12 replies to this topic

[meb58]

If track width is increased over a front axle and not the rear roll resistance is increased over the front axle and we get more understeer, correct? And, as a result, the decrease in roll (rate) allow the front tires to perform more work, correct?

These two elements, more undertseer and more traction, seem to be at intuitive odds and I have great difficlity describing how we get more potential grip while increasing understeer - I am in general being razzed pretty good by some of my track buddies.

The confusion begins, I think, when they think of roll resistance in terms of how a swaybar works; too much front bar for example and the inside front wheel comes off the ground in a tight turn and we get understeer and front traction is reduced...

My fanny dyno tells me that I am correct; increasing front track increases initial turn-in undertseer but steady state grip increases...this has been my experience. I guess I am asking two questions...I am I correct? If so how do I explain the physics? I thought describing roll resistance and a wider track was easy enough...

[Fat Boy]

Your first sentence is written oddly, so it's difficult to know exactly what you're asking. However,...

By increasing the front track width, you've increased the average track of the car. This reduces total lateral load transfer. Reducing lateral load transfer will increase your potential lateral acceleration. That's 1/2 of it.

Now, as far as your statement of increasing understeer. Generally speaking, this is not the case with an increased front track width.

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Out of interest, are you relating karting to real cars? Karts often have opposite reactions to changes on a car. This is because the overall controlling factor on a kart is the live axle and mechanisms involved in getting the inside rear unloaded enough to let the kart turn. Karts are great for driver training, but the absolute devil for engineer training. Do yourself a favor and completely separate the two.

[gruntguru]

If track width is increased over a front axle and not the rear roll resistance is increased over the front axle and we get more understeer, correct? And, as a result, the decrease in roll (rate) allow the front tires to perform more work, correct?

Usually when you increase roll resistance at one end you will reduce grip at that end because you increase lateral load transfer at that end. However when you increase track, sure that increases roll resistance (which is a moment per degree roll) but you have also increased the length of the moment arm so the weight transfer does not increase in proportion to the increased roll resistance.

[meb58]

Fat Boy,

My first sentence wasn't confusing, it was horrible!!! Sorry. My brain is working faster than my hands...poor roll couple

Here is my fanny dyno evaluation. After increasing front track by 5mm per wheel - 10mm totoal track increase - I noticed a spike in initial turn-in understeer afterwhich steady state front end grip improved. Although the car continued to display understeer - more roll resistance - the threshold for front tire saturation was higher. Make sense?

I know this dialogue can go all over the place because there are so many other potential variables. When the uninitiated consider the term understeer we sometimes fail to evaluate the degree of understeer - a little or a lot - and at what threshold it eventually occurs. We can also get undertseer from a nose heavy front driver with a very low RC and very narrow track. We still get undertseer in this last example but the threshold is potentially differnent from my example...

gruntguru,

I am not sure if you agree...I sense you do in your reply...not sure i follow this, however. but you have also increased the length of the moment arm so the weight transfer does not increase in proportion to the increased roll resistance.

Edited by meb58, 05 April 2010 - 16:33.

[gruntguru]

gruntguru,
I am not sure if you agree...I sense you do in your reply...not sure i follow this, however. but you have also increased the length of the moment arm so the weight transfer does not increase in proportion to the increased roll resistance.

By "moment" I mean the overturning moment tending to roll the car. The longer arm resisting this moment is the wider track.

[meb58]

That is what I thought you meant.

Also, my observations about increased understeer hold true for a Mac Strut...increasing track on an SLA setup affect motion ratio and effectively reduces spring rate. So those folks have to get back to thier original wheel rate first, I suspect.

This is a quesion...

From the perspective of rotational displacement and rotational velocity, I gather that a wider track decreases displacement - theta.

Edited by meb58, 07 April 2010 - 20:14.

[lateralforce]

For the same CG height and roll centre, increasing track width will reduce lateral weight transfer and thus potentially, increasing the lateral force on both inner and outer wheels at a given turn radius. This equates to increasing overall axle grip, and on the front axle, it should mean reducing understeer ie you will need less hand wheel angle for a given turn at the same cornering speed.

[Greg Locock]

I know what you mean, but I haven't actually got any tire data that shows a DECREASE of lateral force for increasing Fz. It could happen, I suppose.

[Goran Malmberg]

If we increase the track width of one axle, the tangent distance of movement of the wheel of the axle in question will raise
compared to the wheel of the other axle, for each dgr of roll. This means that the axle that is given larger Tw will take a
larger part of the weight distribution, even as the total weight transfer has become less from greater average Tw.
Goran

[meb58]

Ah...that clears up at least one misconception of mine; weight distribution increases over the axle with a wider track... Does a car's static weight distribtuion matter? If we have a 50/50 car vs a 60/40 car and all else is equal will the increase in distribution over the front axle affect the rear in a different way? I know this is a slightly different question and may not really have much affect.

[Goran Malmberg]

Ah...that clears up at least one misconception of mine; weight distribution increases over the axle with a wider track... Does a car's static weight distribtuion matter? If we have a 50/50 car vs a 60/40 car and all else is equal will the increase in distribution over the front axle affect the rear in a different way? I know this is a slightly different question and may not really have much affect.

The static weight distribution does not matter, the cassis is supposed to be rock solid and roll the same amount of degree over its whole length, so the
roll force that occur is a product of CGH and Tw only.

Hope I can explain this following in some short words...

Since a greater Tw over one axle will raise the average Tw for the car, the total Wt will get somewhat less. This will result in that the axle that is given
greater Tw will have the same Wt as before, and the other axle will get all the reduction in Wt from the average greater Tw.
Regards
Goran

[meb58]

I poured over chapters 16-18 in Milliken this past weekend but there isn't really a study or discussion about track width changes per se.

Thank you for your reply.

Increasing front track width reduces rear weight transfer and vise versa? - just re-writing to make sure I understand. If I crudely add an increase in front roll resistance from greater front Tw to a reduction in rear weight transfer I expect more understeer. And this is my actual observation on track. I know there are so many other variables but when I experimented with track width I change nothing else.

[Goran Malmberg]

Increasing front track width reduces rear weight transfer and vise versa? - just re-writing to make sure I understand.

That is right. I made a counter for it, mail me at hemipanter@hemipanter.se and I should send a copy to you.
It has a number of input like CGH, rear and front weight etc. If anything raise a question, just mail me and ask.
Regards
Goran