* * * Bike Designs and Theories * * *

[Reid Welch]

As I intuit his words, the effect of ultra-low CG on a two-wheeler
will cause, when it breaks away in xtreme cornering, to break away
without feed-back warning to the driver.

Otherwise--no problem.
And it's that lack of warning/feel that makes, I suppose,
the theoretical ground-level CG bike "dicey".

Look at it this way: say the bulk of mass were far below axle height;
at the ground (just for an logical extreme example), then...
the slightest slip of tire grip will cause a sudden flip-out,
just as if a giant boot had side kicked the bike out from under you.

That's just my take.
Safe or other persons, please correct.
My notion may be entirely wrong.

________________
______

addendum: have not read through all the pages in this thread.
Looking at the past few, I come to this practical real-life conclusion
which fits my bike and situation:

-my ride is a cruiser bike. It sets me high from the ground.
I choose a seat-way-back postion (the leaned-back Thud Buster)
and have a reversed high-rise steel stem and lonnng cruiser bars
so...my bike's weight is grossly set to the rear.

70lbs bike, 150 lbs me, weight on the front tire? Is about thirty pounds.
And the bike handles great, it's nimble enough for turns and stable at high speed.

I prefer this because because because when I brake, it's the front brake that I most use, and the dynamic tranfer of mass from front to rear mandates my sitting far back when I brake hard at speed,

because otherwise the pitching forward of weight would tend to throw me,
if not throw the bike.

This is why I prefer and feel safe on a heavily rear-biased bike.
And I find the handling is just grand as it is,

and bear in mind I'm on soft, huge balloon slicks and so traction is never a problem on pavement; it is almost impossible to lock the front wheel in braking because the rubber patch is so grippy,
soft and wide.

Practice trumps pure theory -sometimes-
when it comes downto real-life needs.
My greatest need is to be able to haul to an emergency stop without locking a front wheel.

Putting the weight to the rear, with myself well-back behind long bars,
prevents rear wheel lifting and precludes the tendency body pitching over the bars too.

Motorcycles of 1910 were still bike-like and had long handlebars.
I like that look and I like the security and comfort of my choice of upright stance, etc.

Personal choices overide cold theory often with superior real-life results.

 

[Malcolm]

Thanks for your interpretation Reid – makes good sense of course. I'm still not totally convinced though. Sorry to hijack your question Lessss, but I'd like to work this out as it's been bugging me for a while. The following is a bit of a ramble, and if anyone can straighten out my thinking I'd be very grateful

First, I think it's important to distinguish between centre of gravity (CG) and moment of inertia.

Moment of inertia is a measure of how "spread out" the mass of a body is. The more spread out its mass (i.e. the greater its moment of inertia) the slower it will respond to rotational forces. (Like Safe says: think of an ice skater doing a spin – except we're mostly thinking of rotation about the horizontal axis here).

Centre of gravity is a point where you can imagine that the mass of a body is concentrated. It's the point through which all the mass acts.

Two bikes could have their CG in exactly the same place but have totally different moments of inertia. A bike with a large moment of inertia will react slower to rider input than a bike with a concentrated mass, or small moment of inertia. I would guess that, for the best handling, you would want a bike to have to smallest possible moment of inertia around the horizontal axis, so that it responds to your body movements rapidly. That means concentrating as much of the mass together as possible, including the rider.

Seems pretty straightforward so far. The problem for me comes when I think about how the bike moves in reality. It doesn't actually rotate around its centre of gravity, it rotates around the points of contact between its tyres and the ground. In this case you can imagine a bike as an inverted pendulum, with the height of the CG determining the length of the pendulum. The shorter a pendulum, the faster it swings – so the lower the CG, the faster a bike will respond to rider input and the faster it will handle.

Like you say Reid, when the CG gets below a certain point the rider may not be able to react fast enough to correct a shimmy, BUT if the CG is low enough any loss of control should always result in a low-side, rather than a more dangerous high-side (flip).

 

[safe]

In this case you can imagine a bike as an inverted pendulum, with the height of the CG determining the length of the pendulum. The shorter a pendulum, the faster it swings – so the lower the CG, the faster a bike will respond to rider input and the faster it will handle.

