front disc hub question
 

All

I have recently switched the fork on my road bike to disc brakes, they are much better during the rainy months here in WA. I am now thinking of switching the front hubs in some of my wheelsets rather than sell them and build new ones. One of these has a front wheel w/ a Velocity Aerohead 24 hole rim. I know that this is sketchy for disc brakes, but am hoping that i will pull it off. My question is: I have found 2 hubs from Cannondale w/ 24 holes. One (Omega) has 2 large flanges, the other (Wind) one large and one small. Any significant benefit to the Wind design? I was thinking it might reduce the diff in spoke angles and therefore even out the spoke tension.

Also, in the highly unlikely event that my hub flange sizes match exactly, can I reuse old spokes or is that false economy?

TIA! Appreciate the help from this forum, I really enjoy doing the work on my bikes.

How did you move to a disc fork without changing hubs?

Since they make 24h disc hubs, I assume (with some reserve) that it's possible to build reasonable 24h disc wheels. HiLo flanges make some sense for rear wheels since they help reduce the tension differential on dished wheels (the benefit varies with the number of crosses), but I can't see a reason for them in the front since most front disc hubs have the same ctr-to-flange distance on both sides.

As far as reusing spokes goes, many people do, I don't. Then again by the time I rebuild wheels they have enough mileage on them that I feel I've gotten full value from the spokes and rims. Reusing spokes is a judgement call, based on how much use they've seen already, vs. their cost and the value you assign to your time. If you do reuse the spokes, try to keep them sorted by outside (head-in) vs. inside, because the elbows will have a different set accordingly, and it'll reduce the change in set with the rebuild.

I took it that the OP already has at least one disc hub front wheel and is considering changing other wheels he also owns.

Hubs with dissimilar flange sizes are annoying to build, you have to bend half the spokes fairly strong to get them to clear the big flange.

Assuming that it's the disc side that has the bigger flange - yes, that will improve the brace angle, just a tad. Haven't done the math for it, but I'd guess it'd only be by a few percent. Whether it's "worth" it or not is really hard to tell.

False economy or not depends on how you value your time, and what you know about the condition of the spokes. I've disassembled and rebuilt a bunch of wheels. For fronts it's a fairly safe bets. Rears are more dodgy.

24 spoke being sketchy for discs is a bit misleading. The big thing is available spoke cross section, which you get from no of spokes times gauge. If you'd do 24@2.3 mm gauge spoke, you'd get as close as doesn't matter to 32@1.8mm gauge, and no one would question that build. Sure, more spoke are safer if one breaks, so you'd have to be OK with that risk.

Really? None of mine has, and that includes one Shimano set.

Thanks for the input guys , here's some more info

1) Yes, I had a wheelset built w/ front disc and rear rim brakes when I switched over (before I was brave enough to make my own wheels!). Now I have my old commuters and my old lightweight weekend wheels, and figured I repurpose them
2) The hub I'm considering is this one http://www.ebay.com/itm/Cannondale-W...59521207464921
the picture makes it look like the large flange spoke holes are further out than the short ones, am I mistaken? At any rate if it's a minor improvement I'll go with the one w/ two large flanges and keep it simple.
3) Thanks for the comments on reusing spokes, if I go with that large flange hub I'll be using new spokes for sure. Thanks also for the 2.3mm spoke idea I'll be sure to use those.

I'd try and re-use the spokes. While I have had many broken spokes on rear wheels I have rarely broken a front one.

The spokes on a 24 spoke wheel may not be ordinary spokes, the kind that cost a buck or less.
They may be expensive and they may be stronger than ordinary spokes since there are fewer of them.

I recently got a disc only monstercross bike and I love the brakes.

What kind of disc are you running?

Hey Geo - thanks for the encouragement! I'll see if they fit when I get my hub

I am using a stock Avid BB7 kit, with the included 160mm disc. In the winters it is vastly superior to rim brakes. In summertime the braking is also fine, but I find on long descents that I use my rear rim brake a lot. Mostly because I get chicken when I can smell the heat coming off that front disc!

No, they don't.

If that's the case does it make sense to get an asymmetrical one for the front too?

Personally, I wouldn't base the front hub choice on whether the flange diameter is the same on both sides or not. Most front disc hubs have the same flange diameter on both sides, including very primo hubs, but some don't, and I'm sure there's a minor advantage to the differing flange diameter. I think dabac's response in post #5 is good one.

I'd consider a hi-lo flange hub or an OCR (there's an OC version of the Aerohead)... but if you were to use both, there's a chance the disc-side spokes would actually be looser.

Don't think so. Last Shimano build I did, front non-brake side had a theoretical value of 66% of brake side tension. Next wheel out was a cycle monkey hub, which had 63%. An OCR won't take much more than a nibble out of that.

I'm sure there's an oddball hub or two out there that doesn't conform to the norm, but IME they're all assymmetrical. Unless you put the effort in to track down something special, that's what you're gonna get.

"Makes sense" is a tricky thing to answer. For most riders, bikes have ample safe margins and will eventually be discarded due to other reasons than simply running out of life.
While building wheels that are theoretically more durable than average certainly makes sense from one perspective, it won't do you any practical good until you've ridden them more than average.
While JRA (just riding along) there's little-to-none benefits from having wheels that are more durable than needed.

I dug out the chart. First row is spoke gauge, First column is spoke count. A 36H wheel built with 2.3 mm gauge is set as 100% cross section, then the rest are calculated as percentages of that.

