richbiker prefaced his questions with, "I'm preparing to order a custom tandem frame to build up as a road tandem with disc brakes." and, "... I seriously doubt if my team weight would be over 300 lbs.". So, we're out of the realm of off-road tandem forks that are subjected to completely different forces and demands vs. what a road bike's fork must be designed to contend with.
2. Although Bullet Bros. (aka, Bullet USA) produced the ZZYZX for Santana with a 20mm through-axle back in '98, several of the other disc brake compatible, dual crown suspension forks that were beefed-up for use on tandems by Cannondale (Moto FR-T), Stratos (FR-4T), Marzocchi (Jr. T) used 9mm quick release skewers and to no great peril by the folks who used them. They had vertical drop-outs with machined recesses for the skewer nut and lever head and over-sized sliders/legs and fork crowns. I personally owned and used the Moto FR-T (OEM on our '98 C'dale MT3000) and the Stratos FR-4T ('00 Ventana), the latter being fitted with 185mm Hope O4DH 4 pot hydrualic discs. The only issues with the 9mm Stratos fork was fork-leg twisting (aka, shear or torsion) and poor wheel tracking in gnarly conditions related thereto. Never any issues with the 9mm fork coming loose due to the disc brake or, for that matter, the fork torsion and the motivation for moving up to a 20mm axle on our '02 Ventana's Stratos S-5T fork was added fork rigidity to eliminate the leg twisting and improve the handling.
Anyway, such was the state of affairs with forks and tandems. 20mm through axles were seen as a way of getting a "stiffer" fork that handled better and offered a higher margin of safety relative to durability vs. single or dual-crown forks with 9mm axles. There were no perceived disc-brake issues with forks until some two years ago when James Annan and his wife Jules unexpectedly lost their front wheel under hard braking while riding moderately smooth hardpacked trail on their Calfee Tetra Tetra Enduro tandem. We thrashed-over the accident and possible causes on Double-Forte and the brake force issue is what came to the surface. It was noted that the rigid fork modified for use on their tandem had odd, somewhat rear-facing drop-outs which, when coupled with their Hope 04DH disc, exacerbated the axle pull-out problem and is likely to credit (blame) for highlighing what some believe is a potentially serious issue. The result has been a two-year long campaign by James to raise industry and consumer awareness of the potential issues associated with the front wheel & fork interface on disc-brake equipped bicycles, which you can read about here: http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/
Personally, I think concerns can easily be addressed by the average, informed cyclist who uses quality components produced by recognized brand-name manufacturers per their instructions. At a minimum, drop-outs on disc-compatible forks should be parallel with fork blade or canted forward and incorporate supplemental tabs or recessed flats to preclude a skewer pull-out while subjected to shearing forces. Ti or super-lightweight quick-release skewers should not be used to secure a disc wheel and the cyclist should make a point to read the manufacturer's instruction regarding proper use and closure of the skewer. Finally, anyone using four or six-pot downhill disc brakes like the Magura Gustav M and Hope's Enduro or Ti-6 with 185mm or larger rotors must be absolutely sure they are using a disc-specific fork with properly aligned drop-outs and supplemental retention features since these particular caliper installations create severe brake force & brake force path issues. IMHO, single piston disc brakes are usually not of great concern assuming all other things have been addressed AND skewer position and tension are checked before and after each ride. For tandem teams, advanced, heavyweight, or super-aggressive riders, closer attention may be warranted. In any event, if anyone finds that their skewer shifts or feels loose, they should inspect their drop-outs for any damage and pay close attention to their skewer on subsequent rides. If the skewer continue to shift or loosen they should replace the skewer and monitor the new skewer for movement during use. If it stays put, resume the prudent process of just double checking the skewer before and after each ride. If it continues to shift, they should carefully evaluate if they have selected the appropriate fork/axle retention system for the type of riding they are engaged in.
If you're going to see him tomorrow, tell him I'm (im)patiently waiting to hear about a replacement crank arm.
I will make mention of it.
Brian
I may be a bit out of line comparing our situation to his, but my point was that the 203mm rotor seems like a very bad idea with a standard QR. His desire to use a low spoke count would probably further compromise safety. Richbiker has left the building as far as I know, so I was just responding to comments in a general way. If we build a road tandem, we will have another custom titanium frame made, but will probably go with something crazy like a rigid custom fork that takes a 20mm thru-axle, with a suitable hub laced to a 700c rim. I like the security of the huge axle, and will probably adapt the Marzocchi QR to the custom fork. Or just swap the wheels on our MTB Tandem for a lighter set with slicks and save a big heap of cash.
