Ride height and track width?

[Birel99]

Instead of starting a new thread, I will post here:

1. Lets say that we have 55% static weight on the rear axle. Why does this induce oversteer? But when a driver applies a small amount of throttle (weight shift towards the back) it induces understeer?

2. Why does having more weight on the rear axle improve rear tyre wear?

Regards,

[Jersey Tom]

55% rear static weight does not = oversteer. Moving static distribution rearward will shift the balance to less understeer / more oversteer however. (There is a big difference).

As you load up the rear tires with mass, they do not have as much tractive capacity per unit mass. Meanwhile, unloading mass from the fronts increases tractive capacity per unit mass. Fronts work better, rears don't work as well -> shift toward oversteer.

Getting into the throttle, you transfer load to the rear axle while the mass stays the same. Same effect as taking aero load off the front and increasing aero load at the rear. Rears now have excess grip compared to the front -> more toward understeer.

...up until the point you break the rear tires loose.

Same way load transfer to the front under braking will free a car up, unless you have heavy front brake bias or until you lock all the tires.

[alexbarwell]

Back on the bike parallel, if the transference of CofG in a bend (centrifugal force as vector sum with 'static' CofG) and assuming approaching the limit of grip, a low-side can develop if the resultant CofG is inboard of the contact patch (inside of bend) and tend towards high-side if the CofG is outboard of the contact patch (outside of bend). The exception being when it slips then bites again...The extra pair of wheels a car has prevents the low-side, but we've seen clips of a car high-siding. A super-low CofG might result in a shear state developing, but a higher CofG with plyant suspension could move the mass more directly over the wheel, more likely to flip, but if grip is poor already it may be a necessary compromise. With increased track (the car, not the black stuff!) CofG is more readily contained, but to support theoretically chronic outriggers requires more strength (weight penalty) or presents a weakness, but also extra car/bodywork that can present more front and surface area to make for more aero to contend with. Also short wheelbase is nimble/twitchy while long can be awkward/stable.
I'm sure someone has a correction to make to this lot.

[riff_raff]

weight transfer to the rear (squat) due to the torque reaction about the drive axle produced by increasing engine power (ie. standing on the gas), may or may not occur depending upon the suspension geometry. It is possible to produce a rear double-wishbone suspension geometry that is either pro-squat or anti-squat under acceleration.

Static weight distribution is relatively meaningless with regards to handling. Handling is more directly a result of the instantaneous forces applied to the tire contact areas due to aerodynamic forces and chassis mass inertias.

The main benefit of a wide track is that it allows the maximum undisturbed flow area for the underbody. Thus maximizing the available ground effect.

[Jersey Tom]

weight transfer to the rear (squat) due to the torque reaction about the drive axle produced by increasing engine power (ie. standing on the gas), may or may not occur depending upon the suspension geometry. It is possible to produce a rear double-wishbone suspension geometry that is either pro-squat or anti-squat under acceleration.

Weight transfer != squat.

Weight transfer to rear on accel will happen, unavoidably.

Static weight distribution is relatively meaningless with regards to handling. Handling is more directly a result of the instantaneous forces applied to the tire contact areas due to aerodynamic forces and chassis mass inertias.

While handling is a direct result of instantaneous force, moving static mass distribution fore or aft will shift your entire "map" of balance. Dynamic loads are a function of the static loads. I'd say that's pretty meaningful

[Belatti]

Static weight distribution is relatively meaningless with regards to handling. Handling is more directly a result of the instantaneous forces applied to the tire contact areas due to aerodynamic forces and chassis mass inertias.

The main benefit of a wide track is that it allows the maximum undisturbed flow area for the underbody. Thus maximizing the available ground effect.

riff raff, I do not agree with you on that (maybe this is the first time) However you used the word "relatively" so I dont know what do you mean by that. A very "fine" driver from arround here (the one who beated R.Schumacher, A.Wurz, J.Trulli, P.dela Rosa, and more) can tell you that there is a static weight distribution difference of 10Kg and be 2Kg away from truth while driving a 1300Kg touring car. That disturbs his driving, so it doesnt seem "relative" at all.

