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GT3 Road Race Chassis Rebuild, S14

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A solid axle suspension is much simpler to tune and offers very reliable, very predictable handling. Set up with a three link and panhard or watts link, you can control the squat/dive characteristics, roll center, wheel hop (usually under braking). Dealing with wheel hop usually involves a bushing or spring setup on the top link.

 

IRS has so many variables that are hard to account for. Bump steer, roll characteristics, dynamic toe and camber change, etc. An IRS is much more likely to have trailing throttle oversteer or snap oversteer than a simple solid axle.

 

For a team with unlimited budget and tons of testing, sure, an IRS is definitely going to work better, but for a budget amateur sports car, you can't go wrong with a solid axle.

 

Note - SCCA doesn't actually specify a solid axle, but the rules make it an attractive alternative.

Edited by Stoffregen Motorsports

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On 8/3/2019 at 9:50 AM, Stoffregen Motorsports said:

A solid axle suspension is much simpler to tune and offers very reliable, very predictable handling. Set up with a three link and panhard or watts link, you can control the squat/dive characteristics, roll center, wheel hop (usually under braking). Dealing with wheel hop usually involves a bushing or spring setup on the top link.

 

IRS has so many variables that are hard to account for. Bump steer, roll characteristics, dynamic toe and camber change, etc. An IRS is much more likely to have trailing throttle oversteer or snap oversteer than a simple solid axle.

 

For a team with unlimited budget and tons of testing, sure, an IRS is definitely going to work better, but for a budget amateur sports car, you can't go wrong with a solid axle.

 

Note - SCCA doesn't actually specify a solid axle, but the rules make it an attractive alternative.

 True True.

A good IRS is better suited for bumpy tracks.

I like IRS but once you commit to a specific geometry you are stuck unless you want to make cut and weld alterations, i.e. not very adjustable for roll center.

 

 At one time SCCA did mandate solid rear axle, but have since allowed IRS with weight penalty. In my opinion SCCA GT cars weigh too much as it is! 

Edited by GT2
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A panhard bar is acceptable for drag cars, as they don't turn in anger,

and a nazkar, as they only turn left.

But a road race car has to turn both ways, and a panhard bar's roll center is off to one side.

May be cheap, but a poor choice for road racing.

Now a Watts link is a good choice, as the roll center is at the height of the rocker.

Bottom of the axle is about as good as it gets.

Center of the diff isn't much better than leaf springs, other than getting rid of all the side to side slop.

 

WattsLink.jpg

 

DSC08381.jpg

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Pan hard rod is only off center if you mount it that way. It is possible to mount it centered.

Mine is off centered a little, this can be compensated for with spring rate if necessary

 

This is an age old argument. The watts link is an excellent way to locate the rear end, however the roll center is not very adjustable and ends up too low for many cars.

I based my choice on personal experience and the advice of many Trans Am ( SCCA) drivers ( who have made a science of the solid rear axle), they say " We have driven all rear attaching types and in the end they all work good, it comes down to personal preference ( drivers feeling) !

Edited by GT2

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Ideally, a Panhard bar would be as long as possible. I have mounted them with an outboard frame mount going all the way to the axle tube end. Basically, the entire width of the axle housing.

 

Also, (Duax) you're talking about dynamic change on a car that uses about 4" of suspension travel. Not a lot is going to change over the course of 4 inches.

 

Yes, a watts link is best, but a properly set up panhard works great too.

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