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[tommyb13662]

Howdy all

This is my first car with a staggered setup. I have noticed the push effect when cutting sharply around corners, mostly 90deg or super sharp corners. However i also noticed that it keeps the rear planted where i want it when flying through twisties unless i feather the trottle to make it slide. Im used to my other cars (fwd) had the habbit of the back skittering when i was whipping though S's and had to watch the speed or the rear could spin around. So my question is why the love for square setup so much for track? dont we need the extra width in the back to get the power down when the right foot hits the floor? most of what i have looked up on the forum is people explaining their setup to become square not so much the why they really want it.


Thank You in advance for answering my noob question.

[wjk_glynn]

It's about achieving a certain type of balance, assuming you want that.

First lets ignore some inherent limitations in tire size choice on the E82 platform.

Here's how I'd frame the general approach.

You start at the rear and choose the amount of tire (size/compound) that is a balance between traction, and whether you want the rear end to always be planted or whether you want (as some do) the ability to rotate the rear under power (allows for more adjustability mid-corner). That tradeoff is down to individual choice. Some people have 275 tires on the back of a stock 128i, while others have 235 on the back of tuned 135i cars (I'm ignoring tire compound choice for simplification).

Once the rear setup is selected, then you need to decide how much corresponding front end grip you want.

Generally speaking, most cars as-is from the auto manufacturers are setup to understeer for a variety of product and liability reasons. There are of course exceptions such as track day specials (e.g. GT3) and seemingly every modern hi-horsepower Jaguar.

So if you have an understeering car and you want to dial that out, there's a few different ways to tackle that - one of which is to increase the front rim/tire size (i.e. going square). You can also do other things like adjusting the front/rear roll-stiffness-ratio (e.g, sway bars). But the advantage of wider front rims/tires is that the total amount of grip available is increased - assuming the suspension balance/setup can exploit it.

The best example I can think of for a square setup from the factory is the 2015+ Camaro Z/28. It has 305 tires on the front and rear... just amazing!

But back to the E82.

Unfortunately there's a variety of clearance issues (especially in the front) that limit both tire sizes and wheel offsets. These have been discussed numerous times on this forum (just look for the "will XXX fit" threads).

So there's only so much tire you can get under it, assuming that's what you want. Because it is possible to over-tire a car to the point that's it's overly locked down and there is no playfulness/adjustability remaining (e.g. the Miata is a great example on the other end of the tire size spectrum).

Karl.

PS: Side note... the E82 wheelbase is on the shorter side. This means when it lets go, it can be "snappy" (or "agile" depending on your point of view) as compared to the E9x cars. Factor that in when making decisions.

[tommyb13662]

I find even with 225f 255r i can still rotate mid corner when i want by just adding some right foot. I

[John_01]

Originally Posted by tommyb13662

So my question is why the love for square setup so much for track? dont we need the extra width in the back to get the power down when the right foot hits the floor? most of what i have looked up on the forum is people explaining their setup to become square not so much the why they really want it.

When you drive on the track you punish the front tires very heavily in the braking zones. Especially with semi-slick tires a very high proportion of the cars weight is transferred to the front axle during braking. It causes a large thermal stress on the front tires. It is also normal to a use stiffer sway bar on the front than the rear which further stresses the outside front tire in cornering. To cope with this, a lot of track cars run square tire setups, but others have a small amount of front / rear tire stagger. A lot will depend on driver preference, power level, and the car setup.

IMO, a square tire setup isn't quite as beneficial for a road car as others may have written. If you want to get good handling and steering response you can do it with good suspension setup and wheel alignment. For example, by upgrading to firmer sway bars and by adding some front negative camber. Its NOT intended as a car setup for Auto-x. I'm talking about a road car used for driving on public roads. Square tire setup is beneficial for tire rotation to extend the tire life, but the fitment is a problem on a stock 135i due to lack of space in the front fenders.

[tommyb13662]

ok so its more of trying to get more meat on the front tires than actually making it square front to back?

[John_01]

With track cars it has a lot to do with keeping the tires in the right temperature range and not getting overheated. So yes, generously sized front rubber is a help. The tire width will also change the handling balance of the car, but the tire temperature is the most fundamental thing that needs to be right for good performance. For a track car, it may be possible to get more rear grip by lowering the rear tire pressure. That gives a larger contact patch for the rear tires even when using a non staggered tire setup.