Also, the wide track main benefit is not aerodynamic from my point of view.

[riff_raff]

Belatti,

The only reason static mass distribution is even considered is because that is how the car is initially set-up. But once on the track, the instantaneous forces due to mass polar moments and aero C of P predominate.

With a car that employs aero ground effects to increase grip, like an F1 car, pitch is very important. Since acceleration "squat" or braking "dive" can instantaneously upset the traction balance front-to-rear. Less aero downforce at the front would cause understeer, and less aero downforce at the rear would produce oversteer.

A few years back, the third spring was added to many open wheel car suspensions. The third spring allowed the chassis to be stiff in pitch, but soft in roll. The high pitch rate minimized changes in underbody downforce under acceleration or braking. The soft roll rate helped to keep both left and right tires on the track during cornering.

I suppose another benefit of a wide track dimension is that it allows long suspension arms. This helps to minimize roll center changes during suspension travel.

Regards,
Terry

[Carlos]

riff raff's posts reminding me of the problems of the early '80'2 F1 cars called aerodynamic porpoising. Aero has been a steep learning curve over the years. To me that's called development. To F1 designers it may have been a nightmare.

" It was discovered the car was generating too much downforce for the driver to cope with. The problem was twofold: firstly, the ground effect's low pressure area under the car was moving around with the car's centre of gravity. The phenomenon was known as porpoising, as the car appeared to be lifting and squatting at different speeds, causing it to lurch violently through corners. Secondly, the slightest difference in track ride height including off cambered corners, kerbs etc affected the undercar pressure hugely. The team experimented by fitting the car with wings, but this had little effect on the way the car behaved. Andretti persevered with the car, but his new team mate Carlos Reutemann refused to drive it and stayed with the Lotus 79."

It's a good article to read right though.
http://dic.academic.ru/dic.nsf/enwiki/2948674

[riff_raff]

Carlos,

If you recall, a few years ago the teams moved the exhaust outlets from just above the diffuser exit, to a point on top of the rear deck. The principle of locating the exhaust exits just above the diffuser exit was that the high velocity exhaust gas flow helped to make the underbody diffuser more effective. It worked pretty well, but as the cars became more refined and sensitive to changes in the aero C of P, the configuration was abandoned. The problem was that as the driver got on and off of the throttle, the changes in exhaust gas flow was actually enough to upset the aero balance of the car during braking and acceleration.

Regards,
Terry

[Belatti]

The only reason static mass distribution is even considered is because that is how the car is initially set-up. But once on the track, the instantaneous forces due to mass polar moments and aero C of P predominate.

The instantaneous forces due to mass polar moments you mention have a lot to do with static mass distribution. If not, why are F1 teams "playing" with ballast locations all the time nowadays?

I suppose another benefit of a wide track dimension is that it allows long suspension arms. This helps to minimize roll center changes during suspension travel.

Indeed. And dont forget weight transfer, wich is the main benefit.

[Gerhardsa]

Sounds like BS, to be honest.. and that guy is mixing up terminology. Not exactly a reputable source (not that there really are any).

Yes, agreed. I swear it works, but the reason? Don't know. What I know is that everybody and his dog rides with rear axle as high as possible (it has only two positions, btw) and raises the front axle in the rain (this can be graduated, unlike the rear). I do and I really like driving in the rain. I think I have an advantage because of my height. You lose a little bit of braking, but, also in the rain, you want to stomp on the brakes, another thing in karts that goes against the full-racing-car theory, btw.

For example, suppose you had a car that was SUPER wide. As wide as the race track! In theory you'd have real great grip on a skidpad. However, the width of the thing would mean your racing line would be stuck going around the center of the track. With a narrow track width, you can take a much broader arc through the corner.

