Board logo

Wheelrate : swaybar stiffness
ettore bugatti - 2/2/05 at 10:27 PM

Is there a general ratio in which these two should appear? I just cannot find any information on setting the correct anti rollbar stiffness

pbura - 3/2/05 at 04:49 AM

Wheel rate is a factor in determining the roll stiffness of your springs. The ARB roll stiffness is added to this to arrive at total roll stiffness, so the more wheel rate you have, the smaller the sway bar you'll need.

I highly recommend a read through this website for some very good tech articles:

Smithees Race Car Suspension Setup

Sad to say, I could once duplicate the calcs on their weight transfer spreadsheet, but the knowledge has slipped out of my head for the present. Probably AU$295 would be well-spent in getting their guidance on roll couple, ride frequency, and roll bar specs. An AU builder who posts to the Locost NA list has spoken well of Dale Thompson of Smithees.

Pete

ettore bugatti - 3/2/05 at 04:51 PM

thanks for the site!
So since the wheelrate has a important influence on the ride. The way to go is to get the wheelrate right for the ride and add a swaybar to get the total roll stiffness right or Im wrong?
I guess the total roll stiffness determins how much roll the car has, but what is than the role of RC?

greetz Richard

britishtrident - 3/2/05 at 05:08 PM

For a flat ride it is better to have the rear spring frequency higher than that at the front, in or to put it another way ideally the front wheel rate should be softer than the rear. To do this most road cars even fwd ones use a front anti-roll bar which allows the use of lower frequency (softer) suspension at the front without upseting the understeer/oversteer balance or allowing excessive roll.

The realationship between spring rate and ARB stiffness has a fair bit of leeway but it has limits -- Robin Hood for example ran into trouble using a standard Sierra ARB on the front of one model which in effect took away any indpendant movement of the front suspension.
Rear wheel rate, roll centre heights and chassis stiffness also have an effect --- use too stiff a front ARB and you can end up cornering on 3 wheels.



[Edited on 4/2/05 by britishtrident]

pbura - 3/2/05 at 08:16 PM

quote:
Originally posted by ettore bugatti
The way to go is to get the wheelrate right for the ride and add a swaybar to get the total roll stiffness right or Im wrong?
I guess the total roll stiffness determins how much roll the car has, but what is than the role of RC?


The swaybar does add some wheel rate when loaded, but that's OK under most circumstances because the weight of the loaded side increases, too, keeping the ride frequency from increasing too much. I think most people disregard the sway bar when determining wheel rate, though I've read in a few places that no more than half of total roll resistance should come from the bar. Too much bar, and it's practically like having a solid axle.

Weight transfer happens no matter where the roll center is, but the higher the roll center, the less body lean you will have. For example, a car with a center of gravity height of 12" and a roll center height of 3" will have a quarter of the weight transfer pass through the roll centers. Instead of pushing down on the springs, this force is expressed as a lateral thrust against the outside wheels. So, too low of a roll center and you're giving away body roll, too high and you have grip problems.

ettore bugatti - 3/2/05 at 08:26 PM

Thanks for the info. This subject is getting more clear to me

Rorty - 4/2/05 at 04:41 AM

quote:
Originally posted by pbura
I think most people disregard the sway bar when determining wheel rate, though I've read in a few places that no more than half of total roll resistance should come from the bar.

I don't have any practical figures in my head at the moment to counter what you say, but I would have thought well less than half the roll resistance should be from the ARB.
The only exception to that, that I've meddled with is the front of my Hammerhead three-wheeler which actually has the roll centre below ground level and uses an ARB to controll most of the roll. The reason being, to use very soft shocks on the front to reduce jolts through the handlebars, and improve traction. But that setup is fairly unique.

pbura - 4/2/05 at 11:52 AM

quote:
Originally posted by RortyI would have thought well less than half the roll resistance should be from the ARB.


The less the better, probably, as ARBs also transfer additional load to the outside wheel, not a good thing.

britishtrident - 4/2/05 at 04:16 PM

quote:
Originally posted by pbura
quote:
Originally posted by RortyI would have thought well less than half the roll resistance should be from the ARB.


The less the better, probably, as ARBs also transfer additional load to the outside wheel, not a good thing.


As with changing the wheel rate at one end or playing with the the RC height ---- same result by other means that is how a designer/develpment engineer balances the handling.

ARB aren't a good thing on rough surfaces beacause they effect traction and braking which is why Land-Rover were so reluctant to adopt them until they realised how few Disco ever went further than the school run.

One of the reasons for using ARBs is they allow lower RC heights without the need for harder springs. Cars with low roll centres tend to be more predictable than those with high roll centres. A lot of the black art of suspension design is not to allow the roll centre position to move around too much as the vehicle rolls. On his successful cars Chapman used low roll centres with soft springs, moderate to sitiff damping and relatively stiff anti-roll bars.



[Edited on 4/2/05 by britishtrident]