TandemGeek
Are we talking about a normal-sized 9mm axle
Yes.
I have always been told NOT to overtighten the QR on a skewer because it will cause fatigue and then failure.
There is "properly tightened", there is "overly tightened", and there is "not tight enough". If you've worked on other people's bikes, you may have an appreciation for how many skewers are "not tight enough" which, for most applications is probably OK. However, get into some rough road conditions or add a disc brake, and it isn't OK anymore.
Properly tightened used to be verified by ending up with a quick-release lever impression in your hand, but that was before steel skewers were replaced with skewers made from lighter-weight alloys and Ti which, coincidentally, have more elasticity than steel. Now days, in addition to having to take into consideration how much elasticity the skewer materials have, the amount of lever pressure required also varies from skewer to skewer as some provide more mechanical advantage than others.
Case in Point: Those of us who happened to own pre-2002 Ventana El Conquistador off-road tandems fitted with disc brakes discovered the value of steel skewers quickly in that, similar to the front wheel pull-out, the rear-wheels would pull-out of Ventana's somewhat unique rear-facing drop-outs under hard braking with the rear disc. It didn't create a life-or-death situation, but it was a PIA to deal with. The rear wheels would pull-out because the rear-facing drop-outs were designed to counter the braking forces at the brake bridge from V-brakes, not the disc brake caliper that was added a few years later. The only solution that precluded rear-wheel pull-outs was the use of a steel skewer tightened properly so as to leave a big old imprint in your hand. Alloy and Ti skewers were subjected to the same installation method and routinely failed to hold the wheel given their lighter, softer construction. The entire problem was eliminated in '02 when the drop-outs were redesigned to work against the disc caliper's forces.
I am not opposed to disk brake use on a tandem as long as the system performs at least as well under all likely conditions as a rim brake system.
The Avid disc brakes, head-to-head, will outperform rim brakes in terms of braking power and heat tolerance. More importantly, the Avid's failure mode is gradual and accompanied by warning signs, if you know what to listen and feel for. The same is not true of rim brakes. However, Avid's discs will not outperform a tandem fitted with an Arai drum brake when used as a drag brake and that is clearly conveyed by Avid and most tandem speciality builders using the Avid brakes.
To your comments regarding OEM brake system performance standards, I would note that on Page 42 of Santana's 2004 catalog they write, "Santana recommends a non-rim brake when your team weighs in excess of 350 pounds, tours with heavy packs or rides lots of mountains". They go on to describe their two non-rim brake options; the Formula Disk and Arai Drum. Now, while recommended in their catalog, it does not magically become an OEM item for teams who weigh over 350lbs. Co-Motion similarly mentions the same thing on page 8 of it's 2004 catalog, i.e., ... we recommend the use of a supplemental Arai drag brake for touring, heavier couples, or moutainous terrain. The Arai is optional as a stand alone item or sold as part of their optional touring package. Thus, even though the major builders acknowledge brake system limitations, none of them market a two-place tandem that comes with an Arai drum brake as standard equipment. Moreover, like Cannondale's rear disc equipped tandems, even Santana's Formula rear disc equipped high-end tandems cannot be fitted with a drum brake when the disc is installed on the rear wheel.
... I am concerned that Cannondale's 2004 tandem, as sold, does not meet this standard for capacity and lifetime and maintenance, and less intelligent purchasers may expose themselves to health risks (high speed crash after brake system failure due to fade or pad consumption on steep, long hills) during operation.
