Frame geometry and handling
Over the years I have heard lots of references to frame geometry which cite "angles" such as 73 degrees (apparently that's what Co-Motion design to). Also references to millimeters of rake eg, 55 for Santana and 45 for a Co-Motion with Woundup carbon forks. But . . . what do these figures mean? Can anyone explain in layman's terms how this affects handling and stability of tandems? Perhaps I'm not the only one who would be grateful for an explanation.
Generally speaking, a seattube angle of 73 degrees is pretty much what a road racing frame has. You'll see alot of 72.8, 73.4, which are a Mfg's way to pretend they're different. Be real wary of the old "proprietary" geometry story. That usually means your being sold a bill of goods. A slacker angle, less than 73, will be more appropriate to a touring bike. Steeper is time trial bike... I've got a 78 degree TT bike. The slacker your geometry, generally you'll ride more upright. That may be more comfortable, but for me I really like steep seattubes as I've riden TT so much. Head tube angles are about teh same: 73 is standard, slacker than that is touring-type geometry with some noteable exceptions. So... this matters depending on how you ride: race/performance vs. touring/comfort.
I will try to help you then Mark can clear up what I mess up.
On any two-wheeled single-track vehicle (bike or motorcyle):
For any given head angle (say, 73 degrees), the more rake in the fork (the higher the offset measurement) the less trail the bike will have. Bikes with less trail will initiate turns quicker and with less resistance. Longer trail (less fork rake) will result in a bike that is slower to turn. Think Harley (lots of trail, wants to go strait) or Kawasaki Ninja (less trail, wants to turn).
Some folks rave about the Co-Motion 'racy' handling which in fact has more trail than other tandems which would result in slower steering. I have never experienced this first-hand. But, I would guess that on high-speed descents, the slower steering imparts a greater sense of control and stability to the pilot and thus the pilot pushes the speed of the descent.
Tandems are very stable at speed anyway because of the long wheelbase, so I can't quantify how much difference it would make.
Useful info on angles. How is seat tube angle measured? And how does it equate to the millimeter figures TandemGeek throws around? Thanks.
Seat tube angle on almost all production tandems is 73 degrees front and back. I think large Burley's might be 72.5. Seat tube angle on tandems has almost no effect on the steering. The bike is too long and changes in how much weight is on the front wheel vs the back wheel will be negligable. Most folks adjust the seat fore-aft to the KOPS of their single bikes anyway.
ElRey is talking about single bikes.
Frame geometry is not the same as fork rake, trail etc.... Best to keep the two separate. Your riding demands should determine what type of frame (touring vs. racing) you lean towards. The fork will probably come with the frame, so you don't have to know anything about it: no good Mfg. will put an inappropriate fork on their product. If you're replacing just a fork then you need some more data. If that's the case, this sort of forum is not the best place to decide: contact your frame Mfg. and get recommendations from them. A racing fork on a touring frame won't make a racing frame.
Frame Geometry IS the same as fork rake, head angle and trail. They are the DETERMINING FACTORS of the frame geometry and how the bike will handle. The combination of head angle and fork rake have more effect on the bike's steering than any other factors. Fore-aft weight distribution, center of gravity and wheelbase will also determine how a bike handles, but to a lesser extent than the front end geometry. After all, it is the fork that does the steering.
We are talking about tandems here, not racing singles or TT singles. If you look at any tandem manufacturer's website and look at the frame specs, you will find that with few exceptions, the frame geometry is identical within the models/price ranges for each give frame size. The seat tube angles are the same, the wheelbase is the same, the fork rake (offset) is the same. Burley offers an upgrade to a carbon fork with less rake but otherwise, everything is the same across the line.
You can change your fork rake and trail by buying another fork. You can't change frame geometry. And frame geometry is not determined by fork rake, trail or anything else related to teh fork. Once the use of a frame is established and an appropriate geometry layed out for that use, then the fork is designed around that. The frame is not designed around the fork. Call a frame builder and ask them.
by the way, I'm not just guessing here: I just replaced the fork on my Co Mo with a Reynolds tandem fork (yesterday). Just went through all this. Done it a million times on racing bikes. Not my LBS, me... research, purchase, installation.
Changing the fork on a given frame will completely change how it handles, even to the point of making the bike unstable.
Of course all other factors are considered but the frame's steering angle and fork offset are primary considerations.
And again I will state that with tandems, with very few exceptions, the frame geometry is the same regardles of it's intended use.
