Men's versus Women's frames - what gives in the geometry with a shorter TT?
I've been trying to fit all my sister's in law with bikes and they all seem to want shorter and shorter saddle to bar lengths. Reading up a bit that seems to be common among women cyclists these days and bike manufacturers I understand are actually making frames with "women's" geometry - i.e. shorter top tubes.
But then a thought popped into my head today.
if you shorten the toptube on two frames with the same seattube length, what other changes happen elsewhere in the frame geometry? if one frame is a 54 x 54 but the "women's frame is 54ST x 52TT, doesn't that mean the headtube angle will be different. does that push the fork out, flatten that fork angle (rake?)?
Hmm, or would the downtube be shortened proportionately to keep the headtube at the same angle? sketching this on paper just now, I expect this has to be the result. The whole front end of the bike is move back Xcm.
But that would then change the wheel base. Hmmm,....
sorry, I am not a frame builder.
Last edited by pstock; 04-19-12 at 01:58 PM.
toe overlap can be a real issue. Most people put a steeper seat tube and shallower head tube angle on small frames. If someone wants a very short top tube on a drop bar bike, they might want to consider a smaller front wheel.
I believe the head tube angle and height are usually adjusted to account for the shortened top tube.
you might want to look at the free version of bikecad and try to design a 49cm frame yourself. http://www.bikecad.ca/
That's quite amazing. thank you!
Originally Posted by unterhausen
TT is one dimension, and the distance from the BB to the front axle is another (called the front center). In designing a mens' version and a womens' version, one probably wants to keep the frent center nearly equal to prevent toe overlap. This will require adjustments in the head angle and/or the fork offset. Both of those will change the steering geometry and hence the handling behavior of one version relative to the other.
Another adjustment in womens' bikes is often a longer chainstay, at least in Terry's bikes, but I'm not sure how it might be related to a TT reduction.
Bicycle Repair Man !!!
Generally, women are built differently than men and carry more height in their legs with shorter torsos and less reach so the saddle to bar distance needs to be shorter and the frame needs to be shorter. This is not an issue on a step though that has slacker frame angles and a shorter saddle to bar distance with swept back bars.
Build a bike with 700c wheels and the changes to the frame will bring the wheel much closer to the down tube and will reduce toe clearance if you retain standard road angles.
This can be overcome by using a smaller front wheel as seen on older Terry bicycles or by running a smaller wheel set like 650C to maintain the optimal frame angles.
My daughter rides a Terry designed Norco road bike, she is 12 and fits the standard mold for body type so this bike (a 47cm) works great as she gets the gearing from the 700c rear and smaller front wheel eliminates toe overlap.
Andrew R Stewart
There's a lot of claimed assumptions about what women need as far as frame fitting goes. What it really comes down to is that each female is different in body proportioning, flexibility and muscular capacity, just like males. But some commonalities do exist, usually. Most women don't have the upper body strength that men do. This means that they often can't support the upper body's weight as well. Women (often) have more weight carried in their hips then men. So the bike's center of gravity (lengthwise) moves to the rear wheel. Women tend to tolerate seat pressure "up front" less well then men do. All this is independent of their femur to lower leg ratio, their torso to arm ratio and those actual dimensions. Then add in the athleticism of many women entering the cycling and you can see why the easy way to design a female frame is to simply shorten the top tube with a steep seat angle and slack head angle, maintaining the frame's front center/toe clip clearance.
This design can work for many women, but not all. The handling result can be poor though with a "truck like" feel, lots of wheel flop. Often the better solution is to have the seat set back (and seat tube angle) reflect the femur length, the top tube length and head tube height deal with the upper body and arm lengths (along with the seat issues) and let the front center fall where is does. pick a head angle and rack to give the handling response wanted. Then choose a wheel size that allows adequate toe clip clearance (which varies with rider experience). The chain stays, if anything, might be a bit longer then the current racing trend to keep the center of gravity well placed. Andy.
Bicycle Repair Man !!!
Terry designed her own bicycles and also shared her designs with Raleigh (IIRC) as I have seen very similar Raleigh models while my daughter's bike is a Norco.
This is a Terry...
Changes in design to road bikes has made bikes like this less common as a sloping top tube and tig'd frame lends itself to allowing women's bikes to be built with 700c wheels with less issues with overlap.
When road bikes had conventional geometry and lugged frames, getting a sub 50cm bike could be difficult and quite a few were custom built or built in very small numbers since the people who needed them composed a rather small % of the riding public.
About 1/2 of my custom frames have been built for women in my 35+ years career. I don't believe the standard women's design commonly used in production frames is anywhere close to optimum. Since this is a forum about making custom frames there is no reason to include the compromises production companies feel they need to make to maintain profitability. For starters, many women prefer their handlebars to be at or near the height of their seat. They don't like the pressure on their crotch that a lower position creates. This higher position rotates the body back that requires more saddle setback which results in a shallower seat angle. A further back seat position also takes more weight and pressure off of their hands. In other words if the distance from the nose of their chosen saddle to the back of their handlebars is 46cm, there will be less pressure on their hands if the saddle is positioned further behind the bb still maintaining the same 46cm. Andy already mentioned other reasons many women prefer not being stretched out to their handlebars.
The problem created by a swallower seat angle and shorter top tube means that the frame is unlikely to have toe clearance when the front wheel is turned. Now racer types don't think this is much of a problem because they turn by leaning and not by rotating the front wheel. However when someone decides to turn around in the road and their foot hits their front wheel and as a result are now sprawled all over the pavement, they aren't going to think whoever made their frame was very smart. The solution of course is to use smaller wheels like 650C. Unfortunately they aren't widely available and wider sizes are hard to find. But the resulting design is superior to the compromises required to use 700C tires.
For my daughter's frame (she is just under 5' 4"), I made the fork and rear brake bridge to be at the height that both a 650C rim with 39-49 mm brakes will be compatible with MTB 559 rims with 47 to 57mm brakes. This way if she goes with me when we do our Ukraine bike ride I can swap out her fast wheels for 559's. Of course I have to change the brakes out as well.