Weight Transfer on an Impreza

[Keith Watson]

I hope you don't mind answering this general question. I'm trying to understand weight transfer, more specifically lateral weight transfer. I've been trying to gain a better understanding of roll bars after Arnie's thread regarding roll centers on the Impreza.

I've been readng Tune To Win by Carroll Smith, Performance Handling by Don Alexander, and How To Make Your Car Handle by Fred Puhn.

All three say that the total amount of lateral weight transfer depends on lateral g, vehicle weight, cg height, and track width. Lots of people latch onto this as the final answer.

But two of the books seem to also say it is more complicated than this and that increasing roll resistance, especially at both ends of the car, increases lateral weight transfer.

I'm having a hard time correlating the two statements. Can you shed some light on it for me?

[Wojtek]

Hi Keith,

This is a very intersting topic, be it a complex one at that.

The information you have found in the books is correct. Total weight transfer is only dependant on lateran acceleration, sprung weight, CofG height and track width. I think what you are getting confused about is that this IS the TOTAL weight transfer, not individual wheel weight transfer or inside ver outside wheels, and front ver rear. To do this, other factors such as effective spring, shock and sway bar roll resistance needs to be added to the equation. Increasing roll resistance does increase total weight transfer, but the trick in suspension tuning is firstly increasing given roll resistance up to the maximum tyre grip, as well as adjusting the front to rear roll resistance in order to maximise the total grip available. Sway bars are the simplest and most effective tool for this, for both road and race cars.

Regards,
Wojtek.

[Keith Watson]

Hi Wojtek,

You said the exact phrases which confuses me. Now I get to ask what you mean. :D

Total weight transfer is only dependant on...Those parameters make sense to me. I can see it's just the simple physics involved.

Increasing roll resistance does increase total weight transfer...It's that word "total" being used in this contect which throws me. Is it just for one end of the car?

For example, if you have a car with low overall roll resistance and everything stays the same except the overall roll resistance is increased, is there more total weight transfer?

The rest about making tradeoffs at each end of the car to maximum overall grip I think I understand pretty well, at least the concept. It is quite an interesting topic -- wheel rates, camber gain, weight transfer, jacking, etc.

Thanks,
Keith

[Wojtek]

Hi Keith,
Weight transfer is dependant on roll couple (which is a function of roll centre, centre of gravity, wheel track, weight) and roll resistance (springs and sway bars). Increasing roll resistance using sway bars increases weight transfer, however this is off-set by the improved tyre contact patch which results in increased grip, and reduced the time vehicle takes a "set" into a corner, which increases the period of maximum lateral acceleration. Overall result is a increased lateral accelaration, and therefore speed, especially for street cars. Another option for increasing roll resistance is by using stiffer springs. The down side to this is however for the same roll resistance (increase) using springs only will generally require excessively high spring rates inappropriate for street use. The trick is to strike a good balance / compromise between ride and handling.

Wojtek.

[Keith Watson]

Hi Wojtek,

Thank you for the great explanation. I understand everything you said. :) My conceptual model was just missing something.

Regards,
Keith

[Wojtek]

I'm happy to hear that Keith. I'll let you do some more reading, and I'm sure we'll have more things to talk about.

Regards,
Wojtek.

[AndyRoo]

First off, thank you Wojtek and all of Whiteline, I am learning a lot.

Originally posted by Wojtek
Hi Keith,
Weight transfer is dependant on roll couple (which is a function of roll centre, centre of gravity, wheel track, weight) and roll resistance (springs and sway bars). Increasing roll resistance using sway bars increases weight transfer, however this is off-set by the improved tyre contact patch which results in increased grip, and reduced the time vehicle takes a "set" into a corner, which increases the period of maximum lateral acceleration. Overall result is a increased lateral accelaration, and therefore speed, especially for street cars. Another option for increasing roll resistance is by using stiffer springs. The down side to this is however for the same roll resistance (increase) using springs only will generally require excessively high spring rates inappropriate for street use. The trick is to strike a good balance / compromise between ride and handling.

Wojtek.


I think I am confused here. I understand that increasing roll resistance will increase weight transfer and I understand that the increased roll resistance will improve the contact patch of the tyre (or tire ;)) and thus increase grip, etc. But then wouldn?t increasing roll resistance in the rear of a car via stiffer springs or swaybars result in more understeer because of the resulting extra grip back there?

Or do you mean that the improved contact patch would be found on the front of the car (if adding a stiffer rear sway bar)? Can you describe the weight transfer that is going on, laterally and longitudinally? I have trouble connecting what happens diagonally between the corners of the car, but I think I am beginning to get an idea of what is going on.


Thanks,
Andrew

[Wojtek]

Hi Andrew,

Welcome to the forum.

Firstly, I think you may be confused about weight transfer, and what happens when it is increased or decreased.

Is this a good or bad thing?

What happens to the grip of individual tyres, inside and outside, and the total grip when weight transfer is increased?

As you can see, if you are not comfortable with the basics, this can be quite complicated to come to terms with. I'd suggest that you refer to a few suspension text books first, as it's not easily explained in a few words and then let me know what you think.

Regards,
Wojtek.

[AndyRoo]

I think you are right. I'm going to go check out some books.

- Andrew

[Wojtek]

Great.
Awaiting to hear back from you.

Cheers,
Wojtek.

[Keith Watson]

Hi Andrew,

Buried inside of NASIOC are a couple of good threads this year going over some of the basics, especially as to how they apply to the Impreza.

In order to speed up the learning process here are some key basics to look for in your reading. Some Wojtek already mentioned.
How does the amount of weight on a tire affect the lateral grip of that tire?
How does the amount of weight transfer affect grip at one end of the car? This is key.
Why does increased roll resistance improve the contact patch?
If you increase the roll resistance at one end of the car is an improved contact patch the only effect?
For an explanation of diagonal weight transfer start with an explanation of adjusting corner weights.
If you don't have a book yet I like how Don Alexander explains things.

Regards,
Keith

[AndyRoo]

Thanks Keith.

I'm gonna check out some of Don Alexander's books tomorrow since I won't be able to get my hands on GT4 until wednesday ;) . I'll keep an eye out for those basics you and Wojtek posted.

- Andrew