I'm more concerned about dealers who sell Cannondale's tandems -- or any other tandem brand for that matter -- who don't know diddle-do-do about tandems and that renders concern over just the Cannondale moot. IMHO, anyone selling a tandem bicycle has an implicit responsibility to make sure the buyer is given sound advise on their equipment selection. Anyone who would sell a tandem bicycle to a 400 lb combined weight team that had only rim brakes or any other combination of brakes (front rim / rear disc, dual disc) and that did NOT take time to understand how the tandem was to be used and to carefully educate the team on the limitation of their equipment and it's proper use HAS NO BUSINESS SELLING TANDEM BICYCLES; it's irresponsible. This is why I will always direct anyone looking for a tandem to patronize an honest-to-goodness tandem dealer, that is someone who specializes in tandems. At the same time, anyone who intends to take the pilot's seat on a tandem has an implicit responsibility to be accountable for their actions as the tandem pilot. Being accountable means full disclosure with the tandem dealer regarding the team weight (including potential team weight, i.e., who else is going to be invited to ride besides your 100lb 14 year old?), your skill level and bike handling skills (the latter is quickly discovered by a bonified tandem dealer who make you take them out for a test ride before letting you take your spouse out), and how and where you plan to use the tandem. Additionally, the buyer should be asking about limitations on the equipment, optional equipment, etc...
Case in point. If a 220 lb man and a 180 lb woman walk into a tandem shop to buy a Cannondale from a knowledgeable dealer the dealer darned well better understand if that tandem needs to go out the door with the OEM discs, a hybrid brake set-up (front disc, rear rim, & Arai drag), or a conventional brake set-up with optional Arai drag brake.
Bottom Line: Tandems and bikes don't hurt or kill the people who ride them; people do.
TandemGeek
... but my point was that the 203mm rotor seems like a very bad idea with a standard QR.
Again, it all depends on the rest of the "system". Remember, the primary and largest braking forces that an axle and fork must resist come from the contact patch where the tire meets the ground and that's the same regardless of what type of brake you use. Remember, it's not the brake that stops the bike from moving forward, it's the resultant friction of the tire's slowed rotation against the road. The orientation of a disc caliper alters some of the forces acting on the axle at the drop-out but, in any event, what holds the axle is the fork is the drop-out and the goal of any front disc brake / hub/ fork system should be to ensure all of the brake energy is directed into the deepest part of the drop-out. The skewer just needs to be strong enough to keep the wheel from popping out due to front wheel torsion or the loss of normal wheel loads, e.g., a bumpy road surface, doing a wheelie, or catching air would be good examples.
His desire to use a low spoke count would probably further compromise safety.
Based on empirical evidence from the field, at their stated weight of 300lbs and for a road application, 32h is still adequate provided the components used are first rate and the builder knows what they are doing. Of course, there's only 2 - 3 years of data from a very small population of custom tandem owners and builders. However, knowing who those folks are I can say that experienced, aggressive teams who weigh upwards of 360 lbs haven't had any problems wth dual discs in the Alps or Pyrenees and one less aggressive couple over 400 lbs successfully descended Alps d'Heuz without suffering a melt-down or wheel durabilty issues. Of course, these folks also understand how discs should be used which is a big part of the equation when looking at suitability for different teams (see my earlier post back to SDS).
However, all that said, discs on road tandems are still evolving and they are clearly not for everyone. Anyone contemplating their use needs to do their homework which should include a candid discussion with a builder who is actually putting their name on tandems fitted with dual discs.
Brian
Remember, the primary and largest braking forces that an axle and fork must resist come from the contact patch where the tire meets the ground and that's the same regardless of what type of brake you use.
I'm afraid that either I don't understand, or I disagree. A rim brake grabs the outermost part of the wheel. If you lock the wheel, where's the force being exerted on the spokes/hub/axle? The road/tire friction would be leveraged against the brake arm to fork junction. I don't see where a spoke or hub woudl be the first to fail. With a disc, there is a tremendous amount of force transferred from the rotor/hub through the spokes to the tire/road. The weak link that will have the most force exerted against it by a high-traction situation would be the spokes. Does that make sense, or am I talking out my *****? Either way, I have a feeling I may learn a thing or two from you, and that's certainly appreciated.
stapfam
I'm afraid that either I don't understand, or I disagree. A rim brake grabs the outermost part of the wheel. If you lock the wheel, where's the force being exerted on the spokes/hub/axle? The road/tire friction would be leveraged against the brake arm to fork junction. I don't see where a spoke or hub woudl be the first to fail. With a disc, there is a tremendous amount of force transferred from the rotor/hub through the spokes to the tire/road. The weak link that will have the most force exerted against it by a high-traction situation would be the spokes. Does that make sense, or am I talking out my *****? Either way, I have a feeling I may learn a thing or two from you, and that's certainly appreciated.
