Mounting your battery, Center of Gravity.

[Tiberius]

Assuming we agree that the actual turn radius is a moment affecting the inertia of the bike at the end of it, moving bike mass along that moment towards the pivot centre yields a reduction in inertia and a reduction in the demand on tyre grip for a given speed.

Hi Hangdog,

I wasn't sure whether it was a matter of terminology, so I phrased it as a general question.

For instance, I'm not sure what you mean in the sentence quoted above. Are you talking about steady state conditions, ie., constant speed, constant radius turn, or non steady state? Non steady state would be entering or exiting the turn, changing radius or speed during the turn.

When you say "pivot centre" (correct spelling, btw ) do you mean the centre of the turn or the roll axis?

Nick

 

[Hangdog98]

Hi Nick, yes I re-read my post and even confused myself. Any dumbing down in this explanation is for my benefit, not yours.

OK, steady state etc etc. What I'll attempt to discuss is the comparison of two identical bikes of different mass location configurations travelling around the same constant radius corner at the same speed, same everything. The only variable is the position of the battery pack. Bike #1 is your MTB with the battery pack at the top, bike #2 is your bike again with the battery pack moved to a position under the bottom bracket. The argument enters the realm of the ball on a string moving in a circle - centripetal/centrifugal force. What I am trying to describe is the effect that carrying weight higher on the bike results in less centrifugal force trying to make the bike deviate outwardly from its path around the corner, (usually on its side accompanied by a tumbling rider).

Even though the bike turns the corner because of the tyres, the forces acting upon it are like those of the ball on the string. The bike draws an arc, that arc has a radius being the centre-point of an imaginary circle, that radius is the piece of string. The bike is the ball.

Bike #1 carries a fair amount of mass in a high position when travelling upright. When it is leaned over 45 deg for the turn, that mass moves one hundred centimetres* towards the centre-point of the imaginary circle. The centre of mass of bike #1 is 'A'.

Bike #2 carries a fair amount of mass in a low position when travelling upright. When it is leaned over 45 deg for the turn, that mass moves ten centimetres* toward the centrepoint of the imaginary circle. The centre of mass of bike #2 is 'B'.

A is closer to the centrepoint of the imaginary circle than B is.
A's velocity is therefore less than B's.
The centrifugal force acting on bike #1 is less than that acting on bike #2 because A is slower than B.
The demand on tyre grip and lean angle is less on bike #1 than it is on bike #2
Bike #1 can now go faster than bike #2 and bring the velocity of A to be same as B. Even though bike #1's speed is now higher than bike #2, the centrifugal forces are the same.

OR, the ball on the string becomes elongated on the string and moves some of its mass towards the pivot point and turns the centrifugal/centripetal energy differential into greater speed. Wheeeeeee.

Does that make any sense or did Sir Isaac Newton just roll his eyes at me?

Of course steering dynamics are much more complex than this, but this, I believe, is one way that weight positioning can affect cornering.

(* for example)

 

[Tiberius]

Does that make any sense or did Sir Isaac Newton just roll his eyes at me?

Hey, you get lots of points for reading the whole thread - some people just dive in with something that contradicts Newton's 1st Law.

I see the point you are making, but I'll try to explain why I think its a small (second order) effect. First of all, in steady state motion, and that includes cornering, there are no inertia effects, because the kinetic energy is not being changed. (Aside: in cornering there is acceleration, but the KE is constant.)

Normally we assume that vertical mass distribution doesn't affect the lean angle in a corner. You are right though that it does affect the effective radius of turn and therefore the centripetal force {mw^2r} and therefore the lean angle {tan-1 (mw^2r/mg)}. But suppose we can move the CG up and down through 300 mm; in a 45 degree lean that is 200 mm difference in r, compared with a starting value of maybe 10 m. It only makes a 2% difference in the tyre grip needed. Yes, the effect is larger in tight, high g turns but you see my point.

The fun comes when we get beyond steady state, first we can look at how the rider balances the bike, which is tiny movements about the steady state point, then we can look at taking the bike in and out of turns, which are large movements. Inertia and mass distribution are important for both. When you take the bike in and out of turns, the forces to roll and yaw the bike have to come from tyres, and I think you'll find in those instances the mass distribution makes a difference of way more than 2% to the tyre grip needed.

Nick

 

[johnrobholmes]

Centralized mass is most important when entering and exiting a turn. With decentralized weight the handling becomes very sluggish.


We must keep in mind that some people may not be able to handle the bike when stopped with 50lbs of lead acid place high. In heavy battery, weak rider cases it would actually be best to place the weight low around the rear of the bike. Horrible handling, but the rider can keep it upright if a gust hits them when stopped.

