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Help from handling gurus needed


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We are running a first gen Rx7, which is inherently prone to snap oversteer.  We manage it by not trailbraking, and being cautious with throttle liftoff in high speed sweepers.  We have acquired a set of wider (225) tire/wheels, and they have an offset which pushes the wheels out quite a bit (we will have to flare the fenders).  We have a large inventory of narrower (205) wheels and tires to work through as well. Additionally we are putting a wing on the back.

So the question is, how would our handling change under a few scenarios:

1. We run the wider wheels and wider offset wheels all around

2. Wider wheels up front

3  wider wheels in the rear.

 

Our car currently handles pretty well under most circumstances (aside from noted snap oversteer), and brakes very well.  We are concerned about possible extra stress on the rear bearings (is this worthy of concern?).  Will the snap oversteer be wildly exaggerated if we run the wider wheels up front?  Will the wider wheels in the rear help the twitchy handling at all?  We prefer to run the wider wheels in pairs for now rather than all 4 so we can keep the handling consistent over a whole race even when we change tires. We will be going to 225s all around once we have worked through the 205s.

Thanks in advance!

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Snap overseer is a bit weird with that chassis in my opinion (we race a 1st gen with multiple podiums in lemons and chump)...

 

I would be checking to see what is binding or touching once the car is loaded.

 

That being said, going to wider tires probably won't change the behavior.  Especially with such a small change in width.  I would expect rear aero probably won't help either.

 

Find out what is binding or if you have full coil compression or the axle hitting the frame or something along those lines.

 

The watts link works Okay.  A 3 bar is better, but the watts link shouldn't be making the car that prone to snap overseer.

 

In fact, we just put an entire 2nd gen subframe under the front of our 1st gen.  Much more front grip and precision with this subframe.  The rear is still capable of producing enough grip that the car is neutral now (instead of a bit pushy).

 

Get a cheap go pro and mount it looking at the rear springs, and axle tube. 

 

Review video to see watts binding or hitting...

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I am not a guru, but my opinion is that the snap oversteer is probably due to binding in rear trailing arms (bushings reaching the end of their ability to keep all the links in a range of free-ish motion). Since this is largely due to loading up the suspension in cornering, anything you do to increase the loads (e.g. bigger tires with more grip, wider track on that end) on that end will probably increase the likelihood of encountering the binding.  There are tons of ways to remedy this issue, look for some of the Fox body Mustang solutions.

 

I could be wrong with first guess, if it is due to coil bind or bottoming out, you got lucky as that can turn out to be easier to fix (heavier springs / bump stops).

 

Take springs off car,  put shocks on, and move suspension through full travel, including one side bump, other in droop.  Pay attention to where and how anything binds or bottoms and then engineer ways to unbind or un bottom it. 

 

Note: I know nothing of 1st Gen RX7s, but I hear they are a miniature version of the Fox body, which I have owned a few of, and road raced another.

 

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As a background where my experience with snap oversteer comes from, I built and raced a 1992 Chevy S10 pickup that was stripped out at about 2500lbs before driver, and almost 70 percent of that weight was over the front tires, with a leaf sprung solid rear axle that even had a welded diff! Aka, terrible snap oversteer....on paper. We had a LOT of haters that said it would handle terrible and they were blown away when we would pass E30s in the corners. A couple of the guys on our team were ride and handling chassis Engineers for GM. We ran a stiff front bar with 800lb springs (keep in mind the motion ratio is high for GM dual a-arm, so extra stiff springs become a lot less effective at the wheel) but stock rear leaf packs with the middle leaf taken out (extra soft) and no rear sway bar. Rear shackle was extended to reduce understeer angle on compression. With 255/40-17 Z2 tires on all corners it was extremely stable and forgiving, just the slightest amount of understeer that made it super easy to drive at the limit. I am a firm believer in a soft rear with no bar for cars that have a tendency to get loose. I don't know RX7s well, but something to think about.

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I am a fan of soft rear also. I had a 3800 pound pure stock oval car (73 Momte Carlo) and it turned into a rocket ship with a pair of 700# fronts a and a pair of 100# in the rear. I had 58% nose weight. I would also look for toe to be correct. It gets overlooked in solid axle cars. I have had to use a torch a few times to get  bent axle back to spec. Eating bearings is a clue sometimes but not always.  Pulling rear springs and shocks and running through the range of motion will tell you if it's a binding problem. Bent shocks have been known to cause this problem so check their stroke. So does too much rebound in the rear shocks.

 

 

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4 hours ago, UglyBoost91 said:

As a background where my experience with snap oversteer comes from, I built and raced a 1992 Chevy S10 pickup that was stripped out at about 2500lbs before driver, and almost 70 percent of that weight was over the front tires, with a leaf sprung solid rear axle that even had a welded diff! Aka, terrible snap oversteer....on paper. We had a LOT of haters that said it would handle terrible and they were blown away when we would pass E30s in the corners. A couple of the guys on our team were ride and handling chassis Engineers for GM. We ran a stiff front bar with 800lb springs (keep in mind the motion ratio is high for GM dual a-arm, so extra stiff springs become a lot less effective at the wheel) but stock rear leaf packs with the middle leaf taken out (extra soft) and no rear sway bar. Rear shackle was extended to reduce understeer angle on compression. With 255/40-17 Z2 tires on all corners it was extremely stable and forgiving, just the slightest amount of understeer that made it super easy to drive at the limit. I am a firm believer in a soft rear with no bar for cars that have a tendency to get loose. I don't know RX7s well, but something to think about.

