bike geometry 101
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bike geometry 101
Post by ue on Sept 28, 2007, 3:35pm
I have been comparing bike geometries in the last little while and would appreciate your input on some of the different bike geometries.
The biggest difference is in Seat tube angle. Of all the "All-mountain" bikes I have been looking at, all have a seat tube angle of 70° or more. There is only one that has a STA of 68°. How much does the STA affect your riding and what part of the riding does it affect the most?
Also , there is quite a difference in wheel base distances. How much importance should that play in the decision making process? Does it only affect ease of turning tight corners ?
What about the chainstay and effective tob tube lenghts? How do these affect the ride? ThAnks a lot for your input.
Ute
Re: bike geometry 101
Post by jackp42 on Sept 28, 2007, 8:00pm
Just to add to the confusion, seat tube angles can't always be taken at face value since a on a number of bikes, the seat tube does not intersect the bottom bracket like on my SuperMoto where it intersects significantly in front of the BB. I believe this is done on long travel bikes to allow the rear wheel to clear the seat tube.
Then of course there is the top tube length. Some times this is actual length, some times it is the theoretical horizontal.
How the combinations of all these different dimensions interact is part of the black art of bike design.
Re: bike geometry 101
Post by brad on Sept 28, 2007, 10:00pm
Jack makes a very important point. Bike geometry and ride quality truly seems to be a black art. Sometimes the numbers on two bikes can be identical, but the two bikes will ride like they're from different planets.
Ute, I think the approach you've taken so far is good. Figure out your true 'target' trail for the new bike and get your hands on as many different bikes as you can and see how each of them feels. When you find some that feel good, try them again.
I think I'm correct in saying that your target trail is Hilton. If I recall correctly, you've tried a 6.6, a Heckler, a SuperMoto, your Devinci, and probably some others too. You could spend another year borrowing different bikes trying to find the exact 'perfect' bike. Or you could just go with what your heart is already telling you, and BUY IT!
I say do the latter, get on the trails, and enjoy the ride. And no, Paul (your boyfriend - not Stoner) did not tell me to post this
Re: bike geometry 101
Post by stoner on Sept 29, 2007, 6:13am
And to make things even more complicated, differenet companies measure things in different ways (with or without sag, sometimes they don't specify if it's horiz top tube or effective top tube, my Stinky's geometry was specified as measured with a 4" fork but was sold with a 5" fork!)
Breaking it down I would say that the most important variable is horizontal top tube length. That is going to be the best measure of how the bike fits you. (It's actually pretty bizarre that bikes are classified based on seat tube length since the saddle is adjustable but the top tube length isn't!)
For Hilton, the next most important variable is head angle. A steep one will be twitchy and a slack one will rock (but you might hate it on the steep climbs).
Almost equally important for Hilton is BB height. A low BB is a bad idea at Hilton unless you are mad at your pedals and want to get back at them.
Chainstay length affects how easily a bike can be wheelied. Short ones are easier and a short chainstay is the sought after item for urban / trials riders I think.
To me things get fuzzier now. Wheelbase is just a consequence of head angle, fork length, top tube length and chainstay length. Downhill people talk about wheelbase and longer ones are more stable and short ones are faster turning.
Seat angle? Haven't a clue. I guess for a given bike when you raise the seat to the right height then it will adjust your c.g. and could affect your climbing (but I always climb standing). You can move the seat backwards and forwards in the rails or get a straight or offset seatpost to adjust for a seat angle that isn't right.
I'd be very interested in hearing what others think about seatpost angle and all the other variables!
Paul
Re: bike geometry 101
Post by jackp42 on Sept 29, 2007, 9:45am
Paul, I agree with your read on head tube angle, top tube length and bottom bracket height. Really I think head tube angle is less important then trail, but since in the bicycle industry they don't measure trail, head tube angle is a good indicator since the axle to steer tube offset on most forks are reasonably consistent.
In terms of chain stay length, I think your comments are correct for a typical hard tail layout, but I think we are seeing a trend towards longer chain stays with longer travel bikes. I think really the important thing is the relationship of the horizontal distance from the saddle to the rear axle. Chain stay length is only part of this. By changing the seat tube angle and/or offseting it, you can cheat a bit, but of course this is all really driven by the ergo position from saddle to cranks.
With regards to seat post angle, I don't think it is that important if intend to always ride your bike with the saddle at 1 height. If you intend to lower it in some instances and raise it back up in others, I think a steeper seatpost angle is a benefit. Otherwise you cockpit sprinks and your knee bend angles can get compromised too much. I find the seatpost angle on my SuperMoto a little too slack for this and ended up running the seat further back the optimum when fully raised so that it is better in a typical position for technical terrain like HF's. Otherwise I think designers change it to get tire clearance and to accommodate rear suspension design (look at a Maverick for example).
Paul, I'm currious to know what you think is a good BB height for say a 6" travel bike. I know mine it higher then most at 14.65" but I can still bash my pedals good at HF's. I know Mo commented that he has to pay particular attention to pedal clearance since his BB height on his 6.6 is considerably lower.
I could go on and on, but I will let others chime in.
Jack
Re: bike geometry 101
Post by morris on Sept 29, 2007, 3:12pm
Ute make note of a few other important items, handle bar width and rise, seat fore and aft position, gearing, fork and shock set up, tire width,type and pressure.
We all set our bikes up a bit different.
Re: bike geometry 101
Post by stoner on Sept 29, 2007, 8:58pm
Don't listen to Sprockets.
It's all about the Q factor. Once you get that right nothing else matters, well, maybe crank arm length.
P