Roll bar spring rate

[TeoMV]

What about the roll bar rate in relation to the springs rate? Does the rollbar rate have to always be less than the spring rate?

 

[TeoMV]

I mean antirollbar of course. I am sorry.

 

[BrianPetersen]

Some context would be nice to have. Some suspension designs have no antiroll bars at all, so the entire roll stiffness is given through the spring rates and the geometry. Transport-truck trailer suspension that uses air bags has a very soft spring rate with an effectively almost infinite antiroll rate; the usual design effectively locks the left and right wheels into moving up and down together with (substantially) the only difference being given by the deflection of the bushings.

 

[GregLocock]

In production cars it is not often used as it leads to 'head toss' which is tiring and annoying.

On a racing car with a compromised suspension design (eg macP) you might well end up with front a/r stiffer than the springs.

The best reason to do so in my experience is to stabilise the front end of a RWD car, so providing better traction for the rear end out of corners. But the tradeoff is that you'll have less effective braking into the corner. You'll also probably get more weight transfer understeer, which may or may not be an advantage.

Other reasons you might do it include that if you have a very low RCH, as racers often do, then you might want to reduce the roll gain.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

With our Locor race car, the macP's are with 300 lb/in springs for vintage racing with the front antiroll bar at 300 lb/in. When we raced it in the old days, the front springs were as high as 450 and the bar was around 690.

Typically, the stiffer the suspension the quicker the lap times but make the car rather unforgiving and require far greater driver skill levels.

Rod

 

[NormPeterson]

Brian - I think that transport trucks rely on unusually high geometric roll center heights rather than elastic constraint. In the rear, you typically find really stout Panhard bars mounted off the tops of the differential housings, one running to the right side frame rail, one to the left. The point of application of the majority of lateral load is in the 5th wheel plane only a few inches above that, so assuming that there is some small degree of X-axis rotational freedom at the 5th wheel pin connection the roll moment applied to the rear of the tractor chassis perhaps isn't as large as might be first thought. Until everything starts to overturn, anyway, in which case the roll center height probably doesn't matter any more. I'll have to ask my son about this later today, since he drives one of these things for a living. Maybe take a couple of pictures.


Teo - whether the sta-bar rate is less than or greater than the spring rate depends on the vehicle and the philosophy behind its design. Of the four cars in my driveway, two have greater front wheel rates from the springs and one has more front wheel rate from the sta-bar even after fitting much stiffer than OE coil springs (haven't gotten around to dealing with #4 yet).

Perhaps it's better to first select the springs based on other ride considerations and then see what sort of sta-bars would be required.


Norm

 

[hemipanter]

I guess we are talking wheel rate of springs and bars here.
If the car has no anti:s the suspension (wheeltravel) of the car will deflect more for the same G load sideways than from acc and braking. That is becouse of the track width is less than the wheelbase. Therfore we can mount a swaybar that add as much roll stiffness as is needed to bring the roll down to the same deflection a we have in pitch. Just a rule of thumb for bar stiffness.
Goran

 

[GregLocock]

That's an interesting way of looking at it, is your reasoning that the ideal state is for a circuit car to use exactly all of its wheel travel? If it doesn't then car is too stiffly sprung, and so will not develop full grip.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

One of the things I get to witness in vintage racing is the difference in suspension setup from the ' old days' to what is considered 'state of the art' today. Many of the formula cars of the early days are fairly stiffly sprung. Often without antiroll bars or no more sophisticated dampers than "friction" or Houdaille, Armstrong type. Wheel travel is rather limited. The 'mid years' you had Lotus and rather long travel, relatively soft suspension with all sort of bars and dampers. Enter the 80's and what I call the EBJ era. That's short for "Eyeball Jiggler". The new stuff uses so much aero add ons that I suppose you could drive one on the 'roof of a tunnel' at speed.
For my type of racing, saloon/sedan racing, as I have said before, stiffer generally yields quicker lap times on most of the smoother (virtually all) racing circuits.

Rod

 

[hemipanter]

"is your reasoning that the ideal state is for a circuit car to use exactly all of its wheel travel? If it doesn't then car is too stiffly sprung, and so will not develop full grip."
Greg

It is a bit of a Catch-22. We want to have a low center of gravity, and a low drive provides a low center of gravity. Thus, it is idea to lower the car as much as possible, and the limit is when it hits the ground. We can then have a harder sprung car that is lower, or softer which must be correspondingly higher for not hitting the ground. But having a high car that is tightly sprung becomes a loss in the CGH, as well as a soft car that is too low will be a loss in that it hits the ground the whole time. Should we adjust the springs to rideheight or rideheight to the springs?

First let me say that my theory is that we must have balance based on the choice between the stiffness of the suspension of the roll and pitch in order to benefit all suspension travel effectively. Then we look at how the track looks, how smooth or rough it is, because that is what determines how much travel is that we need. Depending on the asperities looks (and curves), we will receive different combinations of balance between the suspension and shock absorbers, which in turn affects the rideheight which must be used. Right adjusted car is using all available rideheight effectively.

It is wrong to say that a harder or softer car go faster or slower in general. We will always be a frontier where the hardness becomes too hard or softness of soft. But if we see it from a purely tiregrip situation, softer suspension gives more grip, but then the chassis movement must be dealt with on the right way, and we have a deterioration of the needed CGH.

Goran