And here are some long-establisthed general guidelines for single bike geometry:
Crit racing bike - moderate to steep head and seat angles, short trail, high bottom bracket for quick steering and pedaling through corners.
Road racing bike - moderate to slack seat angle, moderate head angle, moderate to long trail, lower bottom bracket for comfort, more control descending, powerfull climbing position.
Road racing TT/Tri bike - extremely steep seat angle, moderate to slack head angle, moderate to long trail, low bottom bracket for aero riding possition and stability.
Through the size range:
Smaller frames have steeper seat angles and slacker head angles and more fork rake. Larger frames are the reverse.
Small changes in rake/trail - 3 mm - can make a big difference in the way the bike steers.
Due to the advent of brake/shift levers, racing bikes in general have less trail than they did 10 years ago IMHO.
Let me attempt to restate since I don't sem to have made myself clear: you can't determine trail and rake from teh frame's geometry. They relate to the fork. Of course rake and trail impact handling, but where did I say they didn't. BUt they are changeable, not frame geometry. Our friend here semed to think that they were all one: he lumped head angle, rake and trail into one grouping and they aren't in the same grouping. Rake and trail can be alterd by buying a new fork. You're stuck with frame geometry so that's where you look closely. Hence, I initially bought a frame set with 73 degree frame geometry (Cdale) and changed out the fork with something a little bit quicker. Changed the handling characteristics completely. Frame geometry didn't change.
Here's something I wrote in Aug '04 during a thread discussing the differences between a Santana Sovereign and a Co-Motion Speedster. I think it's perhaps closer to what you're looking for...
Originally Posted by JayB
It's probably an overstatement to say the Co-Motion geometry is "unstable" at slower speeds in much the same way it is to say that the Burley, Santana, or Trek tandems are "unstable" at high speeds.
Originally Posted by ottodog
Not to put words into other folks mouths, but having read how various folks have attempted to put riding impressions into words over the years -- myself included -- I think a more accurate way to characterize it is....
A Co-Motion demands more attention and steering control by the captain at speeds under 10 mph and is very sensitive to weight shifts or movement by the stoker. This is sometimes disconcerting to new teams during initial test rides or even experienced tandem teams who have ridden tandems with less steering trail when they first ride a Co-Motion. However, it's also something that almost any but the largest teams or teams with stokers who move around a lot will assimilate into their normal riding experience in short order (like a couple of rides). The other, shorter trail models (Santana, Burley, Trek, Bilenky, and most others) have a more refined feeling at slow speeds and are less sensitive to weight shifts or minor movements by the stoker. The Bilenky is perhaps the most refined in this regard as it has very short steering trail. At the opposite end of the performance spectrum, the Co-Motion comes into it's own as it is very responsive to leaning inputs for directional control and does not exhibit any oversteer throughout the speed range. The tandems with less steering trail will begin to exhibit understeer at higher speeds and do not inspire aggressive cornering as much as the Co-Motion. In the mid-range, the Co-Motion is a livelier ride than any other the others, whereas the Santana and Bilenky are perhaps the most refined. In short, comparing tandems is like comparing fine wines (or coffees and cigars, noting that is what Co-Motion names its tandems after). Some are more sweet, some have more body, and some are just plain awful. Everyone's tastes vary, so you try to find a good match.
If you can find it, my column from Issue #16 (August/September) of Recumbent & Tandem Rider Magazine also addressed tandem steering geometry without getting too tied up in the numbers. However, as others have noted, unless you are having a custom tandem built or changing the fork, a given tandem's steering and frame geometry are factored into the tube selection (material, diameter, thickness, internal or external butting, and shaping), and even the wheel and tire selection to create the steering, handling, and comfort that the designer or marketing department believes are what its customers are looking for and/or what they believe exemplifies how their products should perform. The latter is why test riding is so important, both when shopping for your first tandem and more importantly after you've been riding for a year or so and start considering the things that you'd like to change about your next tandem.
... and if you haven't figured it out yet, steering geometry and frame design are the most controversial subjects in the bicycle sciences.
And I went the other way on my old Gitane with 74 degree head angle. I had the steel fork straitened by about 3 mm thus increasing the trail.
Originally Posted by ElRey
Both of us, by our fork modifications changed the frame geometry: We changed the rake, we changed the wheel base and I changed the steering angle slightly by raising the head tube a smidge.