 

[recumbent]

Are you guys saying the lower seat height on my recumbent bike is adversly effecting handling of my bike in turns?

Because twice this summer my rear tire slid out from under me on a gravel road, turning at low speeds. The fall was uneventfull because the ground is only 16 inches from my butt and the chair is wider to protect me also.
Still I wonder if i was riding a regular bike, would the same thing of happened?

 

[Hangdog98]

Are you guys saying the lower seat height on my recumbent bike is adversly effecting handling of my bike in turns?

Yes... and No. The chair is the problem too. On a regular bike your weight is spread along the 3 contact points hands, feet and butt. When you're riding in the gravel you will unweight your butt and most of your weight will be carried on your feet. This positon allows faster and finer adjustments to address balance and traction loss. On the bent, your weight is spread out along your body and your ability to make those adjustments is compromised because you're lying down, you lazy bugger. :lol: A regular bike rider would pivot the bike at his feet, you have to pivot on your behind. In the gravel that way is slower, even without seeing the math.

This is why I was banging on about weight position and intended use. As a post said some pages back, BMW R series bikes have a low slung engine to assist with stability. They're fabulous at slow speed turns and riding across the desert and gravel. They don't go so good on the race-track. BMW jacked them up about 200mm (8 inches) to make them steer for the BMW-only road race series.

 

[recumbent]

"snip"... On the bent, your weight is spread out along your body and your ability to make those adjustments is compromised because you're lying down, you lazy bugger. :lol: A regular bike rider would pivot the bike at his feet, you have to pivot on your behind. In the gravel that way is slower, even without seeing the math.

So true, and that is exactly what happened, I could not react in time it seemed, before i was on my arse already.

I haven't been on a regular bike for many months and forgot how quick we can react because we are essentially on our feet, sort-of.
Thanks for clearing that up. I was getting worried it was my balance, but now i know, it's just because i'm a lazy bugger

 

[Tiberius]

Hi Recumbent,

I assume we're talking about the bike in your avatar? You also said it was at slow speed.

As the speed gets slower, balancing a bike gets more difficult. This is simply because at low speed the tyres move sideways more slowly in response to a given steering input, so the corrections you make have less and less effect the slower you go. (You may read explanations of this based on inertia, but they are wrong.)

Another thing that happens is that the difference between the paths followed by the front and rear wheels becomes more noticeable. This is worse on a long wheelbase bike; the effect on balance is worse if the weight is at the back, and the balance corrections are less effective if the weight is low down.

You can probably see where I'm going here. Long wheelbase, weight at the rear, weight low down are all working against you for low speed stability. Add in some gravel and I expect you are spending a lot of time with your legs stuck out rather than on the pedals.

At high speed the balancing becomes easier and you gain from the lower aerodynamic drag.

Nick

 

[Papa]

As the speed gets slower, balancing a bike gets more difficult. This is simply because at low speed the tyres move sideways more slowly in response to a given steering input, so the corrections you make have less and less effect the slower you go. (You may read explanations of this based on inertia, but they are wrong.)

Another thing that happens is that the difference between the paths followed by the front and rear wheels becomes more noticeable. This is worse on a long wheelbase bike; the effect on balance is worse if the weight is at the back, and the balance corrections are less effective if the weight is low down.

Said another way - as the CoG is placed further aft, steering inputs must be initiated quicker and more agressively to maintain balance, because the front contact patch needs to move a greater, lateral distance.

For example; The front contact patch on a bike carrying 50/50 weight distribution needs to move 2" laterally to counter a 1" lateral shift in the CoG. Likewise, if the weight distribution is, say, 75r/25f, then the front contact patch needs to move 4" laterally to counter a 1" lateral shift in the CoG. At the other extreme, unicycles are easily balanced because the CoG is directly above the steer wheel.

 

[recumbent]

Yep, you guys are correct, my bike is 7 feet long total about 70"inches at the axles and it's a bull to handle at low speeds under 5km/h, but once over that it gets better fast. I still have to work the steering back and forth a bit even at 10km/h (6mph) above that it's pure heaven though and tracks like an arrow.

Thanks for explaining the physics to me, I'll be more attentive next time

 

[johnb]

A bike wheel turns when you lean it over because the part of the contact patch inside the turn rolls on a smaller tire circumference than the part of the contact patch outside the turn, In the same way an ice cream cone rolled along the ground draws an arc. So, the rear wheel does more than simply follow as some have attested.

Perhaps I have misunderstood you here...

Are you saying that this is the only turning force on a bike, a major factor or a minor factor?

I'm not sure about a motorbike with fat tyres, but I would suggest that it is at most only a minor factor on a bicycle. Lean is obviously necessary to prevent the bike rolling to the ground but surly the turning force is established by the position of the handlebars/front wheel.