OMG. I'm getting dumber by the minute. I swear I thought you both were talking about the racetrack width... damn english.

How would you define "grip"?Is it the friction factor*weight?Or just the situation,when car has a feel of more grip?Or sth. else?

Grip, I think, is the amount of lateral force a tyre can develop before sliding.

I know I am Beating and old dog ( old forum topic) with a stick here, but in karting, if your rear axle sits at its highest point (As you suggest here), it actually means the chassis is at the lowest it can be.

Same with the front. Raising the front stubs, means you are putting the spaces between and underneath the stub and C joint, lowering the front of the kart.

I wouldn't really do that to the front especially in the wet because it means you have less leverage to turn the inside front "underneath" the kart to create the jacking effect at the rear.

Another point about lower rear ride height. It also means you are moving the CoG a bit rearwards, placing more weight on the rear of the kart percentage wise. This, to me (logically) means that you now have a bigger percentage of weight at the rear (At your disposal) and will therefore be able to transfer more weight to the outside rear whilst cornering, creating even more side bite on the outside rear tire (if you use your body weight correctly during cornering to lift the inside rear).
just my 2 cents

[olefud]

As a general rule equally loading all four tires generates the maximum traction. Thus the end that increases the roll couple resistance, i.e. stiffer antiroll bar, generally generates less traction if originally properly set up. However, in some cases, i.e. rain, the more heavily-loaded tire will better expel water under the tire and increase overall traction. Often unpaved racing surfaces also do better with higher tire loadings.

[crbassassin]

Soft springs mean low tire load variation which generally gives more grip.

I might add that stiffer springs give a high peak load on the tires

[crbassassin]

I suppose another benefit of a wide track dimension is that it allows long suspension arms. This helps to minimize roll center changes during suspension travel.

Regards,
Terry

The further the tires are from the body, the less its wake can interact with the body work.

[JT A.]

Not always true. Haven´t you driven a kart in the wet?
Even in F1 ride hieght is increased for wet weather.

Proof? Source? Other than maybe if they're running softer springs in general and just to offset for how much aero is gonna compress everything.

Nothing about it being wet (that I can think of) makes an axle want more load transfer for better grip. There may be other reasons, but they're going to want to keep it as low as they can get away with.

So, you havent driven a kart in the wet!

Proof, source, verse and chapter, you dear and agnostic Tom (page 15):

CRG Kart Tuning Manual

Simple: "by raising the front ride height, you effectively raise the center of gravity in the kart and will be increasing the side bite on the tire, making it grip harder. If you go too high however, the raised CG will make the kart "bicycle" or pick up its inside wheels."

I don't know why, but it happens. I imagine it happens because you have a large lever and the weight transfer to the outside wheels is larger. If anyone can put this in an equation, I'll applaud.

Actually, I'm tall for karts at 1.83 m (6 feet). Just by stretching my spine I can feel the extra grip (extra lever you get, read page 6). On the other hand, by crouching (or, better, by lowering the front ride height), you will take side bite away from the tires and allow them to slide instead of grip. This will decrease the grip in the front-end.

I assume that the extremely low weight of karts has something to do with this "effect".

Finally, I was totally adrift with "narrower track allows you to take a broader line, which means for a given grip level you can carry more speed through". Could you ellaborate? I know JerseyTom knows tons about that stuff (I have no idea of the best way to "slalom").

I guarantee you that the extra grip you feel when you sit up higher in your seat is literally "all in your head". When you raise yourself up, your head (where you sense the lateral acceleration) is farther away from the CG of the kart. Since your head is effectively on a longer pendulum, it will feel like higher lateral acceleration. If you put a real accelerometer exactly at the CG height of the car, it would read the same acceleration.

Another issue - drivers, even a lot of good professional drivers, are notorious for "swearing" they feel things that just flat out aren't real. In higher levels of motorsport you have data aquisition to filter out what the driver feels and what is reality. In karting, you don't. Hence why BS and myths like this persist.