My Gitane was much less twitchy on rough descents after the change.
You didn't change frame geometry, you changed steering geometry! They're not the same, which is my point. You can only really change "effective" frame geometry through the offset of your seat post; head angle can't be changed. If you straightened your curved blade fork, not only did you change the rake, you changed the blade length, so the frame may not ride plumb anymore. I don't mean to be combative here, but I think the guy really wanted to figure out what some of this stuff means. My intent was to point out that fork variables like rake and trail are true variables within a frameset (frameset: frame and fork in concert) since the fork can be changed. Headtube angles and seatube angles are fixed (until you begin to offset your dsaddle one way or the other with a rear or forward ofset seatpost). Good thing JayB didn'st ask about how all this relates to stem length.....
Actually, you need the head tube angle based on a given fork length + the fork's rake + wheel diameter to calculate the trail. You can unintentially alter the frame's geometry when you change fork to obtain a different amount of rake if the fork length is NOT essentially the same as the original. With regard to the change you made, replacing a steel Cannondale fork (53mm of rake) with a Reynolds Ouzo Pro tandem fork (55mm rake) would not have been much of a change as they most likely have nearly the same fork length @ 395mm (+/- 5mm): a small change in trail. However, given that most tandems use forks that are longer than solo racing bikes, a lot of folks who switched over to AME's (now True Temper's) Alpha Q forks lost nearly an 1" of fork leg length which had the net effect of change the head tube & seat tube geometry by a full degree, as well as lowering the front bottom bracket.
Originally Posted by ElRey
Thus, the suggestion to discuss fork changes with your frame's designers or builders is always an essential step as some changes -- if not fully understood -- can really alter how a tandem will handle, possibly making it better or in some cases making it worse. Moreover, when head tube angles are altered by fork leg length, the net head tube angle needs to be understood before you can decide on what amount of fork rake to use to achieve the desired amount of trail. Burley worked with True Temper on it's V-brake tandem fork (essentially an Alpha Q CX fork that was beefed up for tandems) to give it the same fork length (and a larger wheel opening) as their steel forks so that they would retain their 73 degree head tube angles and used something even more aggressive than Co-Motion's steel fork spec. of 50mm of rake to achieve what they call their "race handling" package.
Again, as a consumer, the only thing you need to understand is that some builders use different steering geometry. Ride different models for yourself and then ride them again before deciding which you prefer and, remember, you're buying to suit your preference, i.e., what's best for you.
Last edited by TandemGeek; 11-03-05 at 11:55 AM.
Time for a change.
It is all very confusing, but the final test is in the ride. Steeper head angles will make the bike more responsive till it gets to the point where it is dangerous, slacker angles make the bike more relaxed on steering till the point comes where there is a time lag on what you do and what the bike does.
Then you change the fork and mess it all up, so providing the bike handles the way you want it to, Don't worry.
Then you worry about rake and what differences that will make, so why be confused. Leave the bike as standard, find out whats wrong with the handling and then talk to a frame builder.
Or just ride the damn thing.
Yes, that was my meaning when I said trail related to the fork: it can be changed if the fork variables are changed. Changing effective headtube angle if you dropped the fork length an inch?? Yes, but you also change the balance point of the frame so much (more weight forward), and BB height (less ground clearance when cornering), that I would wonder if you're not better off buying another frame with a steeper headtube. (you noticed I got the exact same fork length to avoid this... I think the rake differential was actually more than the pulished numbers). Saw a guy riding a $9,000 carbon tandem yesteray from a custom builder... he had aerobars mounted up on an 80mm stem. Aerobars on an 80mm stem... try it sometime. You better not catch a crosswind when you're on them aerobars. I don't think he asked the right questions when he got his custom frame.
We got JayB so confused now he's off buying a sled.
So you replaced a Co-Motion (steel?) fork with a Reynolds Carbon fork. Wondering why you went with Reynolds rather than Woundup? What differences have you noticed with the carbon replacement fork?
Originally Posted by ElRey
Well, have to admit some of this went over my head. Interesting though. TandemGeek says Santana etc are "most refined in this regard as it has very short steering trail". I was interested in his comparison about the way a Co-Motion and a Santana handle at various speeds. But on this "trail" business: let's say I want to measure the trail on my old 531 tandem. I've got a measuring tape in hand. What do I do now?
pan y agua
There's another way to get at this: Ride the bike, and see how you like it. How the bike rides is going to be determined by a lot of factors including the geometry of the frame, but also including the type of material, the weight of the team, the thickness and butting of the tubes, the type of wheels, the thickness of the tires, the length of the stem, whether it fits you, etc. etc.