When I was contemplating Discs brakes for the Tandem, I was advised by a very experienced off road tandem owner, that he had experienced rim failure soon after he had changed to discs on his previously satisfactory rim braked wheels. His theory is that the forces excerted on the rim with V brakes, work in the opposite direction to disc brakes, and if you think about it, this is true. Perhaps the rims had already taken a "Set" of force in one direction, and when then used with disc brakes the force is acting in the other direction. The problem he was experiencing was cracking on the rims between the spokes. It may be coincidence, but he had it happen on 3 rims, two of which were older rims, but on one new rim brake rim, he had it fail within 6 months of fitting.
Since he changed to disc specific rims he has had no failure, but once again it may be coincidence. On the spoke front, I have only ever broken one spoke for no reason but as this was on an old set of wheels that had done a great deal of milage I will put it down to luck that it had not broken before.
Going onto the 20mm bolt through axle. I used to have rim brakes and no problems on the forks.(Marzocchi Freerides with quick release) When I changed to Disc brakes, or rather when we gained confidence in them and started using them in earnest, We became worried about the contortions the fork was going through under severe braking. We changed to triple crown forks with 20mm axle (Rockshox Boxers) and we cured the Fork flex. Although the 20mm axle has given a certain amount of rigidity, a certain amount must also come from the triple crown, but these forks do not flex at all under braking. The pilot now has so much confidence in these forks, both on the steering and braking, that Downhill speed has increased tremendously, and Previously "No-Go" routes over rough ground are now the norm.
TandemGeek
I'm afraid that either I don't understand, or I disagree.
First of all, just recognize that you've shifted the focus from the brake forces acting on just the fork drop-out to both the wheel as a unit and the fork's crown. Recall that the original concern was how a disc brake caliper mounted behind a left-hand fork blade alters the path of brake enerty to act more down and rearward against the axle instead of mostly up and rearward as it does from a fork-mounted rim brake. It is this change in the direction of brake energy that causes the quick release -- which is really not designed to work against brake forces -- to go under shear loads since the combined paths of brake energy coming from the caliper and the tire/road contact patch is no longer pushing directly into the deepest part of the drop-out.
Now, back to where the brake energy from the tire/road contact goes... to the fork drop-out which, in turn, transmits it to the fork crown via the fork legs. To get a graphic illustration of this, just magically remove the wheel from the fork on a bike travelling forward and imagine what would happen as the fork contacted the ground and stopped the forward momentum of the bike. Brake energy is also transmitted from the brake arms to the fork through both the wheel (rim/spokes/hub) and, to a lesser extent, the brake's mounting point/fork that, in turn, transmits through the fork blades into the fork crown. So, yes, assuming the drop-out works as designed and retains the axle, the fork crown is where all that energy meets the rest of the bike and rider's mass and, coincidentally, that's where most non-suspension forks fail.
Back to the bicycle wheel itself, while it's often said the as part of the brake system on a bike the wheel is a giant disc, the wheel is not a rigid disc attached to the hub. Instead, a bicycle wheel is an elastic network of spokes attached to a rim that hold the hub in suspension, all carrying an equal amount of tension. Any force that acts on the wheel is immediately transmitted through the network. So, when you apply a rim brake, it is primarily working to stop the rotational movement of the wheel, and to a lesser extent the forward momentum of the bicycle, e.g., imagine riding on ice and applying the brakes -- the wheel might stop rotating but the bike would most likely not. Thus, brake energy associated with the operation of the rim brakes and the stopping of the bike (tire/road contact) is transmitted between the rim/spoke network/hub and the axle/fork drop-out as well as up and down the fork legs.
