Weight Distribution Question
 

I'd been doing some research on fore-aft weight distribution on my bikes since an auto-racing friend of mine loaned me some books on race driving technique. Race car drivers seem to be highly concerned with how a car balances thru braking, accelerating, and turning, and they spend time weighing each wheel to ensure there's proper balance.

I did some measurements on my single bikes. I put the front wheel on my bathroom scale and my back wheel on a couple of phone books set to exactly the same height as the scale, sat on the bike, and measured. I then reversed the bike and did the same. Not surprisingly, weight distribution hovered around 40% front 60% rear.

This piqued some further interest regarding my steel tandem. I took front and rear wheel weights with no load and got 19 lbs front 20 lbs rear. I then had my stoker sit on the bike, then I sat on the front. It was a bit tricky getting us balanced with zero weight on the cabinet we were using as a side support, but we managed to get some decent readings. So with a 175 lb driver and 135 lb stoker, the static weight distribution was 43% front 57% rear.

So I was wondering if anyone had any strong opinions on how weight distribution affects handling, especially on tandems. If 50/50 is a desirable ratio, then I can see why you'd want to put the heavier rider on the front. if 40/60 is desirable (from single-bike experience), are tandems designed for riders of equal weight? What's the effect of a longer rear top tube on weight distribution? (My 1980 Kuwahara tandem's specs: f/r seat tube: 23.2"/21"; f/r top tube: 22"/25.2"; wheelbase: 67.125") (Although I prefer metric, I gave Imperial measures for you Yanks!)

Thanks. - L.

dude, how are you going to alter the weigt distribution appreciably with riders on??? Are you going to adjust the distribution by selecting your stoker by weight alone? I don't think so.... we have what we have as teams.

we're 175/125lbs., so I'll bet the weight dsitribution we have is similar to yours. As you haven't told us what kind of road frame and what kind of tandem frame geometry you have, it is sort of hard to expect that the balance poitns are going to be the same. A criterium frame is going to ba;lance a lot diferent than a touring frame, etc....

I can take just about any corner on our tandem at similar speed to my single bike and I have raced in the Alps, Pyrennes, Dolomites many, many times so can corner pretty well. It's more about set-up, captain proficiency, tires, quality of the frame, fork, wheels, than weight distribution. I'd much rather have a stoker who's nice and light, if you get my meaning.....

Now... if yer loading up a touring bike for long work, then perhaps distributing the weight of the load makes sense. but I didn't read yer question that way.

First, let's bound the discussion with fore-aft weight distribution relative to your original comment: "Race car drivers seem to be highly concerned with how a car balances thru braking, accelerating, and turning..."

Aside from the nose-bleed level concepts of weight distribution (50/50 is always the benchmark for good handling in performance machines) and basic cornering and braking concepts (friction and inertia are what they are), there are a few things regarding road-going motorcycles such as steering geometry and steering (to a certain extent) that can be transferred to bicycles, but that's about it for the motorsports connection. This is primarily because: (1) the operator to machine weight ratios are vastly different [bicycle ~9:1, motorcycle ~.3:1, sportscar ~.05:1], (2) CG is not limited to the horizontal and the vertical CG on a bicycle is significantly higher than on motorcycles or sportscars, and (3) the amount of power, how it is used, and how it can be delivered to the tires for different effect is obviously quite different, e.g., you can use the throttle and brakes to drift a motorcycle or car through a turn as well as to weight and unweight the front tires for different handling effects: a bicycle limits you to the use the brakes.

That said, as Elrey notes, bicycle and tandem rider weights are what they are. Moreover, because the rider's weight makes up the vast majority of a moving bicycle's mass, it also means that fore-aft distribution of weight is dynamic as riders can sit back, remain neutral, or shift their weight forward, as well as to the left or right of centerline while cornering and intertia introduces yet another variable. A bicycle rider can also exert downward torque on their bicycle's frame/wheels/tires via the handlebars, saddle and pedals to influence cornering. Now we need to factor in bicycle frame design, rider fitting, steering geometry, and of course the rider's ability to apply their knowledge of bicycle handling to the machine they're riding and the road conditions.

Do you see where this is headed...??? In the big scheme of things, fore-aft weight distribution on an upright, conventional bicycle is not something I'd spend a lot of time being concerned with.

Now, when it comes to tandems they are not, at least to the best of my knowledge, designed to compensate for significant fore-aft differences in rider weights beyond the basic need to increase torsional stiffness / reduce frame twisting. Therefore, as rider weights become significantly skewed to one end of the bike vs. another, handling will change... sometimes dramatically. For example, a tandem with an adult captain and a small child stoking will be far more prone to oversteer and/or a rear wheel wash-out in a fast, aggressive cornering situation. Reverse the situation and put a very lightweight rider up front as captain with a significantly heavier stoker in back and you could reasonably expect the tandem to understeer. As to how severe and or noticeable these conditions would actually be to the captain, that gets back into all of the other variables mentioned above.

OK, now lets look at weight differences associated with tandem handling qualities outside of braking, accelerating, and turning..."

It goes without saying, in most cases once you have a stoker of adequate weight to provide good rear wheel traction, any additional weight -- and the higher that weight is carried -- degrades the overall handling qualities of a tandem. However, it's not the fore-aft weight distribution that degrades your handling, per se, it is the exaggerated steering control needed to deal with the stoker's side-to-side weight shifts and body movements that come into play and that torque the tandem's frame which result in unintentional steering inputs. Captains with regular skilled stokers who are smaller and who have never had a larger "guest stoker" will not usually have an appreciation for how significant the impact on handling can be from a larger stoker, stoker's who move around a lot, or stokers who "stiffen-up" every time the tandem leans over for a turn. Likewise, captains who have larger stokers or stokers who are, shall we say, not all that well disciplined or who don't ride cleanly, will never appreciate how differently a tandem will handle with a smaller or more skilled stoker aboard.

To your last question regarding rear stoker compartment length, moving the stoker back will amplify any problems that a team has with highly skewed stoker weights or "stoker steering". Short tandems trade-off stoker comfort for better handling (period). Long tandems trade off handling for stoker comfort. The handling quality penalty for a longer tandem can be mitigated to a certain extent by builders who can figure out how to use materials or design frames that are more resistant to flexing or twisting, but that's about it.

Weight distribution is sort of hard to figure in, if you ride with various weight stokers/pilots.
You're sort of stuck with what you've got and captain has to be able to handle things.
Have ridden with sub-50-lbs to 250-lbs stokers. I'm rather light (135) but able to handle the situation. A bit tougher with the clydesdale stoker, but very do-able.
If weight distribution is a big issue, there are other designs than just 'upright' tandems.

I dont know that much about tandems yet but in bicycles, weight distribution doesnt really matter that much. It dont in a car that weights a ton or more. You want the weight spread out so that each tire is take about equal weight. This matters a lot on the corners. It is totally different on a bike.