Weight Distribution on Bikes

Sunder

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Sydney, Australia
Hey all,

I am trying to decide how to distribute the weight of the battery on a bike. In the past, 2-5kg on an eBike wasn't an issue, but now I'm talking about 50kg, and I think it will make a difference.

The rules of thumb I've heard are:

1. Lower on the bike is more stable.

2. More weight over the back wheel = more traction - to a point, after 60/40 or 55/45 or even 50/50, some magical number, there seems to be an agreement that more weight on the back isn't advantageous to handling.

So I've thought of four ways to mount it and its pros and cons, and I was wondering if anyone has any experience racing or designing motorcycles could give any advice?

Option 1: Diagonally
Pros - Easy. There are already mounts for engine and gearbox exactly where I need them, and I can do all metal fabrication myself.
Front/back balanced - battery is mostly weighted towards the back.
Cons: A fair bit of the weight is high.
View attachment 2

Option 2: Diagonally at the back
Pros - Almost all weight over the back wheel
Cons - Gonna be an awfully tight fit, if it fits at all. Don't know if I need custom metalwork.
Weight very high
Too much weight over the back wheel, given the wheel itself is already getting a few kilos of magnets, copper and stataor?
View attachment 3


Option 3: Low and centered:
Pros - Weight low and centered.
Cons - Need to modify the fairing (The box will come fairly close to touching the front wheel. Will look ugly.)
Will definitely need custom metal work to make a frame.
View attachment 1

Option 4: L-Shaped box
Pros - Weight low and centered.
Cons - Electrically annoying to build an L-Shaped battery.
Will probably need custom metalwork.


Thoughts would be appreciated.
 
Option 1.
But where will your motor be located then?

A rough rule of thump is the higher the speed the higher the weight can be. You write the weight will be around 50kg so that equals an ICE engine and would be at roughly the same location in option 1.
 
SlowCo said:
Option 1.
But where will your motor be located then?

A rough rule of thump is the higher the speed the higher the weight can be. You write the weight will be around 50kg so that equals an ICE engine and would be at roughly the same location in option 1.

It's a hub motor, so inside the back wheel.

20180616004401_344.jpg


Option 1 was my favourite too, (Actually, I posted in order of preference).

If that's what the consensus is, this would make my day - The build will be a lot easier, look a lot cleaner, and you're right, it will only be about 5kg heavier than the engine+gearbox together, and I'm taking about 15kg from the fuel tank (Top front), and adding about 10kg to the rear wheel.

So all in all, I should be keeping the same balance of the bike, perhaps with a very minor bit of tweaking from front to back.
 
It's always a compromise. But with a hub motor in the back wheel you'll have enough weight on the rear tire to rule out the options with the battery further back. That would put the center of gravity too much rearwards IMO.
Motorbike manufacturers try to get the CoG more forward for sportier bikes by using lighter aluminium swingarms and putting the shock low and in front of the tire.
 
Thanks mate.

If anyone else has opinions (Or can even see a 5th option), I would love to hear them before I start building the box.
 
markz said:
:thumb:

Clipboard02.jpg

The pack is being made out of very large prismatic cells. I won't be able to get funny shapes like that.

But thanks for the input.
 
The previous weight distribution threads I've seen here tend to discuss having the mass in a line from the steering tube (or bottom crown) diagonally down to the rear axle.

Think of the bike as a pendulum (or lever), with it's pivot point at the ground (contact patch). For you to be able to more easily shift the mass, the closer that mass is to the pivot, the more leverage you have against it. The farther from the pivot the harder it is to change its' direction in maneuvering.

Conversely, the farther from the pivot the mass is, the more energy it takes to change it's direction, so the more stable it can be under conditions that would cause the bike to lean/steer on it's own, or to oscillate (like death wobble).
 
If the motorcycle handled well, then I think option 1 more closely represents where the center of mass was with and engine and full gas tank. The crankshaft and transmission are heavy, but then you have another 30 lbs of fuel sitting up higher. The caveat is, that some sport bikes have a fake gas tank on top with the fuel tank lower in the chassis; so I guess lower is better, but who wants a big box hanging off the bottom of their bike?
 
Never built one, but where did Harley-Davidson put the weight. Unless you want to do wheel stands, I say low and centered.
elechd.jpg


:D :bolt:
 
e-beach said:
Never built one, but where did Harley-Davidson put the weight. Unless you want to do wheel stands, I say low and centered.

:D :bolt:

I was thinking the same, but differently. This frame could be a better starting point since it looks like it has built in support for the super low battery:

gw.jpg

and it would be cool to see a shaft drive e-bike, maybe a first.
 
Sunder said:
markz said:
:thumb:

Clipboard02.jpg

The pack is being made out of very large prismatic cells. I won't be able to get funny shapes like that.

But thanks for the input.

Ah yeah makes sense, then I'd angle it down abit, maybe have the top row level with the ground so the prismatics kinda shape that front wheel a bit. You'd have to eyeball it and see if it works, but then have another row angled. Or just the entire thing angled getting it lower. You will know when you mock it all up.
 
Well, low mass does make a bike easy to balance with precision, while higher mass does make it quicker to shift weight.

Compare Trial motorcycles and racing machines. Trial does require low mass, racing bikes are built with higher COG.

The idea is that low mass does require more amplitude from the rider to shift his weight, giving balance precision but slowing the action. Thus low COG is best suitable for slow, precise handling in complicated situations.
 
Thanks guys. Some good info here.

What I can do (Don't have time to draw it now), is mount the rear of a battery on a pivot point, and the front near the steering tube, adjustable with different lengths of connectors. That way, I can change the angle the battery is sitting at, and therefore lower and raise the centre of gravity to a degree.

I think if I can do this and ride an hour or two with each setting, I can figure out which is best for me, and lock it in that position.
 
That is the way to do it!


Sunder said:
is mount the rear of a battery on a pivot point, and the front near the steering tube, adjustable with different lengths of connectors. That way, I can change the angle the battery is sitting at, and therefore lower and raise the centre of gravity to a degree.

I think if I can do this and ride an hour or two with each setting, I can figure out which is best for me, and lock it in that position.
 
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