kevbo

I put this in frame building, because it seems to me that frame builders have excellent motivation to understand the ins and outs of frame geometry.

Conventional wisdom seems to be that short chain-stays make a bike a better climbing bike. I read this all the time, but I never see any discussion as to WHY that is so.

I can see two possible reasons:

-Stiffness: Short members flex less under high chain tension, keeping the rear wheel (and hence the whole bike) running on a straight line, and absorb less energy in the tubing itself.

-Balance: Short chain-stays put more weight on the rear wheel, and prevent wheel spin in low gears.

In particular, I'd like to know if this same wisdom applies to very large frames....64cm and up. If this is something "you can just feel" then it may well apply to more "normal" sized frames, and less so as the frame is scaled up....if most riders ride normal sized frames, then most riders might agree on something that doesn't apply to not-normal sized frames.

I'm fairly tall (6'5") but extremely long of limb. I've never been able (or willing, I guess...I am Scots!) to shell out the dosh for a custom frame. The factory 27" frames (no, not the wheel size...think 67cm frame) of the late 70's and early 80's fit me pretty well, and I enjoy riding them when I'm not truing or replacing the rear wheel. These frames typically scaled up all tubes _except_ the chain-stays. The result is that the rider's tail-bone ends up over the rear axle, or nearly so This shifts much more of the riders weight to the rear wheel, and tall riders are also heavy....It has been a long time since I weighed 190# and I was damned skinny at that weight. Pumping newspapers up and down a hilly route through my teens has given me huge legs that are not balanced by a heavy upper body...so my weight is in my legs, and the front wheel carries little weight.

Pretty much the same thing happens when I run the seat post out 8" on my modern 64 cm frame....modern wheels are a bit better(rims are lots better, but increased gear count causes too much dish) so I've had fewer issues. Running a tandem spec wheel is a possibility, but I still think my bike would handle better with a better weight distribution, and a tandem wheel seems like a case of treating the symptom rather than the underlying problem.

I've also been a bit leery of having a custom frame that is off the beaten path built. As I explain above, it seems like big companies with lots of resources don't quite get it right. I can judge a builder's craftsmanship by samples of their work, but their wisdom and insight with regards to dimensioning a frame for a tall rider is trickier. Leonard Zinn may well understand the ins and outs of a tall frame, but he is justifiably proud of his work (and prices it accordingly) , and as for others, who knows? I'm sure some would build whatever I spec....If it doesn't work out, I've got a frame that is too big for 95% of the potential market where I might unload it, and probably not well suited for those who can ride it. My finances are such that it would be a real stretch, and I'd only have one shot in my lifetime to get it right.

I'm finally about to the point where I'm ready to try my hand at frame building, and I'm thinking I want to build myself a tall frame with ALL the tubes scaled up. The cost of the parts, and my time, I could risk. I could try a geometry (or three) with modestly priced tubes and rattle can paint, ride it a while, and build another with the fancy tubing, lugs, and Imron when (if !) I get it right.


If the goal of short chain-stays is to put a "normal" rider's weight back, then this should be a non issue, as the F/R weight distribution should be maintained. If the concern is stiffness, then beefier stays should address that. Oh yes, and if I run panniers on it, then the longer chain-stays help clear my size 14 (Eu 50) heels that are turning 180mm cranks.

Any help understanding the trade-offs one makes with regard to chain-stay length will be appreciated.

merlin55

back in the 70's ultra short chain stays bike were a micro-fad. The thought was shorter stays resulted in a stiffer bike, which just had to be better, It just wasn't so....

Now people like Cervelo (and just about everbody else) has figured that you can have a bike that is stiff (not much lateral swaying) when out of the saddle, yet comfortable over the bumps, by building carbon frames with long head tubes, large downtubes and chainstays, and smaller seatstays. Such bike, like my Douglas Matrix, is comfortable on rough roads, yet is stiff when sprinting or climbing out of the saddle.

NoReg

That said I still see bike with tucked wheels, I think the NAHBS winner this year may have had one. Stuff like that is too edgy to affect my performance. I build for long range comfort. I know of guys who do long range mountain touring on the road and use the longest stays they can manage. Now we have these folks using the Big Dummy for mountain tours. Not tour stage winners in the mountains, but still makes you wonder how many points there are in these things.

kevbo

Thanks for the replys. Glad to know I'm not missing something really obvious. I have ridden our tandem (Bilenky I found used) some without a stoker, and aside from the low traction and weight, it ain't bad at all.

Another thing I thought of is that normal sized folks can often swap bikes with thier riding buddies, take test rides at bike shops or factory events, etc. When all the bikes are too small for me, this isn't possible, so I am missing out on the best way to determine what works well for me.

crock

Okay, this is going to be hard to describe. When you stand up to climb or sprint, it is more efficient to move the bike under your center of mass than to move your center of mass over the bike. The shorter wheelbase bikes do this easier than longer wheelbase bikes. I don't know how else to describe it. A long wheelbase bike always wants to go straight, where a short wheelbase bike will turn beneath you so you can throw your body weight over the pedals.

kevbo

Ahhh. I hadn't thought of that. It takes a while to learn, but you can propel a skateboard by swerving back and forth, and pumping your weight up and down at the right time. ("Grapevining")...or just watch a speed skater.

MichaelW

Some of the handling characteristics of a LWB vs a SWB (in the normal sizes) is that SWB is usually given a sporty steering geometry whereas LWB are set up with stable touring steering geometry.
For big guys, every bike is a SWB, you have to figure wheelbase, like everry other bike dimension as scalable and proportionate to rider size. Small riders have the opposite problem where almost every bike is LWB.
I just looked up an example from a brand with a big size range (Willier goes from xs to xxxl), they all have a chainstay of 40.5cm

IbelieveLiar

Sounds like your talking Mtn bike. You are perfect for a 29er period. It will be a total epiphany. Maybe a custom build as well. Research Zinn as he builds only for tall folks.... I'm sure he has some info.

A guy your size is ouside of the traditional frame numbers spectrum. You should be on a heavier bike with larger wheels, and I would venture, longer stays. It's all relative!

Thylacine

I'm 6ft 4ins and honestly, there isn't an off the shelf bike I've found where I think the numbers are vaguely right.

I have 420mm stays on my roadbike and wouldn't dream of anything shorter, yet the mainstream companies insist we all need 405-410 stays. Why?

As you rightly state, the taller you are, the more rearwards your seat is, therefore you need longer stays to keep your centre of gravity somewhere reasonable. It's just logical, right?

At the risk of blowing my own trumpet, you'll never get a stock bike to fit you right, and although you may not like the lack of test bikes to ride or the chances of you ever selling one of your second hand bikes, that's just the reality of the situation.

The lack of test bikes can be remedied : Indulge yourself in a custom bike and let your chosen designer do what they do best.