more rear bar -- does it help total grip?

[superuser]

one of the first suspension upgrades people recommend is upping the rear bar, on most FWD cars. I have a typical FWD, mazda3, and it does push, so I am looking for ways to mitigate that a little.

Upping the rear bar is supposed to increase the weight transfer in the back, thus sabotaging the rear grip, and making the car closer to neutral.

Now my question is, does this effectively reduce the total grip, bringing the rear grip to the same low level of the front grip? Or does the higher weight transfer in the rear mean there's less weight transfer in the front? Can i get the same effect simply by overpressurizing the rear tires to get less rear grip?

[curtis73]

In general, the grip goes somewhere. If you equalize weight transfer, you generally maximize handling, since all four tires are carrying the same lateral load. Without it, you're giving one or two tires much more load than they can laterally grip and handling suffers.

Using a sway bar in the rear is a an effective way of neutralizing handling for the street, but if you are going to race this car, I might suggest more negative camber on the front if you can get it. You could run up to 1 or 1.5 degrees on the street if you don't mind a little tire wear, but 3 and 4 degrees is pretty normal for weekend autocrossers. You could experiment with camber in the front until you're more neutral... just make sure you test it in the rain, too. The grip that you get from camber comes from tire deflection and you won't get enough traction in the rain... hence wildly oscillating handling.

[superuser]

right, i would love to get some negative static camber... also bigger front bar would reduce the camber loss, right? i understand the weight transfer will be higher, gotta find the right trade-off there. for now, i will assume the factory people found the right compromise value

i am looking into some hardware to allow me to change front camber, I was thinking about -1.5° static. the factory spec is -0.4±1.0°, so this shouldn't be too bad.

you think this would be the most cost-effective thing for autox?

[TheSilentChamber]

Make sure your still going to be within the rules of the class you plan on running in autox. You may dip into a differnt class where you would not have a chance.

[Black Lotus]

Originally Posted by superuser

Upping the rear bar is supposed to increase the weight transfer in the back, thus sabotaging the rear grip, and making the car closer to neutral.

Now my question is, does this effectively reduce the total grip, bringing the rear grip to the same low level of the front grip? Or does the higher weight transfer in the rear mean there's less weight transfer in the front? Can i get the same effect simply by overpressurizing the rear tires to get less rear grip?

Basically there is an excess of grip in the back-- the tires are under-utilized. With a FWD car, the front tires have the burden of accelerating, turning, and with the usual stock anti-roll bar setup, they have the task of being at the butt end of fighting body roll too. If you remove at least some of the weight transfer to the outside front caused by a front only anti-roll bar--by installing a rear anti roll bar, some of the cars "total lateral load transfer" is taken by the rear bar, and the front tires can do more work-- as some of the weight of fighting body roll is actually removed from the outer front tire. This weight is literally being taken up by the outer rear tire. The ideal setup for a FWD car would be that the front drive wheels would have NO front anti-roll bar. ALL the body roll control would be from the rear bar. Just the opposite is true of a RWD car. The idea is that the rear bar for a FWD car would stabilize and keep the chassis level, and the front wheels would be free to do their thing. The limit of the ability of the rear bar to help you (in a FWD car) is when the inside rear lifts off the ground in a corner. You probably will never get to remove the front bar completely, because the body roll would be excessive still--which would hurt cornering.
After all this, the car should get around the corners a bit faster, and be able put a bit more power down coming off the corners.
At the nitty gritty level of all this is the concept of "load sensitivity" (slip angles relative to vertical load, etc.) of tires.
This post is too long already, so I'll skip that.

[superuser]

hokay, i had to do some research to get this:
my car (mazda3)
has 19mm bar in the front, and 17.5mm bar in the rear, with the weight distribution 60f/40r.

The static camber in the front is -0.4°, and in the rear it's -1.5°.

for the first mod, would you recommend changing the bar size (which?) or try to get more front camber? With the stock config I do have excessive grip in the rear, I set the tire pressure to 38 / 32 (the minimum level that stops rollover) and I can't get the car to rotate even with trail braking.

One point, I have seen reports that upping the front bar to 22mm will decrease the tire rollover, so the front pressure can be decreased to 36 psi. This must be evidence of better dynamic camber control, and reduced body roll. I also understand there's more weight trasnfer with the larger bar, and the car might actually be understeering more than before.

Is there a way to figure out whether this is happening? I would like to change the front bar first, if it makes sense at all, the camber control might outweigh the weight transfer, no?