Help "center" of a saddle?

jdd

Maximum Pace
Hardest Crash
Jul 26, 2008
2,519
650
133
Kanazawa
#1
I'm in the process of measuring my bikes, and I've been trying to pair those measurements with the data I got from the shinjuku Y's when I was measured there. Y's mapped their measurements of me onto a diagram of a bike.

One of the numbers on that diagram is called サドル水平距離 (saddle horizontal distance) that the paper says is measured from サドル中心 (saddle center) to a vertical line that goes thru the center of the BB. No problem with that BB line, but is there some accepted way or formula of finding the effective center of a saddle? (other than just guesstimating)

For example, one of my saddles is a brooks B-17 which is 280mm overall, and to my mind, the effective center is well back of the physical center, more like 185mm back from the tip. (give or take) The other saddle is a brooks swift, if finding the center is model/type-specific.

Even on Y's diagram, the saddle measures about 28mm, with the point they call 中心 being about 16mm back from the tip. So by 'center' they are clearly not using the physical center point.

TIA,

John D.
 

GSAstuto

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Oct 11, 2009
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tokyo
www.roadfixie.com
#2
OK - the easiest way to determine your saddle center is this:

1) Make sure your cleat position is already set. You may need a pro touch for this to measure correctly foot angle (cant and shim).

2) Make sure your saddle height is already set - more or less where it should be.

3) Either use a stationary trainer or slight slope road where you can get to full power over a reasonable period of time.

4) Warm up really well, so your muscles are balanced and you are well within your cardio zone 4.

5) Bring up your power and keep a good cadence as you'd normally ride. Probably somewhere between 80-110. Relax your upper body as much as possible with just very light touch on the bars. Keep the power on.

6) You should be able to feel the dynamic balance center of your saddle - so, for example when you take the weight off your bars, if you find yourself drifting forward on the saddle, then you need to move the saddle a little forward. The same if you feel yourself drifting back - move the saddle back a little until you are centered and balanced on it.

The things that will affect this most is poor pedaling technique. Either too much toe down or up will tend to drive you off your saddle. So, make sure you are pedaling level and not overreaching the flexibility of your ankle.

More forward positions favor more slow twitch muscle firing and lower energy utilization at some expense of acceleration and brute mashing. But, if your intention is riding TT, Tri or simply alot of distance at sustained pace, this is generally a good position.

More rearward positions favor fast twitch recruitment, using the 'big muscles' under higher loading or mashing.

This is why the test above works so well - you are checking position based on the power / balance point YOU have. You'll probably find you have several balance points based on type and terrain of riding.

So - this is why longer saddles are sometimes favored - because you can have a longer 'sweet spot'. It's also why some people prefer saddles that lock you into position (like SMP), cause once you have 'your fit' , you can apply max power without slipping fore or aft.

Last thing to take in consideration is simply the frame size. If you have undersized your bike, then the top tube will be shorter than your fit and require longer stem and more saddle setback for you to achieve proper reach. Conversely for larger frame you may be reducing setback and stem lengths.

Remember - the biomechanic formulas just get you in a ball park. In order to really fit properly you need to visit a professional that has the right gear to check and experience and knowledge to apply it.
 

jdd

Maximum Pace
Hardest Crash
Jul 26, 2008
2,519
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133
Kanazawa
#3
Tim,

Thanks for the reply. I appreciate that that took some time to spell out so clearly. (And I guess you know what the measurements are for.)

However, by measuring my bikes, and comparing that with the measurement data that the bike shop came up with (which I realize is open to interpretation), I'm trying to assess/describe where I'm at now, and hopefully see how that differs from the (probably) standardized fit scheme that Y's used.

On the one hand, any differences may be telling me to make some changes to what I'm riding now. A longer stem, move the seat forward/back/up/down, etc. On the other hand, maybe the quirks of my present setups show something important about me or my riding style that should be kept in mind when switching bikes.

So unfortunately I guess it'll have to be a judgment call--to spot the 'dynamic' center as I feel/perceive it without going thru the steps that you have so clearly described.

**

Some of the measures I've done have been eye-poppers (one bike vs the other, or either vs the data on the Y's fit chart), while a few others do line up okay. The two bikes are very different kinds of bikes tho, so that may have something to do with it.
 

GSAstuto

Maximum Pace
Oct 11, 2009
945
242
103
tokyo
www.roadfixie.com
#4
For what it's worth - I personally just :

1) set saddle height - everyone should know theirs. Mine is 73.25cm
2) ramp up and get a dynamic point check - this takes me about 30s now - I know where I want to be. Move saddle fore/aft and that's it!

The beauty of this is that it's seatpost angle independent. Unless you are setting up for TT / Tri - where you won't have enough forward position anyway on a standard roadbike - it will work for 99% of the bikes out there.

As for stem / stack. There are alot of positions on a bar you can ride. This will be more fine tuning and not as easy as saddle. But - in some ways more flexible - so don't sweat it.

Most of the bike fit things are just approximations and don't really mean that's exactly where you should be . If you're seriously interested in a perfect fit - then get one - and do a FTP test accordingly to verify - along with pedal power analysis. Otherwise - it means rather nothing, in my opinion. GIGO!