Question about center of gravity and stable handling @speeds

[auraslip]

I used this picture as the end all be all for my ebike designs when deciding where to put my batteries.

But I read something in a thread that piqued my interest they other day. Someone said that having the weight higher on the frame actually makes it more stable at speeds, while having the weight lower makes it more nimble at lower speeds. And this kinda blows my conception of bike handling out of the water because I always assumed that you wanted the weight as low as possible.

Low weight on a trials bike for better handling?

Anyways, my question is: does keeping the weight up higher really make a bike more stable at speeds, and if so, why?

As a follow up, besides positioning weight, what else can be done to make a bike more stable at speeds? My latest build is so twitchy that it weaves back and forward when I pedal it at 20mph. The frame reviews said the frame was twitchy (which seems dumb considering it's meant to be a loaded tourer). I have the batteries right on the bottom bracket going halfway up the bottomtube. I also have a fork that measures 120mm of travel when the frame calls for 80mm of travel or a max of 100mm. I assumed that the addition trail a taller fork provided would make it handle better at speeds, but I was wrong. I actually have the washer to drop the fork back down to 100mm, but I'm not totally sure that is the answer. Also, I could use a smaller front tire, but again I'm not sure less trail is the answer.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Trail

 

[cal3thousand]

I just found the wonders of moving my battery weight from my rear rack to the frame And I will never go back.

I can't tell you whether it would have been better to keep it lower, but what you posted makes sense. The weight too low at high speed might allow for some more wobble in the frame. I think it would depend on the frame though.

 

[migueralliart]

I just found the wonders of moving my battery weight from my rear rack to the frame And I will never go back.

I can't tell you whether it would have been better to keep it lower, but what you posted makes sense. The weight too low at high speed might allow for some more wobble in the frame. I think it would depend on the frame though.

On a MTB I would stay away from a rear rack, specially at higher speeds. For a commuter, you can't beat a removable style rear bag. It just makes your life sooo much easier.

 

[cal3thousand]

I just found the wonders of moving my battery weight from my rear rack to the frame And I will never go back.

I can't tell you whether it would have been better to keep it lower, but what you posted makes sense. The weight too low at high speed might allow for some more wobble in the frame. I think it would depend on the frame though.

On a MTB I would stay away from a rear rack, specially at higher speeds. For a commuter, you can't beat a removable style rear bag. It just makes your life sooo much easier.

I can agree with the ability to remove it!

For now, my LiPo pack is temporarily in a frame bag that holds 4 cells perfectly and just straps on. The pack I'm building for the frame will somewhat resemble a motorcycle's gas tank in it's location and rough outline. It will slip onto the top tube into a mating section that will use magnets to let it "click" into place. Then it gets locked down to the frame.

 

[gwhy!1]

I am no expert and can only go by what I think feels better and the reasons I think. the additional weight of anything 2 wheeled needs to be inline with the riders normal riding postion COG and as low down as possible near to the axle line without going below it. this is the basics for good weight distribution then its the wheel base,rake and trail that effects the twichyness and speed.

Edit: trial bikes steer and track verywell but they become very unstable at anything above 40mph, but they need to steer and track good for what they are designed to do at low speeds

 

[amberwolf]

The bike is balanced (stabilized) by pendulum effect with you as the bob. So the farther the weight is from your center of mass, the less effect your own mass has on it.

But...the closer the mass is to your own center of mass, the slower it is to move when you do your thing balancing the bike.

Depending on your mass, the bike's mass, and the other mass, you can work out the effects of different placements of it on the frame.

Theoretically the most stable place for it is to attach it to you (backpack tied tightly/securely to you), but then your muscles have to get stronger to move it as fast as they used to move just you.


For nimbleness, the worst place to put it is anywhere that must pivot with the front fork, because you can't move the wheel as fast as you used to. It can make it more stable, if it's fixed down properly, but until you do exercises to make your arms a LOT stronger moving that fork back and forth quickly, it's less nimble.


The rear rack wouldn't always be sucha horrible place to put it but most racks have tremendous slop in them, though even if it were perfectly stiff it'd still affect your balancing act by slowing down the bike's reaction to your moves.

 

[cal3thousand]

The bike is balanced (stabilized) by pendulum effect with you as the bob. So the farther the weight is from your center of mass, the less effect your own mass has on it.

But...the closer the mass is to your own center of mass, the slower it is to move when you do your thing balancing the bike.

Depending on your mass, the bike's mass, and the other mass, you can work out the effects of different placements of it on the frame.

Theoretically the most stable place for it is to attach it to you (backpack tied tightly/securely to you), but then your muscles have to get stronger to move it as fast as they used to move just you.


For nimbleness, the worst place to put it is anywhere that must pivot with the front fork, because you can't move the wheel as fast as you used to. It can make it more stable, if it's fixed down properly, but until you do exercises to make your arms a LOT stronger moving that fork back and forth quickly, it's less nimble.


The rear rack wouldn't always be sucha horrible place to put it but most racks have tremendous slop in them, though even if it were perfectly stiff it'd still affect your balancing act by slowing down the bike's reaction to your moves.

The bike used to turn it's head around quick, but the ass would wallow. The flex of the rack just compounded things even more. I feel like the bike is ultra nimble now, as if I didn't even have the batteries on there. Of course the whole bike is heavier and I can't bunny hop it like a push bike, but it feels like I would be able to with enough pop. Never would have been able to with the rack mounted batteries as the back wheel was cemented to the ground.

 

[-dg]

I can agree with the ability to remove it!

For now, my LiPo pack is temporarily in a frame bag that holds 4 cells perfectly and just straps on. The pack I'm building for the frame will somewhat resemble a motorcycle's gas tank in it's location and rough outline. It will slip onto the top tube into a mating section that will use magnets to let it "click" into place. Then it gets locked down to the frame.

