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Slowtwitch Forums: Triathlon Forum:
static tri-fit system and average proportion?

JulianW

Nov 16, 09 16:59

Post #1 of 10 (476 views)

static tri-fit system and average proportion?	Can't Post

From the article on this site "Devising a static tri-fit system" in the cockpit distance section:

But we aren't measuring that way. We're taking as axiomatic a truism we've often found, that a rider of average proportion, fit properly, will have a cockpit distance equal to his saddle height. So, that distance from BB axle to the top of the saddle (halfway between the saddle's fore and aft) will equal the distance from the saddle nose to the pivot bolt on the bar end shifter.

But, on a longer torso'd person the cockpit distance would be longer than the saddle height; on a long-legged person the saddle height would be longer than the cockpit distance. We consider the saddle height as a function of your overall height, determine whether you're leg-long or torso-long, and calculate your cockpit based on saddle height and this leg:torso ratio.

What is Average Proportion?

how do i determine if I am long legged, long torso'd or average? (ie what is an average leg:torso ratio)

My measurements are:
Height: 171 cm
Inseam: 77.5 cm
Torso: 64 cm (top of inseam to notch)

cheers
Julian

lrobb

Nov 17, 09 6:26

Post #2 of 10 (432 views)

Re: static tri-fit system and average proportion? [JulianW] [In reply to]	Can't Post

Formula from Mark Hodges, as found in the book Serious Cycling:

height / inseam

below 2.2. is long-leg
2-2.2 is normal
above 2.2 is long torso

Seems to have too much of a range though.

You can have a 35" inseam, and the formula considers you normal whether you're 6'5" or 5'10"

JulianW

Nov 17, 09 15:10

Post #3 of 10 (398 views)

Re: static tri-fit system and average proportion? [lrobb] [In reply to]	Can't Post

Well that has me at 2.206 so slightly long in the torso.

Bit that has me confused is that the cockpit distance formula comes out out at 76.8 cm whith a saddle height 68.5cm an 8cm difference.

the cross check mentioned is saddle height = cockpit distance as i am slightly long i would understand 2cm longer for the cockpit but 8 seems a lot.

Slowman

Nov 17, 09 17:19

Post #4 of 10 (372 views)

Re: static tri-fit system and average proportion? [JulianW] [In reply to]	Can't Post

caveats: dynamic fits are much better than static fits, and, this is a new system for us, we're accumulating data, and the formulae might change. that established...

68.5cm of saddle height seems low to me for a rider who's 5'7" or 5'7 1/2". maybe there are riders around that height who can chime in on what their saddle heights are. so much of this has to do with the technique of taking measurements. is the top of your saddle, measured to a point midway between the front and back of the saddle, straight to the bb axle, 68.5cm? or are you just getting 68.5 from the lemond formula we mention for saddle height? this assumes a correct measure of your inseam, which may or may not be occurring.

i'd go back and take a look at your saddle height on your bike, see if it's really 68.5cm.

Dan Empfield
aka Slowman

JulianW

Nov 17, 09 21:46

Post #5 of 10 (345 views)

Re: static tri-fit system and average proportion? [Slowman] [In reply to]	Can't Post

accept the caveats, the bike fitters I have seen in Canberra, Australia tend to set you up in more of a road position, and I know I raced better in a forward position. I am trying to use the stuff you have to get me close and tweak it from there.

I am pretty happy that my inseam is correct at 77.5 which at .883 gives 68.4. My seat heights have been in the 68-71 range but i suspect 71 is too high. I had some pretty major ITB issues that kept me out of Ironman and triathlons for a bit, just getting back into it now and do not want a repeat of the ITB issues.

I have set my saddle up to 68.5 and measured it BB to center of saddle, then again i have never managed to get the armrest drop in the formula either it is at 5 cm now and after this morning which was my first hard effort on the TT bike in a few years i am going to raise it a bit.

I am going to borrow a video camera next to have a look at position in motion.

Can anyone suggest a program that will allow me to measure an angle on a picture or video?

Slowman

Nov 18, 09 6:09

Post #6 of 10 (313 views)

Re: static tri-fit system and average proportion? [JulianW] [In reply to]	Can't Post

i wouldn't worry about measuring the angle in the video. just take the video and slap it here on the forum.

Dan Empfield
aka Slowman

JulianW

Dec 15, 09 16:00

Post #7 of 10 (231 views)

Re: static tri-fit system and average proportion? [Slowman] [In reply to]	Can't Post

Ok have a camera at last, now the ignorant question,

how do i slap a video on the Forum?

what is the best format to save the video as?

cheers
Julian

Slowman

Dec 15, 09 21:00

Post #8 of 10 (206 views)

Re: static tri-fit system and average proportion? [JulianW] [In reply to]	Can't Post

you're asking the wrong guy.

Dan Empfield
aka Slowman

Supakage

Dec 16, 09 5:42

Post #9 of 10 (163 views)

Re: static tri-fit system and average proportion? [Slowman] [In reply to]	Can't Post

In Reply To:

68.5cm of saddle height seems low to me for a rider who's 5'7" or 5'7 1/2". maybe there are riders around that height who can chime in on what their saddle heights are.

I am 5'6", and have my saddle at 70.6cm. 79.5cm (a bit more than 31.25") cycling inseam. I am probably on the "leggier" side according to the above formula.

JBIL

Dec 16, 09 6:10

Post #10 of 10 (152 views)

Re: static tri-fit system and average proportion? [Supakage] [In reply to]	Can't Post

At 5'8" I have my saddle at 73.5cm. This is measured from BB through center rails to top of the saddle.