Dummies guide to optimal weight distribution

There are so many places batteries and motors can be placed on the frame it is difficult to discern where the optimal placement of weight should lie.

If there were no mounting or other constraints where is the most balanced place to have the weight and COG?

Does the location change based on use? Is it different for a road bike vs an off-road bike, low speed vs high speed?

I read recently that a high COG (weight along the 4-5 axis) would be better for stability and turning at high speed whereas weight at 3 would be better for low speed quicker turning.

EDIT: Added V2 diagram Comments start here http://endless-sphere.com/forums/viewtopic.php?f=2&t=63665&p=953998#p953998

The best place for the battery is on the rider. This is the same lesson we learned decades ago motorizing hang gliders. If the weight shifts with the operator, it's far more responsive. Mounting then becomes trivial, as any backpack will do, and it resists theft. It's only a problem for heavy batteries on long rides.

This does remove the distribution problem altogether but it seems a small percentage of ebike riders go this route and seem to find a place on the frame to attach batteries. Even with the advantages you point out, there must be enough other disadvantages to not make this more common.

This is quite a complex subject, of which I'm not up to scratch on all the physics. I can tell you however that the best place for the weight is in the frame triangle on your diagram.

There are more aspects to this than COG and weight distribution though. One important issue you haven't mentioned is 'moment of inertia', or 'axis of rotation'. When we change direction on a bike, what we are doing is changing the patch of tyre that is in contact with the ground by moving our body mass. If you imagine a line running from the rear axle to the headset, this is the axis around which we move the bike. The closer the bikes mass to this axis, the easier it is to maneuver the bike. The best point along the axis is the point of equal weight distribution - somewhere around the mid triangle.

I'm not the best at explaining this sort of thing. Hopefully someone else will be able to expand on it.

Best for bikes is definitely the center of the triangle. If for no other reason, it's impossible to put a large battery only in 3 4 or 5.

What happens is you fill the whole triangle, and possibly some outside the triangle, along the top tube or downtube.

For low speed handling, which is all of it for "bike class" ebikes, weighting it as much as possible to 3 works best.

Not mentioned is the rear rack carry. This sucks if you have 20-30 pounds of battery. But it's easily tolerated at 0-30 mph if under 10 pounds, and barely noticed at 5 pounds.

If you must carry 20 pounds or more on a rear rack, locate it as low as possible in panniers (2) instead of high up on top. Chances are, you have the motor in 2. For street riding, it can handle pretty nice if the motor is in 1 and the battery in 2.

I have bike with battery@#5 & motor@#1. The weight@#5 was attached directly to the frame (similar to the popular bread basket design on a Yuba) vs. fork/handlebar . I've taken it on road descents and it handles quite well. What it doesn't do well is lift the front off the ground. That maneuver requires some grunt work. Also, since it's essentially 2wd (pedal power in back) it can tackle pretty steep sections. The front weight with motor adds more traction & assist.

I've experimented with a few battery options. My hard tail has 20s10ah of lipo bricks located inside the triangle. I can actually feel the difference depending on the orientation of the brick of batteries. As it's rectangular, I have the option of placing its long side against the seat post tube or the top tube, effectively choosing between high/slightly further forward or lower/more rearward. The high/slightly further forward option is surprisingly much better and despite the relatively small mass (6.5kg) and tiny amount of variation in placement it's easily discernable. I've tried this several times to confirm.

I have also tried that same pack in my backpack. That was not good. I'm quite used to the backpack as I carry my charger in it as well as other things for work, but the additional weight placed comparatively far back on the bike unsettled the front end and had a negative effect on handling. To take that option to its logical conclusion, I carried home about 20kg of NOS laptop batteries (that I'm harvesting for their cells) in my backpack. This ride was *very* sketchy. Front wheel would lift under power and just generally awful handling.

My other/new bike has gone the other direction - batteries mounted in pelican case rigidly mounted to the front fork, oatnet style. I have actually ridden this around with the full battery weight in the box (11kg) but with no hub on the rear and it's pretty interesting, the faster I go the better it feels. With the cromotor on the rear wheel it will actually result in weight distribution virtually identical to the standard bike (DH Comp), whereas my hardtail has its distribution shifted rearward by the addition of the hub (HS3540, so ~7.5kg) and the battery.

Initially had to laugh because I was thinking “yeah, I want to redistribute the tire around my waist up to my arms and shoulders” :lol:

Seriously though, if we’re talking about batteries and supplies and not instrumentation, then I have had mine placed in the following areas below.

On my back: Terribly bad choice if riding cross-country. For the second road trip to California, I reduced the weight on my back to the lowest possible number to eliminate the fatigue on my wrists. The only thing you should wear on your back are items of extreme convenience such as Water, bike lock, snacks, maps, wallet, sunblock, and cellphone.

In terms of the Bike-Legend, numbers 1-4, the entire model is inaccurate; it should be:

• Center of Triangle (present system)
• Panniers over the Rear Tire (done this as well; a little problematic due to getting bounced around)
• Panniers over the Front Tire (difficult to achieve if using shocks)
• Trailer (very handy for storing all sorts; I have quick access compartments for rain gear/fleece storage)
• #5 is the exception because it could be saddle bags over the forward section of the top-tube (which I presently use), or for a handlebar bag (done that too).

