For my old buddies from the "other" forum

[John Learmonth]

quote:

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Originally posted by Steve F:
I'm with Larry on this one, karts dont have the physical components to even determine where the front and rear roll centers would be. Great discussion on car-theory though.

Steve Frank

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Steve, I think you almost got it right, IMO you should have said; ‘karts don’t have the physical components to EASILY determine where the front and rear roll centers would be’!!

Merely because they’re hard to find doesn’t mean they don’t exist! The physical components to create roll centres are all there (ie a means to allow the main masses of the kart to move independently of the contact patches, in other words a suspension), we just don’t know how to measure them properly. This is simply because the suspension articulation points are not fixed as they are on a car (or at least a car with zero compliance suspension pivots).

Assuming zero compliance pivots (ie spherical rod ends or metal to metal bearings / bushes of some description) a car’s individual suspension pivots are located at singular finite points, but the ‘pivots’ of a kart suspension are spread over indeterminate flexing sections of the chassis. This doesn’t mean that the movement (relative to the CG) of the outer ends of the kart suspensions won’t be in arcs that have actual instant centres, even if these instant centres will tend to move around somewhat.

Even on cars its not so much the actual finite points of articulation that directly determine roll centre geometry, but the location of the instant centres of the virtual swing arms created by the finite geometry that directly do so. The effective VSA instant centres on a kart will be very hard to define / locate due to the complex nature of the forces fed into the chassis rails that constitute the kart suspension, and the complex manner in which these rails are bound to behave in reaction to those forces.

With a good handling car, the chassis rigidity and fixed nature of the suspension pivots merely adds relative stability to the location of the VSA instant centres and roll centres. This is the reason that racing cars don’t use rubber bushes in the suspension, and one reason why a cars with flexi chassis tend to not handle so well as cars with a rigid chassis, ie soft bushes and/or flexi chassis introduce instability into the location of the suspension pivots (relative to the CG), consequently the VSA instant centres are less stable (relative to the CG), and consequently the roll centre locations become less stable. A flexi chassis will also work against tuning the spring and damper rates, but this problem is in addition to the effect on the stability of the suspension geometry etc.

On a car having soft suspension bushes and/or a flexible chassis, the actual location of the roll centres will also be very difficult (if not impossible) to accurately determine, just as they will be with a kart. This doesn’t mean that roll centres don’t exist on such a car!

[Jaye Bass]

I would propose that, practically speaking, if you can't measure it and hence can't use the concept for tuning...then it doesn't exist.

[Paul Kish]

Continuing on....

I think on a sprung car you have a chassis that rolls. And you can figure out exactly how that sprung weight chassis is going to roll within the unsprung wheels axles etc..

Now here's a question. Once the sprung chassis hits the limit of its ability to roll doesn't the total car when it actually "DOES" start rolling immediatly have a change in its roll center?????

If I'm thinking correctly about that then that's how a kart works. It's normal condition is that it combines both the actions involved with just the sprung portion of its weight rolling and the roll of the total vehicle once the roll limits of the sprung chassis have been reached. The controlled combination of sprung chassis roll and total vehicle roll is what is necessary to operate a gokart chassis. That is why kart setup is so difficult to do and especially to understand.

[Todd Bellew]

John,
I beleive Ferrari has made the rigid car that you speak of. I thought I remember ferrari F1 cars from several years ago using basically rigid suspension. No springs or shocks, it was basically hard plastic bushings. And even the cars with suspension hardly move.

I would bet if you took the amount of front wheel deflection or travel of a kart based on size and weight it would be close to proportional to the ratio of defelection for an F1 car.

Just a thought,
Todd

[John Learmonth]

quote:

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Originally posted by Todd Bellew:
John,
I beleive Ferrari has made the rigid car that you speak of. I thought I remember ferrari F1 cars from several years ago using basically rigid suspension. No springs or shocks, it was basically hard plastic bushings. And even the cars with suspension hardly move.

I would bet if you took the amount of front wheel deflection or travel of a kart based on size and weight it would be close to proportional to the ratio of defelection for an F1 car.

Just a thought,
Todd

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Todd,
I don’t know about Ferrari, but years ago I remember Alan Jones saying that he drove a Williams in practice with a ‘solid’ suspension. He reported it as very difficult to drive, very uncomfortable, and slower.

This was in the days of full ground effects with very very high spring rates giving almost no suspension movement, but even that slight movement must be important. The ‘spring rate’ of the tyres must play an important role as well. If we ignore the ‘spring’ effect of the tyres, with no suspension the entirety of the car must become unsprung weight.

I think a kart has quite a bit more ‘suspension’ movement than an F1 car at the front (relative and actual), but you may be right about relative rear rates.

[Jack Burroughs]

Hey oscar when did you ever see a race car with active suspension lift the LR tire? only when it was "bicycling" or about to flip. IMHO a kart has inactive suspension, no shocks, spring only in tire sidewall flex chassis tubing design and size and axle flex. The power in most race cars is applied by an engine that is in one plane bolted to chassis and changes direction to apply to racing surface. Active suspension allows a tourque reaction that almost allways lifts the LF wheel under acceleration. (unless reverse rotation of motor) Kart has power application in paralell planes. As to your orig.question not as much comparison as contrast. Paul Kish I'm surprised you didn't point this out, your mostly on top of these things. Jack Burroughs.