1969 Torino

[Ron Sutton]

Originally Posted by carbuff

In a similar vein to the TW / WB ratio question, at what point would having a wider front TW compared to the rear TW start to decrease handling performance? Or stated a different way, is there such thing as too much front TW compared to the rear?

In case new people drop in on our conversation, track width is center of tread to center of tread. Tread width is outside tread to outside tread (not the sidewall).

I don’t think of track width difference as decreasing handling performance. I think of it as a tuning tool … if the track width is adjustable … or a tuning challenge if it is too far biased on one end and NOT adjustable.

While there are exceptions to almost every rule, I typically don’t want to see the track width difference more than 1” on either end. Even that can be a challenge if it the difference is the wrong direction.

Within reasonable differences making the front track width wider than the rear … or the rear wider than the front can be a practical tuning tool. Making the front track width wider than the rear = tightens the car … too much makes it push. Wider rear track width than front track width = frees the car … too much makes it loose.

Put even more simply, regardless of what is going on at the other end …
a. Moving the front tires out tightens the car.*
b. Moving the front tires in frees the car.*
c. Moving the rear tires in tightens the car.
d. Moving the rear tires out frees the car.

*Note: Adding spacers to the front hubs, increases the scrub radius. So when I can use … either adjustable LCA’s … or simply replace the LCA’s for the track width I want … that is how I prefer to do it, versus adding wheel spacers to the front. I have used wheel spacers in the front many, many times. It’s just not my preferred method. Sometimes class rules dictate the decision.

A LOT of racers use small (1/8” to 1”) wheel spacers to tune … and in fact, many design & build their rear end & front hub combination with spacers as a part of it, so they can be removed to go “in” if needed … and of course, go “out” with more spacer as needed.

For the car I designed for myself with an AutoX priority, the lower control arms use adjustable threaded ends to allow up to 3/4” width change per side … 1.5” total track width adjustment, without affecting the scrub radius. With 335 tires on all 4 corners, my “standard track width” is 55” front & 54.5” rear. I can adjust the front from 54” to 55.5” without spacers.

The rear end in my car will “start” with spacers that can be removed & of course, spacers can be added, for a 2” adjustable track width range in the rear from 53.5” to 55.5” As far as spacers go, I prefer to use them on the rear end only, so as not to affect the scrub radius.

Additional tips: For cars with “high travel/low roll” suspension strategies, the front to rear “track width split” favoring the front works best. For cars with “low travel/high roll” suspension strategies, the front to rear track width split favoring the rear works best.

If your car is already “designed & built” … and the track width is a wider in the front, that will “contribute” to the car being tight/pushy a degree. If the track width is a narrower in the front, that will “contribute” to the car being free/loose to a degree. I use the word contribute, because there are a lot of other suspension & geometry factors affecting the grip at each end of the car. If the track width is not adjustable at either end, you will need to tune with roll center, springs & sway bars to balance the car.

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[Che70velle]

Great post Ron. Valuable info for free guys!

[Ron Sutton]

Originally Posted by Che70velle

Great post Ron. Valuable info for free guys!

Hey Scott ... thanks. From page 118 to here we've had a pretty good tech discussion going.

Feel free to join in.

[FETorino]

Originally Posted by DOOM

Rob I like all the progress you've been making with this beast! Keep it up!

Thanks Mario. I hear there are bowties that will be prowling the streets with Twin Turbo Big Blocks in them so I need to be prepared.

Originally Posted by Ron Sutton

If your car is already “designed & built” … and the track width is a wider in the front, that will “contribute” to the car being tight/pushy a degree. If the track width is a narrower in the front, that will “contribute” to the car being free/loose to a degree. I use the word contribute, because there are a lot of other suspension & geometry factors affecting the grip at each end of the car. If the track width is not adjustable at either end, you will need to tune with roll center, springs & sway bars to balance the car.

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Uh Yea? How about that?

[Ron Sutton]

Originally Posted by FETorino

Uh Yea? How about that?

I'm not sure what you said, let alone how to respond.

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[FETorino]

Originally Posted by FETorino

Uh Yea? How about that?

Originally Posted by Ron Sutton

I'm not sure what you said, let alone how to respond.

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Hmm I know of a car as you described above It sounds familiar.