--------1.5-------1.8------2.0------2.3
18-------21-------31-------38------50
20-------24-------34-------42------56
24-------28-------41-------50------67
28-------33-------48-------59------78
32-------38-------54-------67------89
36-------43-------61-------76-----100

Blissfully ignoring things like differences in build quality, spoke quality, hub configuration and rim influences, this is a decent starting point for comparing different build options to each other. A 24H using 2.0 mm spokes is 50% of max "strength" while a 32H using 1.8 mm spokes is 54% of max "strength".
24H and 2.3 mm spokes should yield a wheel that's stronger than a 32H using 1.8 mm spokes, and a dead match for 32H@2.0 mm gauge.

As I can get them, I'd probably build brake side 2.3 mm and non-brake side 2.0 mm for 24H. It'd give me a wheel with about the same(lowest)lateral rigidity as a 32H@1.8 mm, and a decent spoke tension balance.

(And yes, I know tension isn't the correct engineering term in this case. But in this setting, the meaning should be clear enough)

dabac

I really appreciate your assistance with this, learning about how wheels work is 1/2 the fun. By asymmetric I meant hi/lo flange, but I went ahead and ordered a hub with 2 high flanges based on your advice in post #5. I like the idea of using diff gauge spokes on each side to even up tension (my current front wheel as diff tension brake vs non-brake, but as you indicate it is not as pronounced as the rear wheel). I'm pretty sure I can get 12 ea spokes in 13G and 14G. One last question - in his book, Jobst Brandt recommends swaged spokes as being better than straight guage. If I could find them in 2.3mm, would those improve the strength of the wheel?

then I'd seek out same spoke count, and same spoke hole diameter, hubs,
and build in the same pattern, then, or the spoke length requirement
may well be different , requiring new spokes.

I'm using BB5s, 160 front, 140 rear. I got the 140 rear because of novelty. It has plenty of power for the rear.
I would have got BB7s but I didn't realize they existed, BB7s are stronger, for sure.
To get the most out of the BB5s I've learned to keep them adjusted as close to the rotor as possible. Also, to have a little more of the inboard pad showing than the outboard pad, with the brake applied.
Set up that way I get a really firm lever feel and good power and modulation.

Abuse that front brake, it's not likely that you could make it overheat and fade on the road. I've used them for years on MTBs and have found them to be reliable.

The OP said he's using the existing 24 spoke rim.

I would imagine the selection of 24 spoke disk hubs is not all that great.

I was thinking you could use a 48 hole hub with one unfilled spoke hole between each spoke.

There might be more 48 spokes hubs to choose from than 24 spoke hubs.

OK.

It's not a big deal really. A Hi/lo hub can be a tad lighter, but symmetric hubs are faster to build. 'Course, there's a balance point somewhere, where the assymmetry is big enough to shave a few grams, yet small enough not to require severe bending of the spokes.
Strength/durability-wise, I'd guess we're talking about single digit percentages in improvement.

If you want a spoke that's 2.3 mm at the thinnest, it'd have to be thicker at the ends. Normally hubs aren't drilled to accept anything thicker than a 2.3 mm spoke. I think you're out of luck there.

We'd better define strength.
If you want strength as in resisting collapse, big amounts of cross section is your friend. The more cross section, the more laterally stiff the wheel becomes. The stiffer it is, the more it'll take before collapsing into a taco. (and sure, the rim plays a part in this too.)

Next thing is durability. A common spoke killer is metal fatigue, which comes from a spoke seeing big changes in its tension.
Horribly simplified, in the finished wheel, swaged spokes AKA thinner spokes will have stretched more(and is ready to spring back more) than a thicker spoke(less stretch, less springback), so as the rim compresses they're more likely to remain tensioned - which protects against fatigue.

If durability is your thing, then you want thin spokes, many of them, in a really rigid rim.

IME, there isn't that much of a point to build for extreme durability. (again, better-than-needed isn't a tangible benefit)
There's usually something else that ends up retiring a wheel before that stage is reached.
What good is an expected life of twice-around-the-world if you hit a bad pothole while riding coast-to-coast? Or theft, or vandalism, or....

There are spokes with a 2.3 mm elbow, and an 1.8 mm shank. The value of this is somewhat debated.
On the good side, it can provide good support and a tight fit at the hub.
On the bad side, bending a 2.3 mm spoke at a right angle means that the material at the outside face of the bend has to stretch more(and the inside has to compress more) than for a 2.0 mm spoke - which some claim creates fracture points on pretty much the molecular/crystalline level of the metal.

It'd be buildable, but odd. With a regular 24H hub, the spoke holes in one flange will be dead center between the spoke holes in the opposite flange. If you take a 48H hub and leave every 2nd, then you get a staggered pattern with the leading and the trailing spokes running at different angles.

ran 24h disc wheels on my commuter last year. no issues. 24h/ 2.0 spokes, stock wheels on a trek portland. rider weight 200lb. probably had 4000 miles on them

Just in the spirit of being thorough: try changing your braking method on descents. Bike rotors have a relatively small thermal mass and you should use the same method as cars when descending a mountain: don't drag the brakes, brake hard at times when you can, and let the brakes cool the other times.

A car known for it's ninny brakes, the Prius, even has hill descent mode for this reason. It drags the gas engine to help provide additional friction and prevent too much speed gain on descents. It can do this because it can circulate coolant to then shed the generated heat.

Since you don't have an extra method to wick away heat, you have to be sure to let the brakes cool since the rotors are not vented.

Yes they are cross-drilled, but not vented like a car, which has 2 flat surfaces and cooling vanes between. This is one reason why car brakes are so good, surface area. They're even taking this farther and farther:
http://wp-content.rpmware.com/wp-con...es-580x561.jpg
http://www.socalevo.net/gallery/albu...ineering20.jpg

So happy braking!

Holy crap! What's that from? It's... just... epic.