Adding track width. Effects?

 I have been reading that by widening the track width, you effectively make it more difficult for that end of the car to lean on to the tires because it is now longer and flatter in that direction. And I have heard that if you do this you can use it to dial in or out understeer and oversteer. My real question comes in as this:  how much of an affect does this have? And how much of it do you have to do in order to gain a noticeable effect? 

The main reason I ask is that I am looking to space my rear tires out approximately a half of an inch, the fronts are fine. And for right now I am not running any sway bars front and back until I  assess more of what the car needs. 

Well, what are the track widths front and rear?  Making the rear wider than the front may just introduce some weird handling quirks down the road.  Adjusting track witdh is usually way down the list of trackside adjustments.  Why are you looking at making the rear wider? Aesthetics?

In reply to Trackmouse :

Increasing the track width does two bad things, makes the car wider so it has to travel further through the cones.  A few inches of added travel may not seem like a big deal  but considering how close some races are that may not be a handicap you want

Wider car pushes more air, that increases the drag. 

Are you talking about stagger? Years ago, I tried “reverse stagger” (i.e., front track wider than rear track) on my E30 track car. It was recommended by my good buddy, who was a regional autocross champ. 

My issue was the E30 had too much grip and not enough HP to rotate under power. I don’t know if it was simply the wider front track or maybe increased weight-jacking effect, but the net effect was the outside rear tire would load up more dramatically. That let me control the rear end better under throttle without having to do so much trailbraking.

Old-school supercars like 512 Testarossas and Diablos used regular stagger (rear width greater than front), presumably to have the opposite effect and stabilize the rear under power. 

Adding track width softens the spring rate and adds grip, assuming the shocks remain in the same location. The opposite affect happens in karts.

In reply to LanEvo : stagger is that he difference between the left and right tire diameter( usually measured by circumference )  not track width.  

Normally a large difference aids turn in ( in one direction)  increases over steer. ( circle track speak loose ) 

A small or no difference will add to the stability increases understeer  ( circle track speak push) 

More track width reduces weight transfer at that end.  So it can add grip.  In a case where you're running a lot of caster (which causes weight jacking), widening the rear track can help keep weight on the inside rear tire in tight turns and let you put down more power.  

If I'm looking to adjust corner entry and corner exit balance, and class rules allow it, adjustable shocks are an easier and more effective route for me.

Also in the past I have tried to address understeer in cars that came from the factory with staggered wheels.  I was told a square setup was the way to go.  It didn't make much difference, but at least I could rotate the wheels around.  Camber plates made a big difference, and brake/throttle approaches came in second.  Ultimately, front engine cars like to understeer and there's only so much I can do about that.

Basic vehicle dynamics: the wider the track, the lower the amount of weight transfer. This means more grip. If you only widen at one end, then that end will gain traction and the other will not which changes the handling balance.

A good introduction to the theory

As always, there are edge cases with regards to range of motion limits and you will change your effective spring rate (aka wheel rate).

loosecannon said:

Adding track width softens the spring rate...

Keith Tanner said:

...and you will change your effective spring rate (aka wheel rate).

A common misconception.  In roll or one wheel bump for a live or beam axle, it will.  On most typical independent suspensions, it will not. 

Well, I'm assuming wheel spacers or offset here, not new control arms. If you move the contact patch outboard without changing the shock location, you will change the motion ratio and thus the wheel rate on an independent suspension.

Motion ratio is based on shock mount location relative to lower ball joint location, not tire contact patch. 

A free body diagram will show that moving only the tire contact patch further outboard will not change the vertical component of the force being fed in at the lower ball joint.  It will however create an additional moment about the lower ball joint that will be fed laterally into the control arms.

On a solid axle, adding spacers will not soften the suspension in roll, only in 1 wheel bump.  Roll stiffness (as far as body lean angle for a given cornering load) is based on spring location relative to the body, not the wheels.  

It’s SRA, not IRS. Thanks for the input guys! I’m still debating on whether I’ll do it, and I think I’ll only do this once sways are on and I need to fine tune it. The tires are the same section width, but the 5x4.5 Bassett’s only come as 3.75 backspacing in a 7” wheel. The fronts are 15x7, but are four lug, so it has the 4” BS.

What irks me is that the rear wheels USED to stick out further than the fronts when I had the 14x8’s on the back. It just looks a little silly to have the back wheels more inboard than the front, especially since the front has such a nice poke! Aesthetics be damned though. Form follows function. 

Driven5 said:

Motion ratio is based on shock mount location relative to lower ball joint location, not tire contact patch. 

A free body diagram will show that moving only the tire contact patch further outboard will not change the vertical component of the force being fed in at the lower ball joint.  It will however create an additional moment about the lower ball joint that will be fed laterally into the control arms.

I don't agree with this. Moving the contact patch further outboard changes the geometry, meaning less spring movement for a given amount of vertical wheel movement.

rslifkin said:

On a solid axle, adding spacers will not soften the suspension in roll, only in 1 wheel bump.  Roll stiffness (as far as body lean angle for a given cornering load) is based on spring location relative to the body, not the wheels.  

My mistake.

Keith Tanner said:

I don't agree with this. Moving the contact patch further outboard changes the geometry, meaning less spring movement for a given amount of vertical wheel movement.

Depending on how much camber gain your suspension has, maybe very slightly, as the wheel centerline has a non-zero amount more vertical movement than the lower balljoint.  But we're talking something like a 0.00X of an inch (insignificant) difference at the center line of the wheel per 1 inch of bump at the lower balljoint, for a 0.5 inch offset increase from a 1 degree per 1 inch bump camber gain suspension. Then add in the actual contact patch movement due to the effects from wheel width and tire deformation, and we're really getting farther in than can be readily determined manually.

Assume a 0 camber gain suspension (parallel equal length double wishbone) though, for the sake of simplicity, and there is absolutely no difference in balljoint movement vs contact patch movement regardless of wheel offset changes.

Driven5 said:

Assume a 0 camber gain suspension (parallel equal length double wishbone) though, for the sake of simplicity, and there is absolutely no difference in balljoint movement vs contact patch movement regardless of wheel offset changes.

That's the part I was having trouble with. I've got it sorted out now, thanks.

Trackmouse said:

Aesthetics be damned though. Form follows function. 

Why not have both if you can?  I'd argue that the performance effects will probably be a non-issue either way.  Realistically, playing wheel offset is about affecting where the wheel sits in the wheel well. Put the wheels where you want them, and get the handling you want by tuning the suspension with springs, sways, and shocks.

Driven5 said:

Trackmouse said:

Aesthetics be damned though. Form follows function. 

Why not have both if you can?  I'd argue that the performance effects will probably be a non-issue either way.  Realistically, playing wheel offset is about affecting where the wheel sits in the wheel well. Put the wheels where you want them, and get the handling you want by tuning the suspension with springs, sways, and shocks.

That’s truth!  I’m supposing with a 1 inch spacer would look about right, considering I lost half of an inch going down to a 1 inch narrower wheel, and then lost a quarter of an inch with the backspacing. So one inch should do it some justice and bring it flush again, just so I can have that JDM yo stance!