Oh yea that would be mine

Pretty much non adjustable track width without a involved.
Track in front a little narrower than the rear. Although I have tightened that up a bit by and looking at .

there has been a fair amount of discussion on the spring, shock and bar package for my set up.

So it seems that leaves us @

Tune with roll center. Yep that's it

[Ron Sutton]

Rob,

I get it now.

Yes ... your car's wider rear track width ... by 2" ... helps us free up the rear of the big girl a bit. This is a step in the right direction to help overcome the bigger rear tires, but by itself wouldn't be enough.

In the suspension set-up I outlined for you, I bumped the rear spring rate & sway bar rate a "tad" ... but raised the rear panhard bar (roll center) more ... also to help free her up.

Once you hit the track, we'll need to "tune on her" to achieve a good balance & optimum set-up.

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[Ron Sutton]

Alrighty then ... let's talk Roll Centers.

I’ll be very basic for any readers following along that are completely new to this & apologize in advance for boring the veterans with more knowledge of this. Cars have two roll centers … one as part of the front suspension & one as part of the rear suspension. I’ll first explain what role they play in the handling of a car … then how to calculate them … and finally how to tune with them.

Think of the front & rear roll centers as pivot points. When the car experiences body roll during cornering … everything above that pivot point rotates towards the outside of the corner … and everything below the pivot point rotates the opposite direction, towards the inside of the corner. Because the front & rear roll centers are often at different heights, the car rolls on different pivot points front & rear … “typically” higher in the rear & lower in the front.

If you were to draw a line parallel down the middle of the car connecting the two roll centers … this is called the "roll axis" … that line would represent the pivot angle the car rolls on … again “typically” higher in the rear & lower in the front.


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[Ron Sutton]

When a car is cornering … the forces that act on the car to make it roll … act upon the car’s Center of Gravity (CG). With typical production cars & “most” race cars, the CG is above the roll center … acting like a lever. The distance between the height of the CG & the height of each Roll Center is called the “Moment Arm.” Think of it a lever. The farther apart the CG & roll center are … the more leverage the CG has over the roll center to make the car roll. Excessive chassis roll angle is your enemy, because it is over working the outside tires & under utilizing the inside tires.

Some people like to look at the car as a unit. I look at it as two halves. Here are some examples … using a typical 3500# Pro Touring Car with 53% front weight… to provide more clarity:

If the CG is 20” high … and the front roll center is 2” below ground … the car has 53% of the 3500# weight with 22” of leverage to roll the front of the car.
If the CG is 20” high … and the rear roll center is 9” above ground … the car has 47% of the 3500# weight with 11” of leverage to roll the front of the car.
* Rolling the car that much more in the front overloads the outside front tire & under utilizes the inside front tire when cornering.

If you lowered the car 2” … the CG drops 2”. The front roll center probably moved too … but it’s not linear … it is based on A-arm angles. Let’s say it dropped 1” in the front to 3” below ground and the rear stayed the same at 9”.

Now …
If the CG is 18” high … and the front roll center is 3” below ground … the car has 53% of the 3500# weight with 21” of leverage to roll the front of the car.
If the CG is 18” high … and the rear roll center is 9” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the front of the car.
* The front now rolls over less & the rear too, making the car run “flatter” … not flat, just less roll angle … working the inside tires better.

Any weight you can remove from high up … or relocate to lower in the car … moves the CG down … reducing the leverage it has over the roll center … allowing the car to have less roll angle during cornering … working all four tires more evenly … and the grip of four tires is faster than two.

We’ll discuss moving the roll centers in the final section. Next, let’s cover how to figure out where your front & rear roll centers are at.

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Front: Measuring all the pivot points in the front suspension to calculate the roll center in the front suspension of a double A-arm suspension car can be tedious … but the concept is quite simple.

Quick Acronyms:
UCA = Upper Control Arm
LCA = Lower Control Arm
BJC = Ball Joint Center
IC = Instant Center
RC = Roll Center
CG = Center of Gravity
CL = Centerline

Your UCA & LCA have pivot points on the chassis … and they pivot on the spindle at the BJC’s. Forget the shape of the control arms … the pivots are all that matter.

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[FETorino]

I just catching up on some light reading before I dove into Ron's posts.