The shorter the pendulum the less force that the tires have to rotate the center of mass. So actually a lowering of mass below the "correct location" will make the bike MORE SLUGGISH. Inertia is always bad... the more spread out things are the more it takes to accellerate it and the harder it is to stop it.

Keep in mind the the RIDER is part of the equation!!!

Where is the majority of the mass of the RIDER?

Obviously the mass of the rider is rather high and so if you placed all the bike mass hugging the ground you would be "fighting" the weight on the other end of the bike.

The "preferred" location for the mass is between the legs and aligned in the center of the bike in the middle of the mass of the rider.

Try to not think about the tires as anything more than an "input mechanism" that works like a lever on the mass of the bike/rider. Once you "comprehend" that concept (and get away from that terrible idea of rotation about the tire contact) it all of a sudden makes sense.

There is a "right" location for the mass.

Too high is bad.

Too low is bad.

Too spread out is bad.

The "right spot" is in the middle...

(one cannot "choose" alternatives, but can really only be "wrong")

 

[Malcolm]

If, like Safe suggests, you should avoid bringing the CG so low that the rider doesn't have time to react to a loss of grip, then that specific CG height must depend on a lot of factors, particularly the rider's skill and reaction times, and the moment of inertia of the bike. That's why I feel it's arbitrary to say that the CG shouldn't be below, say, axle height.

I don't have a lot of motorcyling experience myself, and maybe you're intuitively right about this Safe, but like you, I have to get the picture clear in my head before I can accept what others say

Lessss: The lower the CG, the more responsive your bike should be in a turn, whatever the speed (I think :wink

 

[Malcolm]

The "preferred" location for the mass is between the legs and aligned in the center of the bike in the middle of the mass of the rider.

That's where I always keep my "mass"

But seriously Safe, you seem to have dismissed everything I said without a reasoned argument. I understand that the mass of the rider is one of the biggest variables, which is why I'm interested in "feet forward" motorcycles, such as this http://negatendo.net/kmc/en_us_feet_forward.htm

 

[Matt Gruber]

i'd like to see a real life example of TOO LOW.

 

[Reid Welch]

Right, Matt. I don't think it could be done without looking like a Michelin Man with all his big sections fallen to his ankles. It'd be weird.
Still, with lots of SLA on outriggers poised one inch off the ground....
...complicating factors of case friction to pavement, spin-outs and scuffs,
acid and lead streaks on the roadway. Ah well, this is silly
and so am I.

 

[fechter]

i'd like to see a real life example of TOO LOW.

How about a Harley Davidson?

I know from my motorcycle experience that my old DR-400 dirt bike, which had an extremely high center of gravity, handled extremely well in tight turns. Heck, I'm 6' tall and I could barely touch my feet on the ground with it stopped. It did well in low speed manuvering and I never noticed any instability at high speeds (up to 90mph).

It just seems like if the CG was too low it would make the steering sluggish and possibly put a lot of load on the tires when making a sudden direction change (making it more prone to skidding). It certainly makes a bike easier to park with a kickstand.

 

[Malcolm]

But if you take the idea to the extreme and imagine carrying a 100kg mass on the end of a six-foot pole supported vertically somehow, which way up would you carry it on a bike?

 

[Matt Gruber]

Malcolm
Thanks for that link!
i'm building a FEET FORWARD scooter right now! Very forward, 6" IN FRONT of the front of the front tire! I don't have any expectations for great handling, however.
It is very low. 2 1/4" off the road. my eyes ~30" or less off road.

 

[Malcolm]

Personal choices overide cold theory often with superior real-life results

Wise words indeedyReid.

Matt: Glad to help. Photos please!

 

[fechter]

Malcolm
Thanks for that link!
i'm building a FEET FORWARD scooter right now! Very forward, 6" IN FRONT of the front of the front tire! I don't have any expectations for great handling, however.
It is very low. 2 1/4" off the road. my eyes ~30" or less off road.

That will be interesting.
It should be easy to do stoppies.
With your butt dragging just off the ground, the moment of inertia will be very low, so handling should be OK. It will be like sitting on a skateboard.

 

[Matt Gruber]

it reminds me of a go cart. Like for a kid, so u try it but, feet stick out in front. so i have 1x6" cantalevered in front to rest feet on.

