V8Mongrel's Custom Suspension (track width questions)

[DDurusky]

Hello Everyone,

First, I'd like to share a friend of mine's very interesting project:

http://www.v8mongrel.com.

Its an old RX7 chassis with a Ford 302. If that wasn't enough it has a big rear end with quick change gears. If that really wasn't enough, we designed and implemented an SLA front suspension, ditching those ugly stock struts.

Here are a few pics of the front and rear suspensions:

http://www.flickr.com/photos/dandurusky//sets/72157613523037598

Anyway, with the front suspension finished, we're now working on the rear end. The current question is on front & rear track widths.

The front suspension is done and the track width is 59" from hub face to hub face. We plan to use 0 offset 10" wide NASCAR wheels. (we used NASCAR spindle and A arms, too)

The rear track width is currently 61" wide. We plan to run the same 10" 0off wheels in the rear.

Now the question we face is do we want to reduce the rear track width?

Here are some more facts. The stock RX7 had a wheelbase of 65" (we plan to keep this, but offset it an inch forward for weight distro reasons), a front track width of 56" and a rear of 55".

We noticed that a lot of sports and race cars have a wider front track than rear. If we were to shorten the rear axle we would have to slightly tub the rear of the car for clearance, get new axles, and shorten the tubes.

What do we need to consider in determining a proper rear end track width? Any advice would be much appreciated.

 

[MikeHalloran]

Uh, about those big ugly holes where the big ugly struts used to be... Where are the new springs, and the structure to react their loads?



Mike Halloran
Pembroke Pines, FL, USA

 

[DDurusky]

Doh, I knew we were forgetting something!

That's the easy part. By done I mean that the arms are done and the track width is fairly fixed at this point. So lets think of the front as a constant.

This baby is still a good year away from touching the ground.

 

[MikeHalloran]

I'd have guessed a stock wheelbase around 95", not 65".

I wouldn't tub a car without a compelling reason.
Fashion is not sufficiently compelling for me.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

From what I understand, front and rear track widths are a trade off between corner entry and corner exit performance (think of it like a three-wheeler with either the single wheel in the front or rear). Most racecars seem to have a narrower track width in the rear to give stability when entering a corner but to allow the car so "squat" on the outside rear tire on corner exit to allow the power to be put down earlier.

I'm sure someone here has a more eloquent explanation and will be able to expand on the track width issue.

 

[DDurusky]

Crap, I typo'ed. You are correct, the stock car has a 95" wheelbase.

The compelling reason is that given the current axle width, the wheels (obviously much wider than stock) have just the right amount of clearance from the wheel and tire to the inner wheel well. If the axle gets any shorter, the inner wheel well will have to move. There is 3" behind the inner wheel well to the frame rail.

The coil over rear shocks will mount to the roll cage so deleting the stock rear strut towers, etc is no problem. The stock spare tire well is already cut out for differential clearance and a fuel cell is going in.

 

[NormPeterson]

FWIW, one of the approaches in asphalt circle track racing is to align the (same size) right side tires such that neither is "further out" than the other. I would look at that and go with symmetry from there.

Is it possible to get wheels with slightly different offset for the rear? GM did precisely that with at least one wheel/tire option on the later-generation F-bodies.


Norm

 

[DDurusky]

Hi Norm,

Sorry, but I don't follow your first statement. Does that mean equal front and rear track widths? I of course want symmetry as this is a road race car.

Well.. its possible. But even with more offset, we could keep the axle, but would need to do something about the inner wheel wells. The wheels can't come in at all as they are now.

But of course if we determine that we must reduce the rear track width, we'll do what it takes to make it happen. So the first step is to determine how to calculate (or even theorize) how wide we want to be in back.

 

[Bribyk]

I think you'd have to model your entire car to determine an "optimal" track width. The rule of thumb we used on our autocrossers was about 10% wider in the front, this was out of a prominent suspension book (Staniforth or Milliken?).

 

[NormPeterson]

"Sorry, but I don't follow your first statement. Does that mean equal front and rear track widths?"

Both right side tires at equal distances off the vehicle longitudinal centerline. Any inequality in front vs rear track shows up to the left side, which is always on the unloaded inside in that environment unless something went rather wrong. I would expect symmetry on NASCAR "Cup" cars built for the Watkins Glen and Laguna Seca races.


Norm

 

[DDurusky]

I see what you're saying, but this isn't a circle track car. Yes, it uses some NASCAR parts, but that's because theyre very well built and can be had for very cheap as take offs.

This car will see nothing but road courses. VIR, CMP, Roabling, Mid Ohio, etc.

 

[patprimmer]

Hopefully soon you might have moved past the emotional and fashion based descriptions and supplied all he relevant real data.

I just noticed a very casual mention of a cage.

If the cage is well designed and tied well to the remaining floor pan and body structure, I would not hesitate to alter the inner wheel wells for clearance. That is provided the cage does more than compensate for the weakening of the floor pan.

If the rear axles and suspension is alrady fairly well set in stone, I would use wheel offset to adjust final track.


As I see it you have more typos or you use a different definition of offset to me. Zero offset to me means the track matches the flange to flange presuming zero caber.

Regards
Pat
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[DDurusky]

I created this thread with the hope of gaining some understanding on the subject of determining proper front and rear track widths for our road race car. I am clearly not an expert on this, if I'm missing pertinent info, please ask. I'll measure/figure/dig it up.

We agree on the definition of offset. 0 offset means the mounting surface is dead center in the width of the wheel. Our wheels are 15x10" with 0 offset.

 

[patprimmer]

If you cut a great big hole in the back floor pan to accommodate the tubs and the differential housing for the live rear axle, will the structure retain sufficient strength and stiffness is the unanswered question.

No matter what track you have, if the torsional rigidity of the chassis is seriously compromised the handling and road holding wil be junk.

If the aluminium clamps that locate the rear suspension lower control arms wear lose and slip, the handling will be dangerously unpredictable.

An inch more or less of track won't make all that much difference.



Regards
Pat
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[DDurusky]

"If you cut a great big hole in the back floor pan to accommodate the tubs and the differential housing for the live rear axle, will the structure retain sufficient strength and stiffness is the unanswered question.

No matter what track you have, if the torsional rigidity of the chassis is seriously compromised the handling and road holding wil be junk."

100% agreed. I think we're ok there. It has a solid cage.

"If the aluminium clamps that locate the rear suspension lower control arms wear lose and slip, the handling will be dangerously unpredictable."

100% agreed. These are steel clamps with a large set screw in the top to prevent rotation. They're also often tack welded in place once all systems are go. They're tried and true by thousands of circle track racers every weekend.

"An inch more or less of track won't make all that much difference."

How do you arrive at this conclusion? I'm not questioning it, but i'd like to learn the whys and hows of this topic. Our suspension design softare doesn't handle this situation. I'm using Performance Trends Suspension Analyzer, but the more limited single axle version.

Thanks again!

 

[MikeHalloran]

For a street car, I might go with 15x8 wheels (and 2" narrower tires) with a 1" offset in the back, to get the outsides even. That also helps when your alignment guy starts by stretching a wire over the outside of the tires to get everything zeroed.

On a race car, I dunno... what class will that mongrel run in?



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

I don't have any theoretical analysis, just experience using different offset wheels.

There are always a few variables and the tests were subjective, but turn in rates were not noticeably different.

Roll rates reduce with increased track, and on the front only, kick back rates increased noticeably when the extra track was gained by offset that lead to greatly increased scrub radius. 1" increase on 1" scrub rad is 100% increase, but 2" increase on 60" track is only about 3" increase.

Regards
Pat
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