You can go on ad infinitum about how head tube angle will, ceterus peribus, affect the handling of a bike. But in the real world, all other things aren't equal, and the only way to figure out wht works for you is ride the bike.
Some resources that you might find interesting relative to your last post:
Originally Posted by JayB
There are many others, but these do a pretty good job and the last one will let you punch in some numbers to see how changing different fork rakes (or head tube angles) will affect trail. Off the top of my head, 28mm tires work out to something like 684mm in diameter for tire size.
To know how much trail your tandem has you could always call the builder and ask.
Otherwise, the easiest way is to get a helper. Lay a 2Ē X 4Ē X 8í stud on the ground, flat side down next to a wall or some other object that you can rest your tandem against. Put the tandem on the 2x4 and slide the entire arrangement as close to the wall or object as you can so that the tandemís handlebars can rest against the object sitting as close to upright as possible without falling away from the wall or object. Take the tandem off the 2x4 and use a carpenterís level to make sure the 2x4 is level, shimming as necessary. Put the tandem back on level 2x4 and with the aid of a helper to hold the bike steady, drop a plub bob from the center of the axle nut and strike a mark on the 2x4 where the point comes to rest. Next, have your helper hold your plub bob's string tight against the stem's fixing nut and take the other end and run it down to the ground in a straight, taught line such that it splits the center of the head tubeís left side to represent the steering axis and mark where it contacts the 2x4. Measure the distance between the two marks; thatís your steering trail. The accuracy will be as good as your ability to eyeball the centerline of your head tube. It may even help to have a third person eyeball the fall of the line against the head tube or to use a set of Vernier calipers to find the center of the lower head setís crown race to guide the fall of your line. Itís not fancy, but it will yield the answer to your question without having to resort to a protractor to measure the head tube angle and removing the fork to measure the off-set/rake.
Oh yeah, and if all of this wasnít confusing enough, changes in tire diameter also alters steering trail.
Hey Jay: Last year I went through exactly what you are going through now trying to understand how "trail" effects handling. One problem, as you can see from reading this thread, is that people have a tendency to get things backwards and say the opposite of what they mean when discussing trail.
To put it as simply as possible, trail is the distance between the bottom point of your tire (where it sits on the ground) and the point where the steering axis meets the ground (an imaginary line down through the center of your head tube to the ground). It is called trail because (on almost any bike) the contact point of the tire trails the steering axis by a small distance.
The more trail there is (i.e. the closer the wheel is tucked up under the frame) the quicker it will tend to steer and the less stable it will tend to be. The less trail there is (i.e. the further out in front of the bike the wheel is placed) the more stable it will tend to be). People constantly get this backwards, though. Probably because it is counter-intuitive to think that the further a bike sits behind its front wheel the LESS trail it has.
As others have pointed out, though, trail is only one factor in how a bike handles and feels. If you are used to riding modern road bikes you will probably prefer the greater trail of the CoMos or Burley Race Package. If a more solid feel and less wobbling when the stoker shifts weight is more important, you'll probably prefer the shorter trail of the regular Burley's Treks, Santanas, etc. There is obviously no substitute for a test ride --- but finding the bike you want to test can sometimes be a challenge.
Last edited by K&M; 10-17-05 at 04:57 PM.
Actually, I believe the problem is one of semantics surrounding the word stability and a failure to put that term into context when describing how bicycles handle, followed closely by the same ambiguity that exists around the expression "fast steering". Since I'm at home now I have access to some of the columns that I've written, to include one dealing with this topic and an illustration that also ties in with this discussion that summarizes my views on how sematics muddies the water.
Originally Posted by K&M
However, despite everything that's been written about steering trail and how well most people understand what they've read an studied in those writtings, tandems tend to defy logic since everything is backwards... instead of the "racing models" having shorter steering trail than the models favored for touring or less aggressive riding, they have longer steering trail. Again, from my column:
Originally Posted by Me in RTR #16
Finally, I included the following illustration and summary of my perspective on tandems, steering geometry, and how they handle: feel free to disagree; after all, we are talking about steering geometry.
Originally Posted by Me in RTR #16
Last edited by TandemGeek; 10-20-05 at 03:18 PM.