 

[Hangdog98]

Hi Johnb, yes, that's a common question.

It is a major factor in turning the bike (or bicycle). The effect of the handlebars is for trim and balance. At lower speeds the need for trim and balance is greater and so the movement is greater. The slow turn with the 'bars at say, 45 degrees is executed like a car or trike although some tyre effect is present. Eg; you can't turn left if you lean the bike to the right, but a car or trike can.

As you start to move faster you use less and less steering input and the turning is done more and more by the circumference difference between the outside and the inside of the tyres. The handlebars only ever cause to force the remainder of the bike (behind the steering head) to try and drive around the steering head one side or the other, at any speed.

If you are turning left and leaning over (even a little bit) and you turn the bars to the left, the bike will try and drive past the right side of the steering head and force the bike to the right. Effectively, movement by the handlebars away from centre only acts to slow the front wheel (relatively) and allow the forward forces of the bike to try and overtake the steering head which has now moved slightly away from centre.

 

[johnb]

Hi Johnb, yes, that's a common question.

It is a major factor in turning the bike (or bicycle). The effect of the handlebars is for trim and balance. At lower speeds the need for trim and balance is greater and so the movement is greater. The slow turn with the 'bars at say, 45 degrees is executed like a car or trike although some tyre effect is present.

You will have to excuse me for still being sceptical about this. It may be that it is a major factor on a motorbike and a minor factor on a bicycle. As you say, it depends on speed. I think I would need to see some maths to be convinced.

Eg; you can't turn left if you lean the bike to the right, but a car or trike can.

Actually this is perfectly possible (if a little perverse) on a bicycle if you shift your body weight enough, probably not possible on a motorbike.

As you start to move faster you use less and less steering input and the turning is done more and more by the circumference difference between the outside and the inside of the tyres.

Clearly this is affected by tyre pressure and diameter (also tyre elasticity and surface friction), but a racing bicycle with large diameter wheels and high pressure has no problem with tight bends.

As mentioned recently in the thread, as you move faster, steering input is less because the bike reacts more quickly to that input.

The handlebars only ever cause to force the remainder of the bike (behind the steering head) to try and drive around the steering head one side or the other, at any speed.

If you are turning left and leaning over (even a little bit) and you turn the bars to the left, the bike will try and drive past the right side of the steering head and force the bike to the right. Effectively, movement by the handlebars away from centre only acts to slow the front wheel (relatively) and allow the forward forces of the bike to try and overtake the steering head which has now moved slightly away from centre.

Agreed.

 

[Hangdog98]

This isn't my theory johnb, it's standard cycling dynamics 101. I'll just have to respect your scepticism and trust you'll make further enquiries elsewhere to satisfy your curiosity. I'm sure you'll agree that the research is half the fun. :wink: enjoy.

 

[johnrobholmes]

Consider that while cornering a skinny tire bike, you will still have a bit of steering input. If you try and keep the bike upright by shifting your weight inwards, you will have to increase the steering input to keep the same radius of the turn. If you shift weight and lean the bike over more, less steering input will be used to keep the same radius of turn. In other words, the more you lean the bike over, the more the turn is executed by "the circumference difference between the outside and the inside of the tyres". Higher speed turns require more lean, thus higher speed cornering is effected more-so by "the circumference difference between the outside and the inside of the tyres".


This is something I have noticed many times while hanging on the inside of turns. The more I hang off, the more I have to steer to compensate.

 

[johnb]

A survey of the literature reveals that bicycle models generally include a knife-edge rolling point contact whereas motorcycle models include toroidal wheels. This appears to confirm what I suspected about the relative significance of the tyre profile.

 

[Hangdog98]

Terrific news. I'll alert the rest of the bicycle design community and we can make the necessary adjustments.

 

[safe]

Proper Shoulder Position

We should look at the difference between the steering geometry of the bike verses the possible body movements that can create counterbalance weighting while riding.

You need to use some caution here...

For example, the proper "road race" position (at least the "Kenny Roberts Era classic") while in a turn is to hold your shoulders relatively flat while your body and knee drops down low into the inside of the corner. This is your classic "knee dragging" position. It's absolutely critical that you keep your shoulders flat (level with the ground) even though your body is going the opposite direction because if a tire begins to let go you need to be able to swing your shoulders DOWN as a counterweight to try to correct the loss of traction and get the tires to stick. (it also prepares you for the high side that is about to come when the tire catches) This also applies to riding on the dirt too, you need to keep your upper body and head flat while your body moves around.

So when we are looking at things from the "big picture" we are far more constrained in our ability to use our shoulder to do the "work" in a turn. We need to keep that shoulder flat as an INSURANCE POLICY against the loss of tire traction.