 

[TylerDurden]

Tony Foale on FFs, snipped from:

http://www.tonyfoale.com/Articles/FF/FF.htm


Compared to conventional bikes the resiting of the rider into a telly watching position has three main physical effects on the machine.

Firstly, the frontal area will almost certainly be reduced and even without a fairing the drag coeffient ( Cd. ) may also be lowered due to an improved aerodynamic shape. Thus giving a double bonus in terms of drag, especially if advantage is taken of the improved fairing possibilities. In contrast the rider on a normal bike acts a bit like a horizontal parachute.

Secondly, the wheelbase is often longer than the accepted norms.

Thirdly, the combined C of G. of rider and machine is usually lowered. Mainly by lowering the rider but often also by lowering the weight of some dirty bits.

Let's see how these changes affect various aspects of performance.

The improved aerodynamics will obviously reduce power requirements to travel at a given speed and will reduce fuel consumption accordingly. The low long nature of these machines allow for the design of vastly more efficient body work to fully exploit the potential, as has been amply demonstrated by the NSUs. For directional stability it is desirable to have the sideways centre of pressure behind the C of G. and fortunately the FF. layout usually lends itself to this more than the latest race replica. The lower height and reduced drag also mean that aerodynamic lift over the front wheel is reduced without the need for drag increasing down force features. The sideways centre of pressure is also lower and this reduces the toppling over moment from side winds.

Longer wheelbase, this increases both the pitch and yaw moments of inertia. Putting it another way, it needs more effort to disturb it's attitude. This leads to a more stable and comfortable machine, and one that is less reluctant to suddenly slide. Disadvantages include slower handling and a greater turning circle. In most cases a longer bike is a heavier one, simply because more material is needed to connect the wheels together.

The lower centre of gravity is mainly beneficial but does have some disadvantages. Advantages include quicker and easier handling, and less weight transfer under braking. This can lead to improved braking because of the better balanced loads on the tyres. Let's look at some numbers to get a feel for the effects. ---- Imagine a hypothetical machine, with a 60" wheelbase and a 30" C of G height, and a 50/50 weight bias under static conditions. Then under the action of severe braking, say 1G., all the load on the rear wheel will be transferred to the front, and so the front tyre will be required to bear the total stopping forces. A machine under these circumstances will also be directionally unstable and only the skill of the rider can prevent the inevitable. Now consider a long, low rear engined FF. with a 85" wheelbase and an 18" C of G. height. Then under the same degree of braking only 42% of the previous weight transfer will take place. Resulting in improved braking and control. A beneficial side effect is that the tendency to dive is reduced.

Initiating a turn is usually a combination of some bodily weight movement and a bit of counter steering. The more secure riding position of an FF. largely eliminates the possibility of shifting the rider's weight and so all of the control function must come through steering input, but I don't see this as a problem, for normal riding, just a slightly different technique. In any case a lower C of G. means that less effort is needed anyway to provide the roll or leaning movements (i.e. we have a lower roll moment-of-inertia). Thus we get a quicker handling machine, which may well more than compensate for the opposite effect from the longer wheelbase. It is in side winds that a disadvantage of a low C of G. may be felt. Under the action of a steady breeze the machine must lean into the wind more, in order to balance the wind force. Of course with the FF. layout the lower mounted, body side area may well compensate for this effect. But the side area may well be greater anyway due to increased length and so negate this benefit. With gusty winds I expect that the FF. will be at a disadvantage. Any lowering of the sideways centre of pressure will probably be approximately proportional to the lowering of the C of G., whereas the roll moment-of-inertia will vary approximately as the square of the C of G. height. In other words the bike's resistance to the wind gusts will be decreased more than the disturbing effects from them. This means that the machine will experience greater roll angle changes, and as we have seen in earlier articles, roll movements cause steering movements through gyroscopic effects, thus aggravating the situation. Under some riding conditions I see the low rider C of G. giving problems of control. Apart from the wind case just described, I think a low machine C of G. is always desirable, but there are conditions where a high rider C of G. is very useful. Trials riders don't stand on their footrests just for the view. They do it because with a high body C of G. they can exert a greater influence over their steed, with a bit of "body English", as the Yanks call it. With the FF. not only is the rider's C of G. lower but he is largely prevented from moving about anyway.

