Extreme Rear Suspension Compression

The spring rate is what?
it should be 450lbs, if you are running the correct spring rate, your shock that's adjustable,gives you the option to turn it up on compression.
The actual compression or movement on body is only around
3 inches, stiff 1 inch. That's with you in it.
I would check the rub interference on a 3" body drop. Their is also the camber, have you checked that? I would recommend neutral camber with that wide tire. roll the fender it will let you move the back spacing a little.

Devin, I am not familiar at all and having a hard time visulising BUT I'll throw out something that may or not be useful.

Besides thinking of the total travel as when the shock bottoms out, I would want to KNOW if the springs are going solid (coil to coil).

You can accurately measure the coil wire diameter, count turns and come up with the spring solid lenght.

Chris,

Running 550lbs in rear and the rebound is adjustable. They were preset by Quella when I purchased them from him if that makes a difference.

Also when a-arms are parallel to the ground the distance between shock ears is approximately 13 1/4 inches.

JFB, I will look at your recommendation also.

Thanks for the input so far.

Devin

I would take the spring off of the shock and reinstall the spring. You can then compress the suspension thru its full travel. If the tire contacts something and you are worried about the contact point ruining the tire, then that should be considered your maximum compression. Mark that point on the shock. You then need to add a bump stop to the shock to make sure the shock cannot compress any further than your desired maximum compression. The bump stop will compress a bit before it acts like a hard stop and different stiffness bump stops are available to help control the hit before max compression is achieved.

You cannot rely on the spring to completely stop the compression of the suspension unless the spring is in full bind which is of course very bad. You'll never know when you could hit something in the road and rapidly overpower the spring via a near instantaneous force. The hard stop should be engineered into the shock via a bump stop.

You could put your old tire and shock back on, and sit on the rear ( with the lid up ) sill and see how much your car goes down, put a couple friends on it and measure, it doesn't move that much.

quote:

Has anyone ever drove their car so hard that the rear lower a-arm has rubbed or hit the stock exhaust? I guess the potential is there but was just wondering everyone's thoughts. I don't think I will ever be in that situation but I don't want to ruin some high dollar tires.

Any thoughts or recommendations? I am considering dropping to a narrower tire. I know Foose's narrow bodied Pantera is running this tire and wonder if his would have the same issue under extreme compression.

Thanks.

Devin

Devin,

In my opinion, unless your exhaust us touching your a-arms at some point, then your exhaust is too small ;-)

FYI, I ran a 3" exhaust through my suspension, and like you, also set the ride height with the lower a-arms parallel to the ground. The a-arms just slightly dimpled the exhaust pipes. My upper a-arm pick-up points were non-stock, but this may still be a valid data-point for your build.

As far as tires, I really like the look and stance of the Foose Pantera. The 19" rears remind me of the race Pantera wheels from the 80's, which as you can see from my post here Pantera Race Wheels where much wider that what you're considering for the street. Although some "slight" modifications to the chassis were required to make them fit.The suspension travel was easily far enough on the race track to impact a (stock) chassis on the inside.

Good luck!
Dave

I guess I have a mental block. My current rear 17 inch wheels are running a 12.4 inch wide tire that looks great in my opinion.

The 325/30/ZR19 is 12.8 inches wide. It is amazing how much wider 0.4 inches looks on a tire in person. I really want this tire to work because of the width.

If I drop down to a 285/35/ZR19 for more side clearance than the tire is now only 11.22 inches wide. If 0.4 inches looks so much larger in person than I am afraid that 1.58 inches (19 inch vs. 19 inch) or even 1.18 inches (my current 17 inch vs. 325/30/ZR19) will look tiny.

Funny because 11.22 inch wide is actually pretty large. Thanks for all the advice.

Devin

Just to throw in another perspective.

I have 345-30-19's on the back of my narrow body Pantera.

I can lower my car to the 11.2 compressed shock length very easily as I have air suspension. I wouldn't drive it that low but is does lower to that height without damage to the car.

The tires WILL rub slightly at that height but it is only there for car shows. I doubt you can compress your suspension to 11.2 with 550lb springs, even hitting the largest pothole available.