If we exclude the ability to use up our counterbalance prematurely (keep our shoulders flat except in emergencies) then we must revert back to the bikes natural steering geometry to determine it's behavior.

The simple thing to remember:

"If you are dropping a shoulder into a turn your riding style is inviting trouble and so to seriously discuss using such bad habits is counter productive to the discussion of bike gerometry."

Just Don't Do It!

 

[Hangdog98]

Mick Doohan used to lead his inside shoulder forward and slightly down to weight the front wheel a little more and then effect trim by weighting the outside (highside) footpeg and rocking the bike (shifting the contact patch) whilst maintaining body position relative to the ground. Bayliss used a square-on shoulder but employed the same rocking action to make finer adjustments at lower speeds mid corner. I worked with Bayliss in the mid 90's and learned a great deal about this method. I found it much easier to find a set-up. This method is less effective on lighter bikes like 125 and 250 weights.

I think the most important thing to keep level is your eyes. When a grip change occurs and the eyes detect a horizon tilt the brain can process it much faster if your eyes are level (with the horizon).

 

[safe]

I think the most important thing to keep level is your eyes. When a grip change occurs and the eyes detect a horizon tilt the brain can process it much faster if your eyes are level (with the horizon).

That's the most important thing. I read somewhere that many motorcycle accidents of novice riders occur in long sweeping turns because the novice rider tilts their head into the turn... they get about halfway through it and lose sense of what is going on and crash.

The "classic" position was level shoulders, level head, but it does seem that people are adding new body positions. And I also agree that for our lighter weight ebikes (compared to the bigger motorcycles) you need to stick to the more "classic" positions because with less weight you can't do the weird contortions that are really using your body as a counterweight to the bike.

In all of this it just sort of reflects back on people that post here about how they can do something "magical" with their bikes if they ride them a different way. Body english can go a long way to correct poorly designed and poor handling machines, but it's better to have just built the bike correctly in the first place.

I've had a hard time trying to explain that there exists a "science" in all of the this... we get a lot of folks that simply want to make up their own ideas about how they want the geometry to work.

Hangdog98, it's great to have you posting here... you obviously have a strong background in this topic.

http://www.fireblades.org/forums/articles-writer-block/49503-join-me-freddie-spencer-school-level-two.html

"The number one fault we see in the videos from lots of people, is being "twisted" on the bike, i.e. butt off the seat, knee out, but head still over the windscreen. That's 30 pounds or so of mass that isn't leaning into the corner like it should, and it automatically leads to more lean angle on your tires. Bad. They tell us to watch the MotoGP guys, and how they get that head down beside the windscreen through the turns. They particularly mention Hayden and Pedrosa being very good at this."

...which is an interesting thought because this is in contrast to the "classic" position. As long as there is enough motorcycle weight to deal with the human body weight becomes like an "add on".

It makes no sense to get into these riding positions on a lightweight ebike because you have less weight to deal with. I have tried experimenting with going into a sweeper turn and "hanging off" like this and it changes the way the bike handles. Less lean makes it feel like you are hanging to the side of the bike rather than being part of the bike. If the front or rear tire starts to let go I feel like I'm more disconnected from the bike.

From my perspective I prefer the "classic" riding style as it's the most forgiving when the tires get loose... but I'm also not going 100+ mph on my ebike! :lol:

Even though people are dropping their heads lower, they still seem to be keeping the rule of more or less level shoulders. It's a very contorted position that's for sure.

 

[Hangdog98]

 

[Hangdog98]

 

[safe]

This seems like as he's preparing to exit the turn and he's starting to move his body up in anticipation of the bike following him upwards. (he's rolling on the throttle right here) Seems that the "excess body movement" follows a pattern where you overcommit on the way in and then you start the body out early in order to quicken the bikes getting straightened out.

The "classic" pose was to stay more in the center all the time, but to have a contorted position with the knee down.

The REALLY old school (like 1960's) had the rider sit perfectly upright (no knee dragging or twisting off) and the only thing that wasn't in line with the lean of the bike was the helmet which was set to level at all times.

--------------

I remember a trick I saw a long time ago where the faster rider going for a pass on the inside of a slower rider would slide his body towards the slower rider to make him see that he was there. It made no riding sense, but it made sense because it tended to make the slower rider give him more room. Once the faster rider slipped by he recentered himself on the bike and started a more normal turning sequence. The faster rider used his body to change the slower riders line. (a creative idea)

There's a lot more body english than just the basics I guess... :?

 

[Hangdog98]

That's just standard Doohan riding. His style was to move his body well forward to weight the front tire throughout the whole turn.

 

[vanilla ice]

How many degrees of lean angle is a few pounds of leg worth? It gets pretty far out there.