 

[Lessss]

long wheelbase: Disadvantages include slower handling and a greater turning circle.
Low cg: Advantages include quicker and easier handling

Hmm what I find with my scooter like bicycle is a greater turning circle unless I slow down. So my real life experience is kinda opposite of what this says.

 

[Malcolm]

TD: Thanks for digging out that article!

Lessss: What size wheels does your bike have? You need to compare like with like, as steering geometry and wheel size can also affect your turning circle.

 

[Lessss]

2.5 x 16

 

[Reid Welch]

3-In-1 replies:

-Thanks, yes. You're walcomeMalcolm :wink:

-Lesss, your sig's now more.
You might prefer "Reid" instead of R.W. as you like or not.
Pop those greentwit lint suckers every time.

-Tyler, I enjoyed the Limey's article.
Your italics always interest.
I like to stand on the pedals through turns and even in straight lines, 7 feet tall (it seems, it's not quite). And I never knew why before.
I just thought it was the Titanic effect:
recall how protagonist Jack stood off at the bowsprit, shouted
"I'm the King of the World"
--standing high on pedals
atop (new name for stufu bike: 'Radiation Danger', or Dadiation Ranger')
does that for me
without need for deep freezing water.

 

[Malcolm]

Apart from making you king of your domain, standing on the pedals has the additional benefit of adding some very effective suspension for the heaviest single component, thus significantly improving the handling over rough terrain.

 

[safe]

In any case a lower C of G. means that less effort is needed anyway to provide the roll or leaning movements (i.e. we have a lower roll moment-of-inertia). Thus we get a quicker handling machine, which may well more than compensate for the opposite effect from the longer wheelbase.

This person is getting things confused again...

While it's true that if you focus all the weight (body and bike) in a long slender tubular shape near the ground this DOES lower the moment of inertia, but it still positions the entire mass close to the ground so that the "lever" effect of the tires is very short. Something like this will provide no feedback to the rider that the tires are beginning to drift outward in a slide because it will happen at a rate that is too fast for the rider to even react to it. What he's calling "quick handling" should be taken as sort of a joke. :wink: (that he might not be aware of) One wants a SLOW HANDLING bike when it comes to loss of traction and a QUICK HANDLING bike when it comes to turning radius. When you place everything low like that you end up with the worst of everything. (and the long wheelbase only further disguises the problems)

It boggles my mind why people don't see this as being obviously a poor handling solution.

If you want to "go low" then "go trike" and forget about two wheel handling issues completely. You cannot get a good handling bike when you are that low... your reaction times are in direct proportion to the height of the center of mass above the ground. Anyone who has ridden a motocross motorcycle knows that being "high up" is not all that bad... (tight turns are the biggest problem because the steering geometry gets "funky" when the turn is too tight)

 

[TylerDurden]

This person is getting things confused again...

Tony Foale B.Tech, M.Eng.Sc, C.Dip.A.F.

Educated in Australia, Tony graduated with a degree in electrical engineering, followed by a M.Eng.Sc. in nuclear engineering. He worked at a research establishment, creating mathematical models and subsequent computer programmes for performance analysis of various mechanical and aerodynamic systems. Some of this period was spent as leader of a team developing mathematical techniques for the prediction of human response in situations not unlike those applicable to motorcycle and car driving tasks.

This was then followed by a period as a project engineer with a car manufacturer. From there he moved to a nuclear research lab. Providing mechanical and electronic design services for research departments. At the same establishment he later moved into a research team as group leader developing original finite element solutions for failure prediction in some unusual machinery.

Moving back to England in 1971 (driven by his passion for racing motorcycles and the desire to compete in the Isle of Man). He followed a period as factory manager with a F1 car racing team, with 18 months working as a structural engineer in the Ocean Engineering field, responsible for developing computerized motion prediction and structural analysis models for floating oil drilling platforms subject to wind and waves. During this employment he was admitted as a member to the Institute of Marine Engineers with Chartered Engineer status.

From his mid teens Tony has been a passionate motorcyclist, both on the road and in competition. He has ridden in road-racing, moto-X, enduros and trials events. With a natural instinct for building a better mousetrap he constructed (often with primitive facilities) several of his own machines, both engines and chassis. With a scientist’s curiosity, during the 1960s. he used the facilities and experience of his work to develop mathematical models to provide him with an understanding of motorcycle behavior in excess of that to be found in the literature of the time.

