New version V8. Biomechanical & sizing & fitting spreadsheet

A light bike doesn't replace good fitness.

Moderator: Moderator Team

Post Reply
dmcgoy
Posts: 43
Joined: Thu Nov 15, 2007 7:23 pm

by dmcgoy

Hmm. Your spreadsheet is surprising for me.

My specs:
-------------------------------------------
Inseam: 78
Trunk: 66
Forearm: 34.5
Arm: 65
Thigh: 56.5
Lower Leg: 52
Sternal Notch: 143
Total Body Height: 173

Elbow-middle finger: 45.5

According to your spreadsheet, I should be on a 52cm effective top tube, 51cm seat tube, and a 11.1cm stem with a no setback seat post.

According to the competitive cyclist, I should be on a 55-55.5cm effective top tube, a 50.5-51cm seat tube, with a 11.2cm stem.

What gives with the extreme difference in effective top tube lengths? Do I have a weird body? For what it's worth I ride a bike with a 54.5cm top tube, a 68cm center of bottom bracket to saddle top (along seat tube), a 100mm stem, and a 1cm drop. I ride on top of the hoods unless I'm doing a climb (then I'll go into the drops).

Jaho
Posts: 1
Joined: Mon Sep 19, 2011 12:01 pm

by Jaho

Hi!

My first message to this forum...and i would like to ask is this saddle-pedalier seatback for the road or mtb-bike?

My rider data was:

Inseam: 84,5 cm
Elbow-middle finger: 48,3 cm
Flexibility:6
Saddle lenght: 27,8 (Selle San Marco Regale FX)

And results:

Flat- hadlebars 52,7 cm
Saddle height 75,1 cm
Saddle-handlebar drop 7,5 cm +-1
Saddle-pedalier seatback 8,2 cm??

All other measures is just that i have in my bike but seatback is almost 3 cm smaller in my bike now. So is this 8,2 cm to road or mtb-bike?

Thanks, Jani

daj
Posts: 246
Joined: Mon Oct 12, 2009 6:36 pm

by daj

This spreadsheet is a nice idea but lacks a little bit of accuracy.
Seat tube angle has a big influence on fit yet it is not accounted for in the spreadsheet.

For example, with a seat height of 75 and a setback of 8 (which the spreadsheet suggests in my case), actual vertical saddle height as measured from the bottom bracket is different.

Seat tube angle = 72 degrees: seat is at 71.3 cm vertical
Seat tube angle = 74 degrees: seat is at 72.1 cm vertical
Now this is a difference in height (Y) of almost 1 cm while setback (X) is given and remains the same. With longer legs this difference becomes bigger.

Now consider handlebar reach. This is a very complex calculation but the difference between a 72 degree and a 74 degree seat tube comes down to a difference in reach of about 2 cm. This means that, given the top tube remains the same, with a 74 degree seat tube the distance between your saddle tip and handlebars will be 2 cm longer than with a 72 degree seat tube. I think 2 cm is quite a big difference.

I do think this is easy to adjust and then you would have a great tool!

User avatar
Giant DK
Posts: 1311
Joined: Mon Jan 23, 2006 3:03 pm
Location: Denmark

by Giant DK

@daj

Try a see my comments in my the post on page 7.
Technical editor at Cykelmagasinet Denmark

My Simplon Pavo II

robikka
Posts: 21
Joined: Thu Sep 23, 2010 8:55 pm

by robikka

.

kiekeboeboe
Posts: 8
Joined: Sat Apr 21, 2012 12:13 pm

by kiekeboeboe

I like it, it sounds like a good starter for newbie cyclists!
I'll definitely take a look at it. :beerchug:

raducanmihai
Posts: 2
Joined: Thu Apr 09, 2009 5:26 am
Location: Romania

by raducanmihai

Thank you for the spread sheet,it is a good starting point for the correct bike fit.
But I have a problem with the setback.It doesn't take into account the femur/tibia proportion.
For example : let's take 2 persons with identical inseam,upper body and flexibility.The result of the ss will be the same for both of them: they both need the same reach,setback,etc.
But person 1 has a shorter femur and a longer tibia. Person 2 has a longer femur and a shorter tibia. But,remember,the inseam (femur + tibia) is the same.Because of that, no 1 (short femur,long tibia) will need less setback and longer stem and no 2 (long femur,short tibia) will need more setback and shorter stem,so for both of them to have the same reach.