Curious if you have the rate on the bar? Factory? Not questioning, just running lighter springs with a very high rate bar.

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I used to race a stock rear suspensioned 1985 GSL-SE.  At that time there were several folks running 1st gen RX7's.  Several of the folks were subject to snap oversteer.  Most of those setups were 375 or 400 fr springs and 175 or 200lb rears.  With watts link. 

 

I never had snap oversteer.  I ran 350lb fr and 150 lb rears.  Watts link.  But every bushing installed in the rear of the car was very well greased to avoid binding.  Even if the suspension mount points are overtightened (get some nylock / locking nuts) you will experience binding, and the dreaded Snap-Oversteer will be there.  The links need to freely pivot.

 

Best change to the car was going to a tri-link.

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1 hour ago, Bandit said:

Curious if you have the rate on the bar? Factory? Not questioning, just running lighter springs with a very high rate bar.

I don't know the bar rate, but to the best of my knowledge its a 1LE Camaro bar we came across.

 

The front springs are "measured" at 800lbs, but using a scale and homemade setup. They are actually 1 Ton HD G-Van front coils cut to height (EX Fedex Delivery Van from the late 80s) The coils are about as big as a coiled up garden hose, lol.

 

It was a redneck setup without doubt, we would have done better, but at the time we were all broke and since it was Lemons, we went for stock appearing parts ; )

 

 

Edited by UglyBoost91
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5 minutes ago, UglyBoost91 said:

I don't know the bar rate, but to the best of my knowledge its a 1LE Camaro bar we came across.

 

The front springs are "measured" at 800lbs, but using a scale and homemade setup. They are actually 1 Ton HD G-Van front coils cut to height (EX Fedex Delivery Van from the late 80s) The coils are about as big as a coiled up garden hose, lol.

 

It was a redneck setup without doubt, we would have done better, but at the time we were all broke and since it was Lemons, we went for stock appearing parts ; )

 

 

Beat Beemers. Who cares if you used 1 ton van springs. 🤣

 

Took an 1100 pound spring out of the Monte. Went a different way.

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45 minutes ago, Al36rx7 said:

I used to race a stock rear suspensioned 1985 GSL-SE.  At that time there were several folks running 1st gen RX7's.  Several of the folks were subject to snap oversteer.  Most of those setups were 375 or 400 fr springs and 175 or 200lb rears.  With watts link. 

 

I never had snap oversteer.  I ran 350lb fr and 150 lb rears.  Watts link.  But every bushing installed in the rear of the car was very well greased to avoid binding.  Even if the suspension mount points are overtightened (get some nylock / locking nuts) you will experience binding, and the dreaded Snap-Oversteer will be there.  The links need to freely pivot.

 

Best change to the car was going to a tri-link.

As I recall, no expert on Mazda's by any means, they are subject to bind like a G body stock rear. The bind spikes the true rate through the roof no matter what springs you have.

 

Get rid of the bind and then you can tune the rear rate.

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One more thing if I remember correct on the old 4 link...Bind became worse as the rear was lower.  Then the rate spikes as Bandit says.  A few band aids....but a tri-link setup and panhard does wonders for a first gen

 

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If it is similar to a Fox, what we used to do on the mini-stock (same thinking as posts above) is not run urethane in the uppers, I did not see it mentioned if you were or were not running urethane so I thought I'd throw that out there... we'd run the softest looking rubber bushings we could find, and (as stated in above replies) make sure they are lubed, use nylocks to avoid over-tightening, etc etc all to keep them moving free. The lowers we would run urethane in, since they are straighter. All a compromise with the stock opposed-arm 4 link stuff. 

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On 5/27/2018 at 3:59 PM, wvumtnbkr said:

 

In fact, we just put an entire 2nd gen subframe under the front of our 1st gen.  Much more front grip and precision with this subframe.  The rear is still capable of producing enough grip that the car is neutral now (instead of a bit pushy).

 

 

So this is a super common swap in general and works great in lemons, provided you can find one for 'free', how did you calculate the vpi for running this in champ?

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Great Question....

 

 

We ended up talking with Mike Chisek at WGI this past weekend and he and the tech guys went over the car.  They agreed with our valuation of 80 pts.  It is detailed in the linked thread.

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3 hours ago, wvumtnbkr said:

Great Question....

 

 

We ended up talking with Mike Chisek at WGI this past weekend and he and the tech guys went over the car.  They agreed with our valuation of 80 pts.  It is detailed in the linked thread.

Off topic, but would a dual A-arm setup then be 100 points, 10 additional points for each upper arm? You got the tie rods included with the rack and pinion for just 10 points total, right? 

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6 hours ago, Ron_e said:

So adding rack and pinion is 10 points but adding power steering is driver comfort?  Sigh, what a fine line.

Have no idea.

 

In this scenario, the rack was 10 pts.  No power steering being discussed here.

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No way.  Both the stock configuration had power steering and so would the new configuration.  Neither one was run with power steering.

 

When depowered correctly: all the seals are removed and the rack is greased, power steering isnt needed.  Looping lines or just removing the pump sucks!

 

It helps if it is a light car with only 225 or 245 tires on it.

 

When changing from a recirculating ball steering to a rack and pinion, it is not that much different  just feel is slightly better.

 

Also, with the subframe swap, the original steeri.g box wouldn't fit, or we would have left it.  That's why we took the points hit.

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