In 1973 he succumbed to his passion completely and started a business making complete chassis etc. for racing machines. Many of these were highly successful, winning several championships in various countries. The range of machines built included championship winning sidecars as well. In the early 1980s. Tony began a project to improve upon the design of front suspension systems for motorcycles which culminated in the construction of a small number (about 20 in all) of road machines featuring these ideas, named QL (Quantum Leap) and the Q2. During the design phase for these machines he embarked on a series of experiments into the fundamentals of steering geometry, the results of which verified his long held beliefs that established practice had many failings. These experiments are detailed in his 1984 book “Motorcycle Handling & Chassis Designâ€￾ , the 2002 book "Motorcycle Handling and Chassis Design" and in some of his numerous magazine articles. Since that time many standard production models and particularly racing machines have gradually moved toward the use of steering geometries as suggested by Mr. Foale. He has also consulted on various motorcycle projects including successfully advising a police force in regard to measures needed to cure stability problems with a large fleet of motorcycles.

Unable to further tolerate the English climate he moved to Spain in 1987 where he acts as a consultant on various vehicular projects, specializing in chassis and suspension design, with particular interests in the development of computer simulation models predicting motorcycle behaviour. He also writes regular technical articles and is technical editor of the US based "Motorcycle Consumer News". Also responsible for the motorcycle content of a masters degree programme in motorsport engineering given by Modragon University at Epsilon Euskadi in Spain.

Tony is also available to give seminars on motorcycle dynamics and similar topics, either to privately or club arranged events, also to race teams and design departments. Please send an email for details.

After a 30 year layoff Foale returned to racing in 2006, competing in prestige classic racing events in the USA at Daytona, mid Ohio and the Barber raceway.


Ummm.... who's confused?

:lol:

 

[Matt Gruber]

Tony does have the confidence to use his real name.
.
Safe
do u have experience with a scooter, 32" wheelbase or so? 10" tires?(4" wheels)

 

[Geebee]

Matt do a search for lowracer if you want to see the foot forward style of bike you are discussing.
Safe, over on 'Bent rider online forum this has been discussed to death (well not quite) a highracer and a low racer represent the idea you are trying to push, in real life everyone who has ridden both say that there is no real benefit either way although the theory says there should be (theory skips one aspect which we have discussed before).
The major difference seems to be that most are happier to fall 6" as opposed to 26" so the lowracer is probally less likely to break something.

 

[Matt Gruber]

Geebee
i almost gave up! 90,000 w/big wheels and pedals.
TA DA:
VERY COOL http://rohorn.blogspot.com/
advanced RACING(i'm limited to 30 degree tilt) versions of my project. the 1st 2 are the only ones vaguely similar

 

[safe]

...specializing in chassis and suspension design, with particular interests in the development of computer simulation models predicting motorcycle behaviour. He also writes regular technical articles and is technical editor of the US based "Motorcycle Consumer News". Also responsible for the motorcycle content of a masters degree programme in motorsport engineering given by Modragon University at Epsilon Euskadi in Spain.

Wow, those are impressive credentials...

I was assuming he was another "nutcase" with a wacky bike design idea. Okay, so maybe there might be some theory that makes the "pavement hugger" configuration work, but is it a good thing to be riding so close to the ground for traffic considerations?

To be so low to the ground is to be essentially "invisible" to traffic and someone in a big SUV could run you over and not even know it. Even if there were technical reasons for the "pavement hugger" concept to work do you REALLY want to be "invisible"? (I scoff at the idea because it just doesn't pass the "that doesn't seem right" test)

The "pavement hugger" concept has no future as long as the world has big cars... (it's bad enough making it through traffic when you are visible, but small)

It's like the difference in the "Gravity Games" area between the "Gravity Bike" (which I used to ride) which is visible verses the "Luge" skateboarders that are invisible. It doesn't take a rocket scientist (or nuclear physicist) to realize that the "Gravity Bike" is safer in traffic around cars than the "Luge" skateboarders are. (I used to pass cars on downhills) The "Luge" skateboarders might have technical edges over the "Gravity Bikes", but they are clearly specialty racers. Strap on an electric motor to a "Gravity Bike" and all of a sudden you have a practical means of transportation. Strap on a motor to a "pavement hugger" and you have an invisible "death trap".

This:

Or this: