The "low COG = high stability" myth

[Tenerider]

Hey guys,

 

this has been on my mind for months now. After lots of recherche I feel finally brave enough to publish my thoughts.

I will need to do it in several posts, hope this won't be too annoying.

 

A lot of us guys here (and even more motorcycle reviewers) very often tell others that a low center of gravity on a motorcycle makes it more stable and/or easier to control, especially when riding slow.

 

For example, @Interceptor posted recently (about the Touareg vs the T7):

"lower center of gravity due to a lower fuel tank in the seat area which makes this bike act and feel very stable, balanced and easy to control."

Original post see here:

(sorry buddy for using your post as am example here, couldn't find another fitting one quickly here )

 

Well, this all of course feels right to us:

A lower COG bike can be flipped from side to side with ease (you can feel it when just sitting on the standing bike), it requires less force putting it around corners, let alone lifting it up is definitely easier when it has been laid down.

It just feels easier to handle and more nimble than a high COG bike like the T7.

 

While these points are true as long as the bike is at standstill, most people conclude that a low COG bike must mean better low speed stability and thus a lower tendency for tipovers.

But more or less the opposite is true: A HIGHER COG means the bike will have stable riding conditions at LOWER speeds already than a bike with a lower COG.

 

There are some caveats though which I will address in the next posts and of course I'm going to prove my point. I will definitely not conclude "higher COG is always better", but perhaps it will alter some people's view on the high COG of our beloved T7.

Please allow some time for a decent writeup, but feel free to start the bashing before

 

Jan

[Tenerider]

OK, I think I already have made a mistake in my first post. I need to be very precise on what "stable riding conditions" actually means.

 

You all know that above a given speed any motorcycle will remain upright due to gyro forces caused by spinning wheels. Any motorcycle will stay upright once these gyro forces become big enough.

In German, the speed range below this given speed is called "Kenterbereich" and can be roughly translated to "capsize range" - meaning that in these low speed ranges, the motorcycle will easily "capsize", which is tip over. It will not stay upright on its own (as we all know from personal experience :D).

 

Of course it's possible to ride bikes in this low speed range - but you need to balance the bike with both your body positioning and turning the handlebar. We all know this, once you try to ride at very low speeds, you'll balance the bike by turning the bars. Riding on a perfectly straight line is very difficult to impossible at low speeds. Small (or sometimes bigger) steering input is necessary to keep the bike upright.

 

In this very low speed range, a high COG actually helps you balancing the bike!

 

OK, you'll probably say this is utter bs. But I assume most of you guys own a spade or an axe or a hammer. Ever tried balancing a hammer on one hand, so that it stays upright?

If not, give it a try:

First with its head on your palm, grip pointing to the sky.

Then with the grip on your palm, heavy side up.

It will be much easier in the second case. The hammer doesn't move as quickly out of the perpendicular position. You have much more time to compensate for those movements. The reason for this is its higher "moment of inertia" in the second case compared to the first case (low moment of inertia).

 

In the case of a motorycle (or the hammer), which roll axis is along the connection line between the wheels contact points with the ground, a high COG also means a high moment of inertia.

In the first case, the COG of the hammer is low, and it's not easy to balance, because it rotates too quickly around its roll axis.

In the second case, the COG of the hammer is high - this leads to much slower roll movements (roll movement = tipping left or right).

 

On a very slow or even standing motorcycle, the situation is exactly the same:

In principle, it should be easier to balance the bike if it has a high COG (equivalent to a high moment of inertia in this case), simply because you have more time to compensate for roll movements. The bike doesn't "want" to roll as easily due to its high moment of inertia/high COG.

 

Yes guys. Your T7 is actually easier to balance than the lower COG bikes like an 890 Adventure ("sagging tits tank") or a Bonneville Bobber.

Unfortunately, this is only half of the truth

 

(tbc)

 

P.S.: I know not everybody likes him, but RyanF9 is spot on here:

 

Edited December 28, 2022 by Tenerider

[Tenerider]

In the last post, I highlighted the following sentence:

 

"In principle, it should be easier to balance the bike if it has a high COG"

There are three bs words which actually seem to take all the fun out of it - "in principle" and "should".

Out on the bike some other things come into play.

 

I claimed a high COG bike to be actually LESS prone to tipping over than a comparable (same weight, same height and so on) low COG bike. This is true.

But WHEN a high COG finally reaches its tipping point, this is really a point of no return. You won't be able to catch it as easily (or at all) as a low COG bike, simply because its high moment of inertia now works against you.

 

Let me give an example which is easier to understand, but actually shows a comparable physical situation:

Imagine two different cars on a flat road, both at standstill. One car weighs 50 kg (maybe a toy car), one weighs 1.5 tons (Americans would call this a toy car, too).

Now, we try to push the cars on the road.

Of course it's way easier to acelerate the lower mass car to a given speed of say 5 kph. You'll need much more force to acelerate the heavier one.

The lighter car has less "inertia" than the heavier one.

If both cars are moving at 5 kph, it's also much easier to stop the lighter car.

 

"Moment of inertia" is actually the same thing as inertia, but refers to rotational movements. This means:

High COG -> bike doesn't want to tip over as easily, but also can't be stopped from tipping over as easily.

Low COG -> bike tips over more easily, but can be "caught" with less force.

 

Our instincts/senses tell us mostly about the second part. Thus the common assumption that a low COG bike is "more stable" - it isn't, but it requires less force to keep it upright once it tips over.

 

The most important question is: What is more important on the trail?

A lower COG bike IS easier to pick up after ground contact.

But at the same time, it requires much faster reactions to counter-act its tendeny to tip over.

 

A high COG bike will atually require be EASIER to balance in the capsize range (see above), and its capsize range will be smaller. This means, it will become stable on its own at LOWER speeds!

But once it has decided to tip over, it'll require a lot more force to prevent it from doing so; and picking it up require again more force.

 

(tbc)

 

OK, I have really skipped a lot of details in my previous posts, but if anyone is interested in more in-depth knowledge and prove of my weird findings, I can unhappily provide them (meaning a lot of text and some formula). I am a graduated physicist (German Diploma, comparable to a Master of Science, and physics teacher meanwhile), but I'm on holiday AND always lazy. Anyway, if you insist, I'll open the gates of hell

Edited December 28, 2022 by Tenerider

[Dougie]

@Tenerider, how is the double espresso machine for Xmas working out? I take it all the good films have finished on tv? 

I suppose it’s a pendulum effect. 
like you said, harder to start moving but harder to catch once it goes. A bit like canoe handling, but for hull shape affecting buoyancy instead. 
 

Edited December 28, 2022 by Dougie

[Tenerider]

4 minutes ago, Dougie said:

@Tenerider, how is the double espresso machine for Xmas working out? I take it all the good films have finished on tv? 

I suppose it’s a pendulum effect. 
like you said, harder to start moving but harder to catch once it goes. A bit like canoe handling, but for hull shape affecting buoyancy instead. 
 

Espresso machine destroyed by kids, they're watching The Grinch, I'm angry cos I've got a cold and the weather sucks! That's why Dougie

[Ray Ride4life]

I balance my T7 much easier as my Honda Crosstourer with a low COG.

[Dougie]

17 minutes ago, Ray Ride4life said:

I balance my T7 much easier as my Honda Crosstourer with a low COG.

I’ve never found the cofg a problem either on this. It feels great. I love the height too. Maybe it’s just with people who have to stretch a bit to the floor, I can see that feeling a bit unbalanced for some. 

[Dougie]

54 minutes ago, Tenerider said:

Espresso machine destroyed by kids, they're watching The Grinch, I'm angry cos I've got a cold and the weather sucks! That's why Dougie

Same here. And I’m fed up of chocolate.

[Ray Ride4life]

Almost done at work, the weather is bogus but taking the mud route home.

Let's see what the COG does hahaha.

[AZJW]

@Tenerider Thanks for detailing what I already suspected.  I just proved this to myself again  yesterday on a local ride with my son. I started out on my T7 on a known rocky ( baby head infested terrain) 2 track with the understanding that when my hands we're about to give out,  we'd switch steeds to my 260 lb Beta 4 stroke.  I reinforced my belief that the T7 has decent straight line stability ( MSC Damper equipped) as long as I kept it moving at at least about 8-10 mph or better. When the going got really tough and I was reduced to paddle walking through really loose baby head rocks,  the T7 became much more of a literal handful as the gyroscopic stabilization effect started going away. After my arthritic hands protested enough, we switched bikes and obviously the 200 lb lighter Beta was much easier to handle at lower speeds through the tough stuff. I kept up a bit quicker pace in the baby heads on the Beta as with 8 years on that bike, I know it well in tough conditions.  Interesting note, my son, who is a better, stronger, more confident rider, plowed right through the tough stuff on the T7 and at every stop to reassess our directions on our now unknown routing, he had a smile on his face.  

My question for your analytical mind is, regarding the T7 at what speed does the higher CG become less of an asset and starts to degrade stability?  I would've been happy to test this theory out for you yesterday,  but I fear I'd find out with the cost being a damaged fairing or radiator! Good subject, keep hypothesizing!

[Tenerider]

OK, here comes the final part of my rant. Yeah, I know, nobody asked about it, but it's Christmas, and at Christmas you gotta tell the truth, or Santa will steal your presents back. Actually, I was annoyed of many "professional YT reviewers" or even magazines claiming the benefits of low COG adventure bikes. Yes, there are some, but there are also drawbacks.

 

In my opinion - and this heavily depends on your riding style and terrain and speed - a higher COG overall might have more benefits to an ADV rider.

 

I can imagine that on extremely blocked, tight terrain a lower COG and thus a more nimble bike makes a lot more sense - but only if it's not too heavy to safely catch a tipping bike with one boot. The latter will rather apply to trial and/or light enduro bikes (below 150 kg curb weight or so). But these bikes are so light that the COG probably doesn't matter much (pure guesswork here).

I really believe an ADV bike shouldn't be ridden foot-out MX style - you won't catch a 200kg+ machine with one leg if it really wants to go, no matter how low or high its COG is. As far as I know, most ADV riding coaches don't teach this technique for this exact reason. But, to be honest, I am NOT an experienced rider!

 

As soon as you are riding on terrain which does NOT require to permanently stop and/or pick up the bike, the higher COG should help keeping the bike balanced during slow speed passages. I haven't made ANY reliable calculations or tests to prove this. But there is some evidence for my claim:

I guess almost everybody here will confirm that such terrain should be ridden standing on the pegs.

Apart from other benefits (bike can move more freely), there is one thing which dramatically changes when you stand up - it's the COG. It goes UP. Don't believe the guys who claim it goes down, because your weight is being transferred to the pegs, it's just wrong - if need be, I can explain why.

So, standing up rises the COG (and thus the moment of inertia), no doubt about this. And standing up makes riding/balancing at low speeds a lot easier.

 

I am sure that the higher COG is not the only reason why it's easier to balance a bike standing on the pegs instead of sitting, but I am confident it plays a significant role.

This concept is quite well known to bicycle track racers performing a track stand, it'S much easier standing on the pedals than sitting down (see https://en.wikipedia.org/wiki/Track_stand).

In any case, the higher COG does NOT make the bike less stable at low speeds - rather on the contrary!

But on the other hand, this also means that more effort is required to push the bike through curves. It actually feels less nimble = more stable.

 

I have to admit that I've never ridden a low COG ADV bike. So, many of my assumptions are driven by theory - but to be honest, the fundamental principles of physics for this are well understood and accepted since 200 years or so. It's not rocket science - but somehow contradicts our "common sense".

 

My simplified conclusions:

- Don't think a high COG will negatively affect your offroad riding. IMHO, it should actually benefit from a high COG, because your bike gains stability.

- Especially at low speeds, balancing a high COG bike is easier PLUS the bike enters "stable riding conditions" at lower speeds already (this means, the capsize range is smaller).

- A high COG does have disadvantages: Picking it up, changing directions (!) and so on require more effort/force. There is no such thing as a free lunch.

- In my opinion (!!!), the benefits of a high COG should outweigh the disadvantages.

 

Thanks for your patience - looking forward to your opinions!

 

Edited December 28, 2022 by Tenerider

[Tenerider]

40 minutes ago, AZJW said:

@Tenerider Thanks for detailing what I already suspected.  I just proved this to myself again  yesterday on a local ride with my son. I started out on my T7 on a known rocky ( baby head infested terrain) 2 track with the understanding that when my hands we're about to give out,  we'd switch steeds to my 260 lb Beta 4 stroke.  I reinforced my belief that the T7 has decent straight line stability ( MSC Damper equipped) as long as I kept it moving at at least about 8-10 mph or better. When the going got really tough and I was reduced to paddle walking through really loose baby head rocks,  the T7 became much more of a literal handful as the gyroscopic stabilization effect started going away. After my arthritic hands protested enough, we switched bikes and obviously the 200 lb lighter Beta was much easier to handle at lower speeds through the tough stuff. I kept up a bit quicker pace in the baby heads on the Beta as with 8 years on that bike, I know it well in tough conditions.  Interesting note, my son, who is a better, stronger, more confident rider, plowed right through the tough stuff on the T7 and at every stop to reassess our directions on our now unknown routing, he had a smile on his face.  

My question for your analytical mind is, regarding the T7 at what speed does the higher CG become less of an asset and starts to degrade stability?  I would've been happy to test this theory out for you yesterday,  but I fear I'd find out with the cost being a damaged fairing or radiator! Good subject, keep hypothesizing!

Hey mate, actually the higher COG should never degrade the stability. It affects the bike's "nimbleness" negatively, meaning you need more force to change directions. However, I haven't considered knee-dragging situations on track. Really not sure quire now what happens there.

 

For me, it's very difficult to compare two different bikes like your Beta (once sat on a Beta, what a cool bike!) and our heavier Japanese steel horses. But actually the Beta should as well have a very high COG - but due to its lower weight, a much lower moment of inertia. I assume that the T7 should due to its higher mass and moment of inertia be less affected of obstacles - but then geometry and suspension playe a big role here, too.

 

I think you mentioned a very important point: Try to stay above super-low speeds, like more than 8 mph or so. From my poor experience, the T7 will reward you with a planted, smooth ride - and the high COG definitely plays its part here.

But even at lower speeds, the high COG should make balancing easier - until she really tips over, then it makes things actually a bit worse than with a low COG bike

 

I once swung my leg over a Rocket 3. This one has a super-low COG, it didn't feel any heavier than my T7 between my legs, rather at the contrary. Due to its low COG, it required very little effort to swerve it from side to side. But this was with both feet on the ground and no need to balance at all!

Riding is a totally different situation.

Edited December 28, 2022 by Tenerider

[Ray Ride4life]

Also remember that when you can't use speed higher rpm's also has an gyroscopic effect, not as much as turning wheels but but just revving can help you regain control sometimes.

[DT675]

And just the same Ray, "don't" use higher RPM's when cornering on technical terrain because your just fighting that gyroscopic effect doing so. And the T7 has a lot!  Bigger crankshaft= more gyro effect! Use it to your advantage, loose it when its not needed! 

[Ray Ride4life]

Good thing the T7 has enough low end power.