Cheers, KF

Generally it seems placing weight within the frame is a good option.

What the two options presented in the V2 image? Location A places the weight lower along on a longer dimension. Location B places the weight along the top bar.

The design objective is more stability and handling at speed (25+ mph) rather than low urban street slogging.

The lower part inside the triangle is first choice and also if adding more capacity, consider below down tube in front of the crank. Low and forward with batteries for stability if your going with a rear motor.

In the V2 diagram, the weight at A is both motor (mid drive) and batteries. Point B is for batteries and A is for the mid drive. No hub motors

I've had experience with all those types of distribution and the very best option, with least instability, is in the triangle as low as possible.

A low COG is much easier to control in all directions. Having weight close to either of the tires increases the chance of tip over during hard acceleration (positive for rear;negative for front). Also having the weight higher vs lower will increase the tipping tendencies from left to right.

I agree that a backpack means the weight is under control of the rider, but it also raises the COG. Imagine a car with bricks on the floor board vs a car with bricks on a roof rack. Which one is going to do better on the race track (all else equal)? Additionally, a heavy backpack means the rider may get pulled off the bike as the weight shifts. I have actually had that happen to me but it wasn't batteries, it was my work laptop and a water bottle. During a leaning turn, the weight goes off center of the contact patch and tends to pull the rider downward.

How fast and just what kind of corners are you guys doing that a 10-15lbs backpack might pull you off course?

I'm thinking handlebar type might have something to do with it. In a leaned forward position with drop bars the riders weight would be more forward, with the added battery weight more in line with the bb than behind it.

I'm a lightweight myself and generally cruise around un-assisted with a sixpack of beer + groceries strapped to my back at almost all times.

My only guess is that at higher speeds the COG relative to the g force of cornering can become more of an issue?

I am seriously thinking of a bbs02 with 48v 15ah backpack build. Plus beer + dog = trailer.

Dog help me.

EDIT

Ah, downward you said, for some reason I was thinking of going over the *other* way. Still though, the lower the weight the better the handling seems legit.

fourbanger said:

How fast and just what kind of corners are you guys doing that a 10-15lbs backpack might pull you off course?.

Click to expand...

I'm cornering 25-35 mph on generally well surfaced roads. Makes a big difference.

Carry as much weight as you can in the triangle. Whether it's carried high or low will depend entirely on the bike frame type.

Most of what I've read does say for higher speed cornering a higher, but centered COG is best. But unless you are actually leaning into corners at 50 mph, I think what bikes do is what motorcycles would call low speed cornering. For that, lower the better, close to crank as possible, like a trials motorcycle.

I double dog dare you to take urban 90 degree corners at 30+ mph. Bet you take them at 25 tops.

If your battery is large capacity, then it will be heavy and bulky, and it won't be easy to carry it's weight in one point or another. It will be carried pretty much filling the whole triangle, or even spread out in multiple locations on the bike.

NO backpack carry for any kind of long ride, for the reasons kingfish said.

NO top of rear rack carry, if over 15 pounds.

Bear in mind, if your battery weighs less than 10 pounds, then it may have little significant effect no matter what the hell you do. Once it's that light, then you can carry it in the worst place, and hardly notice it cornering on pavement at 30 mph.

If you are dirt riding, then you likely are doing low speed cornering. Put 15 pounds of battery high on a rear rack and go dirt riding, and the bike will buck you off like a stubborn horse. You'll crash all day. But I carry a 4 pound battery the same way and have no trouble at all riding the bike on twisty trails.

Ideally for dirt, put that battery as low as possible, on the down tube. Hard to do on a Y frame though.

danielrlee said:

When we change direction on a bike, what we are doing is changing the patch of tyre that is in contact with the ground by moving our body mass. If you imagine a line running from the rear axle to the headset, this is the axis around which we move the bike. The closer the bikes mass to this axis, the easier it is to maneuver the bike.

Click to expand...

If you want the bike to "flick" back and forth with a fast roll response then center the mass along the roll axis (like performance motorcycles). If you want slower roll response and more stability, move the weight below the roll axis (like a Goldwing). If you want to feel like tipping over, put the weight above the roll axis.

Thanks for all the replies. The emerging theme is weight distributed relative to the horizontal axis defined by the wheel axles and the vertical axis running through the bottom bracket. The intersection of these is very near the bottom bracket so the general consensus is to put the COG nearest this area and several recommendations are to place in the lower part of the frame triangle as that is closest to the intersection.

What is not accounted for is the weight placed on the handlebars, seat, and pedals. Varying the distribution of the rider weight shifts the COG forward or back. If sitting with the rider weight on the seat and pedals that places COG vertically between the seatpost and the bottom bracket. Therefore battery weight placed forward of the BB helps offset the slight shift rearward of the rider.

Weight from a rear hub motor is also offset by forward placement of the battery in the triangle.

Seems like the triangle area is a pretty darn good place from a physics and a practical standpoint. If anything more forward in the triangle is even better.