Clue me in on geometry

[o2dazone]

I've only been into cycling for about a year, and I'm still pretty green on geometry. I can spot certain things, longer head tubes, shorter stays...but nothing comprehensive. Does anyone have some good reads on bike geometry, and the differences between them? If it helps, I'm more interested in road bike geometry, less so with mountain.

[OtayBW]

Here's a good read on one small piece of the puzzle: http://www.spectrum-cycles.com/geometry.php

[vqdriver]

i like this point made by david kirk.
http://www.kirkframeworks.com/blog/2009/06/19/riding-tip-3/
it's more rider position than geometry but one leads to the other eh

[Peter P.]

Here's a link on bike geometry as it relates to handling:

http://cyclingtips.com.au/2011/02/the-geometry-of-bike-handling/

[carpediemracing]

Here's a good read on one small piece of the puzzle: http://www.spectrum-cycles.com/geometry.php

Interesting. I liked my compact frame TCRs, especially the aluminum one, which was 4 cm shorter in height than the same size frame in carbon (size S). I had my custom frame modeled after the TCR in terms of seat tube length to try and replicate this "snappy responsiveness" without really thinking of why the TCR aluminum felt so responsive.

Now I realize that it felt responsive when out of the saddle, so much so that I had to adjust my riding style a bit. In the saddle there is no difference and I didn't care.

The above article explained to me what I instinctively thought but couldn't explain. Basically I was rotating less mass around the BB area, which is basically what follows a straight-ish line when sprinting or climbing out of the saddle.

I did realize that a light saddle really makes the bike snappier out of the saddle, as does a light post, and I've made a moderate effort to keep my saddle light and my post reasonably light.

Finally lighter bars will help a bit but right now I don't have particularly light bars.

[carpediemracing]

As far as my own thoughts on geometry:

1. For handling you should consider, at least for factory frames, head tube angle and fork rake. This basically determines how responsive the frame will be when initiating any change in turning arcs (whether initiating a turn or adjusting mid-turn) and how stable it'll be in a straight line.

1a. Wheelbase, often quoted as something affecting handling, isn't huge to me. My current bikes, due to the super long top tube, have wheelbases well over 100 cm, yet they are plenty fine for technical crits. Any problems I have in those races are due to my fitness and tactics, not the bike.

2. I never messed with BB height other than noticing my track frame, with a significantly less BB drop, felt sort of unstable, like I was sitting too high up. The rest of my bikes have been in the normal range of 65-70 mm BB drop.

3. Seat tube angle really only has to do with fit. There won't be a performance difference between two seat tube angles except in terms of optimal fit. For example if you need a lot of saddle set back then having too steep an angle will force you to use a big set back type post and shove the saddle all the way back. A shallow angle will be more efficient. For me, with a very forward position, a 75.5 deg seat tube angle means I slide the saddle pretty forward on a zero set back post.

3a. If you have shorter quads or a stiffer back (I have both) then a forward position might work well for you. Jens Voigt has/had a super forward position, a few cm more forward than the similar height Tom Boonen. Voigt's set back was something like 5 cm, which is tiny when you consider how high his saddle sit. The forward position allows you to get lower while staying relatively comfortable.

4. Chainstay length really affects out of saddle responsiveness. I was lucky enough to have an A-B-B experiment with my two custom frames. On the first one I specified a 40.5 cm stay, which is sort of standard for a smaller frame (other important stuff - 73 deg HTA, 43mm rake, 75.5 deg STA, no mention on BB drop, 9.5 cm HT length aka "as short as possible"). I found, to my dismay, that the rear wheel chattered in turns, even while coasting, due to the massive front weight bias my long torso frame exhibited.

On my second custom geometry frame I kept everything the same except asked for a 39.0 cm stay, so same HTA, STA, TT length, etc. Obviously the wheelbase was about 1 cm shorter. Perfect, and as a bonus the bike was super responsive when out of the saddle, I felt like I could point the bike anywhere instantly, like I was riding a unicycle.

Then the critical bit - I sent my original custom frame back and had the stays shortened "as much as possible". This resulted in a 39.3 cm stay, and the rear tire stays planted in turns and the bike is really responsive when out of the saddle. Since it was literally the same bike from the BB forward it was a real "before-after" type test. The dropouts, chainstays, seat stays, and brake bridge had to be replaced, and I have the frame painted locally after I received it.

5. Stem length and bar reach affect weight over the front wheel so you want to keep your hand position the same relative to your front wheel, at least on a custom frame. This means that if you have a 12 cm stem with an 11 cm reach bar and you change to a compact 8 cm reach bar, you'll need to get a 15 cm stem to maintain the hand->front-wheel relationship. For me a 12 cm stem with an 11 cm reach bar works well for weighting the front end. I rode basically the same size bike but with a 9 cm stem (the Missus's bike) and the bike felt really wobbly once up to speed - the problem was that my hands weren't forward enough (I was sitting further back on the bike as well) so my weight wasn't really up front.

In fact I ran into this problem with my own bike when I switched from the older 15 cm drop + 11 cm reach bars to newer compact 12 cm drop + 8 cm reach bars. I really liked the shape of the 12 cm reach bars (FSA Wing Compacts) so I decided I'd make them work. The problem was that even with a 14 cm stem making up most of the reach difference, I couldn't get 3 cm more drop. Therefore my hands were 3 cm higher than I was used to, and that meant that my BB->drops relationship was wrong (important when out of the saddle), and I didn't have enough weight on the front wheel. When I did sprints out of the saddle I literally almost crashed a few times, losing control of the bike, etc.

I wanted to make the bars work. For me that ended up meaning a custom stem to get 3 cm more drop and 2.5 cm more reach (I just didn't feel right asking for that last 0.5 cm - remember, at this point I'm already asking for a 14.5 cm stem). Once I got the new stem the bike handled great again from the drops. The stem is ridiculous but my hand position relative to the front wheel/tire is the same as it's ever been.

[classtimesailer]

ST and HT angles range from 71.5 to 74.5. Fork rake goes from 40 to 55. Chainstays from 40 to 46. BB drop from 60 to 75. All this makes for wheel base from 99 to 103.
If you are average, go 73, 73, 45, 41, 65, 70, 100.

Race bikes are shorter smaller faster sensitive.
Non Race bakes are not.

You are new. Get a Race bike. Go fast.

[cmg]

If you've got some time read the articles about fit from Steve Hogg. http://www.stevehoggbikefitting.com/blog/ buy 2 cheap ebay bikes of similar size and experiment.

[tigoat]

I would get a decent bike off the shelf and ride the heck out of it to establish a baseline. Don't read too much into some experts opinions, at lease at the beginning, as every rider is different, what may work for others may not work well for you. I personally have to toss the rule book out of the window when it comes to designing a geometry that works for me. Have fun!

[o2dazone]

hey tigoat, thanks for the concern! I've clocked in 5300 miles this year so far at an average of 18 mph. Definitely not overanalyzing the hardware, but it's one of the few things about modern bikes that mystifies me.

Thanks for all the other reading material and information. This is exactly wanted I needed to clear the air on geometry. I've got some reading to do.