With disc brakes, what happens is that the brake energy path from the caliper is dramatically shortened by nearly 1/2 when the brake caliper is moved from the rim to to a point much closer to the hub axle. This concentration of brake torque at the caliper on the lower 1/4 of a fork leg is what drives concerns by some regarding the robustness of fork legs and tips, without regard to the direction of those forces. It's a valid concern; however, in practice, fork legs tend to be over-built to begin with and are seldom the weakest link in a fork: the steerer interface with the fork crown holds that distinction. But, what is different with a disc is that the brake energy developing during the application of the disc brakes is applied asymetrically to one side of the hub instead of symetrically at the rim. This, in and of itself, isn't as bad as it sounds in that the design of disc hubs take the asymetrical loading into consideration. However, what a hub brake like a disc (as well as internal hub brakes, coaster brakes, and the Arai drum brake) does is place all of the brake loads on the spoke network and none of that energy is bleed-off on the rim as it is with a rim brake. Moreover -- and this is a point that is often times over-looked -- disc brakes are also able to produce greater braking forces than rim brakes and stop bicycles in shorter distances than rims brakes. In doing so, guess what? The spoke network and rim must deal with more brake energy than is normally generated by rim brakes. This is what drives the need to consider disc-specific rims when using disc brakes -- rims that have less robust sidewalls which are no longer required since the sidewall isn't pulling double duty as a braking surface that needs to stand up to side-to-side compression by rim brakes -- but beefer spoke beds. This is why off-set rims from Velocity and Ritchey, to name a few, are usually recommended for use when someone is converting a 700c wheeled bike to discs. Off-set rims correct the asymetrical dishing issue at the front wheel which helps to equalize tension across the entire spoke network while beefing up the spoke bed. As for spoke life and reliability, the wheel builder has the burden of making sure that wheels to be used on disc brake equipped bikes use adequate spoke patterns (3x or 4x), the proper tension, and are properly pre-stressed to minimize spoke deflection at the hub, the root cause of most spoke breakage.
Again, the key to getting the most out of disc brakes without compromising safety is to look at the entire "brake system" which includes the fork, the drop-out design, the wheel components and construction, the wheel retention system, and of course the actual disc brake caliper and rotor.
TandemGeek
When I was contemplating Discs brakes for the Tandem, I was advised by a very experienced off road tandem owner, that he had experienced rim failure soon after he had changed to discs on his previously satisfactory rim braked wheels... Since he changed to disc specific rims he has had no failure, but once again it may be coincidence. [snip]
When I changed to Disc brakes, or rather when we gained confidence in them and started using them in earnest, We became worried about the contortions the fork was going through under severe braking. We changed to triple crown forks with 20mm axle (Rockshox Boxers) and we cured the Fork flex. Although the 20mm axle has given a certain amount of rigidity, a certain amount must also come from the triple crown, but these forks do not flex at all under braking. The pilot now has so much confidence in these forks, both on the steering and braking, that Downhill speed has increased tremendously, and Previously "No-Go" routes over rough ground are now the norm.
Ref. my previous post, there is some degree of truth to an increase in tension in a wheel's spoke network that can be attributed to a change from rim brakes to hub brakes, but given how the spoke network functions, the point of origin of brake energy isn't a big deal. That said, let me digress a moment to point out how often times the durability of a wire-spoke wheel is taken for granted; it is truly an incredible, technological marvel that warrants a great deal of respect. Just think about how flimsy and weak the rims and spokes are individually compared to how strong they become once laced to a hub and tensioned.
Anyway, your last sentence while addressing your confidence in your 20mm dual crown fork is quite telling in regard to where the significant increases in brake loads come from that lead to non-disc specific rim and wheel reliability issues, "Downhill speed has increased tremendously, and Previously "No-Go" routes over rough ground are now the norm." Hmmm? How many folks who have switched over to discs on 1/2 bikes have experienced increased confidence and upped their downhill speeds as well as the amount of brake energy required to arrest those higher speeds?
Just something to consider. After all, rim brakes seemed fine until suspension came along. Suspension begat higher descent speeds and more aggressive riding which pushed rim brakes and 1.125" headtubes to their limits. Disc brakes have, in turn, upped the ante again by allowing increased descent speeds and even more aggressive riding that pushed 9mm forks and the 1.25" headtubes to their limits. Where is the weak link now that we have 20mm forks with dual crowns and 203mm disc brakes? Perhaps that's why the 1.5" headtube has been heralded as the next standard for off-road frames...???
TandemGeek
If you're going to see him tomorrow, tell him I'm (im)patiently waiting to hear about a replacement crank arm.
Message delivered. Alex hasn't forgotten but he is apparently having some problems getting email through your ISP's firewall; the several communiques have all bounced.
Brian
Message delivered. Alex hasn't forgotten but he is apparently having some problems getting email through your ISP's firewall; the several communiques have all bounced.
I got an email from him today and responded already, thanks.