I noticed this problem in my case (short femur,long tibia),for my MTB (Trek 4300 frame). My numbers are:
inseam : 88 cm
arm : 49 cm
flex : 0
height : 182 cm (irrelevant)
shoe-size : 46
saddle length : 28
horizontal frame size : 58.3 cm
no setback seatpost

The setback recommended is 8.9 cm with a 8.4 cm stem. I have 5 cm setback with 12 cm stem. In this position I have knee over spindle.

I also have the saddle-hablebar drop = 1 cm with no sag,so about 3 cm while riding vs 8.4 +/-1 recommended, which brings me to the other thing I noticed : the recommended saddle-handlebar drop is the same no matter what the flexibility is,which I think is wrong.

Anyway,thank you for this ss,it's a good tool in finding a good position on a bike. With a few tweaks it can become an excellent one.

vtspot
Posts: 48
Joined: Mon Jun 02, 2014 4:34 am

by vtspot

Although it is an old post, but a great reference for bike fitting. I am using it right now and still testing the new measured fitting.

User avatar
beatnik
Posts: 1237
Joined: Thu Aug 03, 2006 6:38 pm

by beatnik

Clever ones did notice that frame angles are irrelevant
Biomechanical spreadsheet. Sizing&Fitting.

http://weightweenies.starbike.com/forum ... 8e319d185b

mountainlion
Posts: 9
Joined: Mon Jul 07, 2014 8:19 pm

by mountainlion

Well that was interesting. Sitting around watching the tdf and browsing the forum, so I decided to try this spreadsheet to see how it compares to my fit that I've settled on over a few years of trial and error.

86cm inseam, 50cm arm-finger, 8-9 flexibility, 45 shoe, not many variables here are there?

Suggested top tube = 56.5cm
current best fitting frame = 56.5cm

Suggested saddle height = 76.5cm
Actual saddle height crank center - sit bone contact point = 76.5cm (really)

Suggested setback = 8.4cm
Actual setback = 8-9cm range

Suggested stem length for 56.5tt = 115mm
Actual stem length for 56.5tt = 120mm

Suggested drop = 8cm +/- 1cm
Actual drop = 8.5-9cm

Suggested crank length = 175mm
Actual crank length = 172.5mm

Interesting... I'm not sure how I would feel about 175mm cranks but maybe I should give them another shot one day. Also not sure if I would be on a 19" mountain bike like it says, my friends' 19" 29er is massive and I feel like 17" might be better. Similarly the 130mm mountain bike stem estimation is probably inaccurate for modern 29er geometry based on what I've seen.

User avatar
beatnik
Posts: 1237
Joined: Thu Aug 03, 2006 6:38 pm

by beatnik

mountainlion wrote:Well that was interesting. Sitting around watching the tdf and browsing the forum, so I decided to try this spreadsheet to see how it compares to my fit that I've settled on over a few years of trial and error.

86cm inseam, 50cm arm-finger, 8-9 flexibility, 45 shoe, not many variables here are there?

Suggested top tube = 56.5cm
current best fitting frame = 56.5cm

Suggested saddle height = 76.5cm
Actual saddle height crank center - sit bone contact point = 76.5cm (really)

Suggested setback = 8.4cm
Actual setback = 8-9cm range

Suggested stem length for 56.5tt = 115mm
Actual stem length for 56.5tt = 120mm

Suggested drop = 8cm +/- 1cm
Actual drop = 8.5-9cm

Suggested crank length = 175mm
Actual crank length = 172.5mm

Interesting... I'm not sure how I would feel about 175mm cranks but maybe I should give them another shot one day. Also not sure if I would be on a 19" mountain bike like it says, my friends' 19" 29er is massive and I feel like 17" might be better. Similarly the 130mm mountain bike stem estimation is probably inaccurate for modern 29er geometry based on what I've seen.


Perfect fit. Congrats, and you did it trial and error ;). As i know 29er are the same only bigger wheels. Yes your size is a 19. 2.5 mm in crank arm is not so much.
Biomechanical spreadsheet. Sizing&Fitting.

http://weightweenies.starbike.com/forum ... 8e319d185b

erikw1968
Posts: 9
Joined: Mon Jul 04, 2011 6:43 am
Location: brunssum
Contact:

by erikw1968

Thanks!

Post Reply
  • Similar Topics
    Replies
    Views
    Last post