noob suspension question

All other things being equal, will more rear bar reduce or increase oversteer? Or, in the case of my specific circumstance, will adding a rear bar exacerbate current understeer, or serve to correct it?

Should reduce understeer.

^what he said. I put in a front bar on my ms3 to take the slop out of the suspension, and put in a rear bar to make it more stable. The car never really understeered much, and it sure as hell doesn't now.

Because more rear bar effectively means less weight on the inner rear wheel mid-corner, right? I'm just thinking this out.

In reply to alex:

Right, that's part of it. If you make the rear do more work in terms of roll resistance, you increase the weight transfer at the rear. There will be less weight on the inside rear, and more on the outside rear. Because of how tires work, you'll lose more traction at the inside rear than you gain at the outside rear**.

At the same time, the front now has to do less work in terms of roll resistance. Less weight will come off the inside front and go to the outside front.

The rear will become less efficient in terms of generating cornering force, while the front becomes more efficient.

** Tires grip more when you press down on them harder (when they have more weight on them), but it's a case of diminishing returns. When you take a pound of pressure off the inside tire and add it to the outside tire, you lose more than you gain.

Increases rear roll rate, which generally means less rear grip, which generally means more oversteer.

I disagree semantically with saying it "decreases understeer", because while the net effect may be a decrease in apparent understeer, you really haven't reduced the understeer, you've just increased the oversteer to compensate. The overall grip is lower, you're just not pushing the front suspension to the point where the understeer characteristics are apparent.

At least, that's how the world works in my head.

Cool. Think I have my brain wrapped around that.

This is on a '66 MG-B, by the way, so if anybody has specific suggestions on rear bar diameter, I'm all ears. Not trying make this an autocross machine, just a backroad burner, which is a relative term with this car, of course. And the reason we're trying to reduce roll in the back is that we're getting a little tire rub on the inner fenders from slightly oversized wheels. Sure we could go back down on tire/wheel size...but where's the fun in that?

ReverendDexter wrote: I disagree semantically with saying it "decreases understeer", because while the net effect may be a decrease in *apparent* understeer, you really haven't reduced the understeer, you've just increased the oversteer to compensate. The overall grip is lower, you're just not pushing the front suspension to the point where the understeer characteristics are apparent.

I agree with that.

I was about to say it depends upon if the car is FWD, AWD or RWD, then if RWD what type of axle and suspension it has versus your preferred driving style.

I have always been a proponent of removing the rear bar completely from a RWD car with a solid rear axle.

But I really prefer a car to oversteer rather than understeer. If you wish to try it out, just undo one end of the rear bar after doing a lap with it secured.

Your experience may vary.

ReverendDexter's use of the word generally is a good call.

I cautiously assert, though, that you really are improving front end grip by making the rear take more of the weight transfer task, though it's very much correct to note that it's a net loss.

...Unless of course the increased roll stiffness causes overall body roll to reduce enough that the tires are being used more effectively... It's not unheard of to have a car understeer less after adding a larger front bar just because it stops flopping onto its sidewalls...

At this point, now that we know what your issue is, I'd half suggest that you start a new thread asking for specific MGB advice, or just google around for MGB knowledge. Even a front bar might reduce overall roll enough to help with your tire rubbing, and may be better for overall balance. I really have no idea, but I'd wager there's a lot of MGB-specific knowledge on this board...

If the car understeers ,a bigger rear bar wil decrease it. If the car is neutral, it will cause oversteer. Maybe stiffer springs all around will fix the problem.

I hear what you're saying on that, but we're trying to avoid going too stiff and sacrificing the ride. We've already gone one stage (so to speak) stiffer than stock, and it's striking a pretty good balance between comfort and sport right now.

Actually, the fender/tire interference is so minimal that we're thinking poly or metal leaf spring shackle bushings will do the job. But we're a little concerned about too much harshness from either of those options, so I'm thinking that a rear bar could be the happy medium (and we happen to have one - along with metal shackle bushings - sitting on the shelf).

We'll try a few things and report back.

Thanks for the input, all.

ReverendDexter wrote: Increases rear roll rate, which *generally* means less rear grip, which *generally* means more oversteer. I disagree semantically with saying it "decreases understeer", because while the net effect may be a decrease in *apparent* understeer, you really haven't reduced the understeer, you've just increased the oversteer to compensate. The overall grip is lower, you're just not pushing the front suspension to the point where the understeer characteristics are apparent. At least, that's how the world works in *my* head.

I'm gonna disagree with that. It's the logical conclusion to "if I stiffen this end, it loses grip". But it's not right. What you're really doing is shuffling the grip around. By increasing the roll resistance at one end, that end will get more of the total weight transfer. So it'll grip less - but the other end will get less of the weight transfer, so it'll grip more. You're not changing the amount of transfer, you're moving it around. Change both ends the same, and you'll get the same grip with less roll.

I wrote a book about this Just because it's got Miatas in it doesn't mean it doesn't apply to other stuff. And there's a sample chapter that deals with just this.
http://flyinmiata.com/pdf/HPMM_sample_chapter.pdf

Now, this is all assuming you're not hitting some boundary condition such as lifting a wheel off the ground (weight transfer at that end stops), landing on bumpstops (big spring rate spike!), having a wheel do something bizarre with geometry or the like. If stiffening the rear bar will keep you off the rear bumpstops, for example, you'll find you get less oversteer at the limit.

Well put, Keith. Interestingly, you bring to mind Keith Code's traction circle concept from his book 'A Twist Of The Wrist,' which is a seminal work on sport/race motorcycle riding.

http://www.sportrider.com/riding_tips/146_0909_traction_circle_riding_skills/index.html