Zinn's Bike Fit

[Tin Pot]

Hi all,

I've read though Appendix C of Zinn's Road Bike maintenance and decided to experiment and analyse my bite set up and frame size accordingly.

This is largely based on body measurements so I don't expect it to be as thorough & effective as a Retul fit, but I hope "good enough" for fitting by yourself.

Measurements as below:

Bike Fit Zinn Method
Body
In seam
Arm
Torso
Inseam + Torso
Torso + arm

Bike
Stand over height
Frame size
Top tube
Stem
Crank arm
Handlebar width
Handlebar drop
Saddle height
Saddle setback
Handlebar height
Handle bar reach
Aerobar width
Aerobar angle

 

[Tin Pot]

Luckily my bike, Cinnelli Experience, has frame details online:

 

[Tin Pot]

The first thing I noticed looking at the book diagrams is that Zinn measurements aren't exactly matching.

Frame size isn't the advertised size, s1, it's s- so in my case not 510 but 465.
Top tube is t1 not t

Then its taking body measurements. Inseam, arm length and shoulder height (inseam + torso). Inseam matched my usual clothing Size, so have some. confidence in the process.

So how does it all work out?

I won't go into all the maths as the book covers that but its thrown up some interesting points I and maybe this bike is not a good fit at all...

 

[Drago]

The most comfortable for the longest period of time is the best fit, and neither me, Zinn or Messrs Retul can tell you what that might be. Its good that you're thinking about it and having a play yourself.

 

[Tin Pot]

Indeed- I'm pretty comfortable up to about 4 or 5 hours, so part of me says "what's the problem?". As you say, I'm curious and it's interesting!

In the attachment I've emboldened the measurements that don't fit. In short,

Stand over height is too high
Frame size is too big
Top tube is too small
Crank arm is too long
Seat is too high
Handle bars are too high

It's a miracle I've survived two years without trouble. . .

What is to note though is that apparently:
1. I need a smaller frame with a longer top tube- can't be done
2. There is no frame Size with a small enough stand over height
3. There is no way to reduce the handlebars so far
4. I'd need a stem 14Omm to compensate the short top tube

 

[youngoldbloke]

The first thing I noticed looking at the book diagrams is that Zinn measurements aren't exactly matching.

Frame size isn't the advertised size, s1, it's s- so in my case not 510 but 465.
Top tube is t1 not t

Then its taking body measurements. Inseam, arm length and shoulder height (inseam + torso). Inseam matched my usual clothing Size, so have some. confidence in the process.

So how does it all work out?

I won't go into all the maths as the book covers that but its thrown up some interesting points I and maybe this bike is not a good fit at all...

s1 can vary according to the length of seat tube above the top tube, and t is dependant on the slope of the top tube. 's' is the more useful centre to centre measurement, and 't1' the virtual top tube measurement - if only these measurements were given as a standard for all frames it would be easier to compare frame geometry. The other most useful measurement would be the virtual seat tube length - BB centre to intersection with centre of virtual top tube.

 

[Tin Pot]

A couple of questions:

I can't seem to find a 140mm stem, longest seems to be 120mm at least in the Cinelli range?
Mine is 110mm, will moving my weight forward 3cm effect handling?

What is the knee flex angle meant to be with pedals at 6 o'clock?
--Zinn says straight with toes below heel, elsewhere he says a maximum in 88.5 degrees. I thought some bend in the knee was needed.

What effect will reducing the crank arm 1.25cm have?

--I'm thinking I will increase knee flex at 6 o'clock and reduce it at 12 o'clock.

 

[Milkfloat]

Take a quick look at http://bikedynamics.co.uk/guidelines.htm. This is the fitter I used and his website has quite a lot of information that may help, including reasons why.

 

[cyberknight]

Have you tried the competitve cyclist website ?
Their eddy fit comes pretty close to what i am at currently .

 

[Tin Pot]

Take a quick look at http://bikedynamics.co.uk/guidelines.htm. This is the fitter I used and his website has quite a lot of information that may help, including reasons why.

Yeah, I did see that and I'm having a look at those angles - I wanted to base it on a single source (Zinn) though.

I've taken some measurements of my angles, but everyone seems to talk in slightly different terms in the matter.

Have you tried the competitve cyclist website ?
Their eddy fit comes pretty close to what i am at currently .

Cheer s - yes, I've seen that too. I might compare it to the Zinn fit results later.

 

[Tin Pot]

Take a quick look at http://bikedynamics.co.uk/guidelines.htm. This is the fitter I used and his website has quite a lot of information that may help, including reasons why.

According to bikedynamics these are the max/min angles:

My maximum is 166 and min is 78.

I'm thinking lowering the seat and reducing the crank length will help.