What frame bag are you using now and where did you get it? I'm looking for a bag for 4 lipo bricks for a stealthy commuter. Thanks.

 

[dogman dan]

Much depends on what kind of handling you are discussing. For trials, that rarely takes place much above 5 mph, the lower the better. For burning around corners at 80 mph on a racetrack, they actually want the weight higher, or so I'm told. The situations are like night and day, and are completely different handling needs.

But one thing doesn't change much, you kind of want the most added weight near you knees. The idea in my mind, is that the wheels can then rocker back and forth some as the front tire hits a bump first, followed by the rear. The big mass, your ass standing the pedals, along with battery, doesn't have to go up and down much. This allows no suspension, or less nice suspension to work better.

In your green diagram, I'd shorten the bar some on both ends, not going over the hubs. In a way, any hubmotor is undesireable, just like any battery on a rear rack is. However, less of it works better than more. I built a 5304 dirt bike, and hated it. No suspension didn't help any, but the motor was just too heavy for that far back on the bike. Switching to much lighter 9c motor, it got a lot better. Sucks still, but sucks a lot less. Just like a 10 pound battery on a rear rack still sucks, but a lot less than 30 pounds of sla did.

 

[gwhy!1]

I can agree with the ability to remove it!

For now, my LiPo pack is temporarily in a frame bag that holds 4 cells perfectly and just straps on. The pack I'm building for the frame will somewhat resemble a motorcycle's gas tank in it's location and rough outline. It will slip onto the top tube into a mating section that will use magnets to let it "click" into place. Then it gets locked down to the frame.

What frame bag are you using now and where did you get it? I'm looking for a bag for 4 lipo bricks for a stealthy commuter. Thanks.

Not the same frame bag but the same principle http://endless-sphere.com/forums/viewtopic.php?f=31&t=44339

 

[Drunkskunk]

Weight in the green zone for best stability. I agree with Dogman that the green zone needs to be shortened up to the center of the bike.

As for weight high VS low, Try balancing a broom by the handle on the palm of your hand. the weight is way up high, and as it starts to fall you have time to react and move your hand to keep it balanced. Now try the same thing with a toothbrush. It starts to fall faster and you probably can't keep it balanced as long or as easily. The idea with a bike is the same, with the weight being the brush end, and the handle being the tires. Higher is mor stable, lower is more nimble. A trials bike you want to be super nimble.

But the front fork angle and trail are even more important than weight height.

 

[gwhy!1]

Weight in the green zone for best stability. I agree with Dogman that the green zone needs to be shortened up to the center of the bike.

As for weight high VS low, Try balancing a broom by the handle on the palm of your hand. the weight is way up high, and as it starts to fall you have time to react and move your hand to keep it balanced. Now try the same thing with a toothbrush. It starts to fall faster and you probably can't keep it balanced as long or as easily. The idea with a bike is the same, with the weight being the brush end, and the handle being the tires. Higher is mor stable, lower is more nimble. A trials bike you want to be super nimble.

But the front fork angle and trail are even more important than weight height.

But..... ( just throwing this out there ) havent you got to look at the broom as being upside down ? ( all the weight at the bottom ) so maybe the more of the weight near the center pivot point the better/faster the handling but more prone to start doing its own thing with the slightest of weight shift at speed, another way to look at it ,,, if you lay a bike on its side and the higher the weight from the wheel axles then the harder it is to pick up.

 

[dogman dan]

Again, it would just depend on what you were doing. Trials or moto gp. Day to day street riding would be guess where? , right in the middle between high is better and low is better.

So hang that weight between your knees and your balls.

 

[neptronix]

p.s. not to be offtopic, but on the idea of frame bags and removeable batteries, that's easy. Keep the bag locked in, and remove the battery..

If you have RC Lipos, you can duct tape them together so that removing them is not a problem. I group my lipos as 10S 10AH units or 6S 15AH units with duct tape. It is real easy to grab those two packs and remove the from the bike.

Triangle mount is great and i design all my eBikes around it, because it makes the handling quite neutral. Upper rear mount ( rack ) is truly awful in comparison. I see how a front mount could be beneficial when you think about how your body controls the motion of the bike by leaning the front at the right angle. But aesthetically, it just looks too weird to me.

 

[recumbent]

I have 6 lipo packs in my panniers (not pictured), 3 on each side and this feels good, but would make more sense to place them nearer the seat or between your knees like was said above.

 

[gwhy!1]

I see how a front mount could be beneficial when you think about how your body controls the motion of the bike by leaning the front at the right angle. But aesthetically, it just looks too weird to me.

If the bulk of the weight is on or near the steerer pivot point then this will make the steering very heavy and very quick if the steerer angle is steep , so the more weight you have at the steerer point then the less steerer angle the better. But is all a trade off , why do you think that some of the motorcycle manufactures have started putting the petrol tanks under the seat and having false petrol tanks with the air filter in/or storage.

 

[dogman dan]

A lower pannier carry always beats the high rear rack if you do have to carry a lot of weight in the rear. Once you are on the ends of a bike, low seems to help.

If you are carrying on a rear rack, the tolerable amount of flex in the rack is Zero. Then you need a good frame, so the non flexing rack doesn't just flex the frame. Put enough weight on the end of the bike, and it has a good leverage on the center of the bike. You want it the other way around, you having the leverage on a lighter end of the bike.

Again, it depends a ton on what you do with the bike. Riding streets under 20 mph, extremely poor handling can be tolerated. Take it to the dirt and ride hard, and the same bike will toss you off over the bars all day.