Frame fitting – weight distribution

Bike_weight-distribution-scalesI read this quote on the mtbr frameforum one day about fitting bikes to people,  “We move weight over the tires when designing the handling. 50/50 is nice for crit. 65/35 is nice for super D. The shape of the rider can have some influence here. KOPS (knee over pedal spindle) approximates a nice road/pedaling fit if you have a very light euro body. Fit with scales for best results.”

So, I tried this out on a pair of cheap bathroom scales and here’s what I came up with on my current mtb bike (frame #13).  The geometry is: 70.5/72.5 (HT/ST angles), 24.3″ top tube, front center of 675mm, 17″ chainstay length, 110mm stem, fork is 470mm with 46mm offset (82mm trail, although that’s useless info for this post).  Seat in the middle of the rails on the no-layback seatpost. The rear wheel is a lot heavier than the front and total bike weight is about 26lbs with the rigid on there. I didn’t take the seat pack off with one tube but that’s probably meaningless.  I have a pair of Watson titanium bars on there with 20 degrees of sweep so that could put me a bit more back on the bike than with using straight bars. If i had a 120mm stem i’d use it.

Results with just eyeing the scales (I need a couple sets of eyes and digital scale readouts to make it more precise!):

Bike only:
12lbs front scale
19 lbs rear scale.

Me on Bike:

76lbs front
127lbs rear

…which translates to:
37.4% front,
62.5% rear

So even though this is a more XC bike geometry and has a rigid fork, the weight distribution is more similar to a Freeride bike.  I like that feel more for all around riding but if i were to go just by my body dimensions on a XC build, I’d use a steeper seat tube angle and put on a 120mm stem to put more weight on the front wheel if I kept the same front center. It’s funny, I used to only ride big 29er (XL) frames since my torso length is disproportionately long compared to my leg length.  But that made for a very ‘Cadillac’ feel which is great on dirt roads and rough downhills, but the more i ride different bikes I like the maneuverability of shorter front centers –  front wheel/front axle closer to the BB.  I’m not sold on short chainstays either. I like the for singlespeeds, but for a more all-around bike where you’ll have long hours in the saddle and varied terrain, I think they can actually be a hindrance. My preference at least, your results may vary…

The trick for builders is to find that sweet spot for each person they build a bike for.  I think road and cross bike geometry has been on a plateau for a long time, and isn’t that dissimilar. Different builders build based on their preference for fit to the rider, but there’s not much variation.  But in mountain bikes, the differences are pretty big in how framebuilders design their bikes for different riders and different riding styles. There’s still some lessons to be learned once you stray from the standard 71/73 head tube/seat tube angled XC bike, and also challenge the current thinking on the the slacker/shorter Super D style bike too.

3 Responses

  1. Remember that your weight distribution changes a LOT when you stand up. Did you do the with-White weighing sitting or standing? In my mind sitting is kinda useless because you don’t usually sit when you’re doing bike-handling type riding. For steep/techy climbing it can matter, of course, but most of the time when you’re in the saddle weight distribution doesn’t matter all that much.

    -Walt

  2. Oh, one more thing – for fun you can add/remove spacers from under the stem. Makes a considerable difference if you move your hands up or down a bit.

  3. So true. I thought about the spacers but that was too much work. Even increasing the bar angle from horizontal (~20deg) for these bent swoopy bars means a little more weight on the front wheel. Better on the wrists, worse on my back. It’s just one tradeoff after another!

    I sit a lot on my geared bikes now that I’m older. I stand on the singlespeed but that’s a given. What I don’t like about more of a “XC” or forward weight distribution is the pressure it puts on my wrists, hands, neck, and back. Slackening the STA, using more setback, and using a shorter stem all have helped me a ton with back and wrist pain & numbness. But climbing on steep stuff with that setup isn’t very cool.

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