Bike Fit and Comparative Geometry

[MacB]

As I'm still dialling in the new bars etc and still looking at a titanium frame, I've been doing some more reading. I thought I'd share a bit of this and invite opinions and comment to hopefully enhance my understanding. firstly I've used this site a couple of times:-

http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO&INTRO_LINK=NOREDIR

You really need someone to help you take the measurements but the results are quite interesting. It gives you a selection of parameters to work from under 3 headings, Competitive, Eddy and French, these equate to serious road, relaxed road and road/touring. I've found that the results for saddle setback, and height, are what I've arrived at via trial and error. but I've also realised that I had things wrong regarding top tubes, reach and seat tube angles. So I found this site very interesting:-

http://www.neuvationcycling.com/bikegeometry.html

Originally I'd made the assumption that a longer top tube, and shallower seat tube angle, would increase reach. Whereas, assuming that you are working on a static saddle setback and height, then it actually decreases reach. Basically more of the top tube will be back of the BB line and you'll have moved saddle forward to retain saddle to BB relationship. So I found the Neuvation method for showing reach to be very useful. They're using BB to head tube, done in virtual top tube style, to provide a 'reach' figure. It struck me that this measurement would be a more useful comparitive tool than many of the others. Obviously you'd still need to look at frames that fell within certain parameters.

Just felt that this would be a good addition to the little geometry charts so loved by all of us

 

[4F]

You need to get out more Al.

 

[MacB]

You need to get out more Al.

 

[Fab Foodie]

Interesting. I always start with this, it's great for beginners...
http://www.peterwhitecycles.com/fitting.htm
peter takes a lot of time talking about 'balance' which effectively means where your arse is in relation to the bottom bracket, the reach is then secondary, this is similar to the Neuvation ideology.
What I've noticed is that modern bikes seem to have more upright geometry for a given TT length than of old, thus we now (are forced) sit more over the BB, as TT lengths are relatively unchanged over the years, more riders have their weight pitched forward over their hands. Look at old TDF riders, they sit further behind their BB's than riders to day.
The Neuvation approach seems more realistic for everyday riders than many other systems.... muy only rider being that it's the measure from BB back to the saddle that's also important. Maybe there's a ratio thing to be determined for TT length split fore and aft of the BB. I probably prefer to sit further before the BB than most being of Anglo-Saxon short leg and long-back.

 

[P.H]

You need to get out more Al.

This won't help.
http://www.bikeforest.com/CAD/bcad.php

But it is a lot of fun

 

[MacB]

This won't help.
http://www.bikeforest.com/CAD/bcad.php

But it is a lot of fun

aha.......I see some time wasting in my future

 

[jimboalee]

I have two methods.

1/ The method Tom Crowther showed me. Written on a piece of foolscap from the seventies and now on a spreadsheet.

2/ The method showed to me by a chap who worked at Dawes Cycles when they were in Wharfdale Road, Tyseley.

They are slightly different. By 0.04mm seat tube length, in my case.
The Dawes method gives a 0.75 Deg shallower seat tube angle; 72.75 vs 73.5 from Tom's method.

Really, there's sod all difference.

My Specialized SWorks is closer to Tom's method ( cus it's a race bike ).

SWorks seat angle is 73.5, same as Tom.
Seat tube L is 54 vs Tom's 21.5" ( 54.6 cm ).
Top tube L is 548mm vs Tom's 549mm.


The Spesh is not bad considering it's from a multi-million dollar research department in the California hills.
Much more than Tom Crowther got when he sold Mercian Cycles.