I had 325/25R20 tires on the back of 6018, 12mm offset.

The outer edges of tires were flush with the edges of the fenders, required rolling the back half of the fender edges on both sides.

After rolling the fenders I disconnected the shocks and raised the tires within the wheel houses with a floor jack, the tires rubbed up inside the wheel houses on the inner edge & outer edge simultaneously, therefore I am under the impression 12 mm offset centered the tires very well within the wheel houses in terms of clearance.

However, like you, I would have preferred if the outer edges of tires were set inside the fender just a bit, say 1/4". So if I were doing it again I would use 18mm offset instead of 12mm offset. This may cause some muffler rubbing issues that will need a little "massaging" for clearance. If you have an ANSA exhaust the "possible" problem areas are going to be the outer corners of the mufflers nearest the tires. On the positive side increasing offset may also negate the need to roll the fender edges.

With the tires jacked-up until they hit the wheel houses the suspension did not contact the tail pipes. Those 20" tires were 26.4" diameter. Your 19" tires are 26.67" diameter, so there should be 1/8" more clearance between the suspension and tailpipes when the tires contact the wheel houses.

I did indeed have tire rub marks up inside the wheel houses, indicating the suspension compressed enough for that to happen from time to time. But nothing ever hit anything other than the tires rubbing. The rear springs on 6018 are supposed to be about the same as those on a GT5, which is around 500 pounds per inch.

Also keep in mind the advice of Dennis Quella passed-on by David Nunn to add enough "rubber bumper" to the shocks to prevent them from ever compressing to the point of the tires actually rubbing the wheel houses ... or the suspension hitting the tailpipes. That's good advice.

Those are my experiences with a tire of identical width and similar diameter.

Devin,

Your Koni's have a minimum length that's a bit less than the OEM shocks, thus allowing the suspension more upward travel than it should have. As George mentioned, the easiest way to deal with this is to install an extra bump rubber in your shocks. There are many different bump rubbers available from Koni but the one to add is the same one that comes with the shocks. This will decrease your suspension travel, under compression, by about about 3/4".

I have 18" x 12.5" rear wheels (ET 16) with 335/30-18 Hoosiers. The Hoosier's are about an inch wider than any other 335/30-18 tire. I had a problem with the tires rubbing the upper frame rail but the extra bump rubber solved the problem and you'd never know they were there. Now, I have absolutely no rubbing anywhere. As far as wheel offsets are concerned, be careful going too far inboard. In the case of HRE, the difference between a 12mm offset and an 18mm offset is the thickness of the center, so if you felt 12mm was too far outboard, the centers can have a few millimeters removed. As I mentioned earlier, my wheels have 16mm offset, so 18mm should be fine with 325 width tires, but I wouldn't go any further inboard than that.

As an aside, one of the benefits of the high pressure gas Koni's is, they control high spring rates very well. In an effort to fine tune the car's balance, I upped my rear spring rates from 550# to 700#! Believe it or not, there was absolutely no perceptible increase in ride firmness. I was very surprised! The car felt exactly the same after, as before. I'm sure other modern shocks are just as good.

Thanks again everyone for the great insight and advice. Sorry for the delayed response but we headed to the remote mountains of Wyoming for the Christmas Holiday. One of the few places left on earth without cellular service and internet. Talk about be disconnected - I had to drive halfway down the mountain to get cellular service.

The offset tool I bought to test fit the new tires is showing 13mm as optimum offset in the rear. I tried going farther inboard to 18mm but the driver's side tire rubs very hard on the upper inner frame rail with the suspension compressed approximately 1 inch from ride height when set at this offset. Likewise, at 13mm offset I will also still need to roll the rear fenders due to rubbing issues.

This is not quite as far inward as I would like (love the tucked in look of the rear tires at 18mm) but 13mm appears to be the best offset on rear wheels of my car. Also at 18mm the passenger rear tire rubs on some the exhaust, however, this would be fixable as George had mentioned.

The front suspension with the 245/35/ZR18 seems to be best at an offset of 19mm. However, the passenger side has issues with rubbing the window wiper motor cover at full lock with this offset. With a little fabrication of a different cover it should be no issue. I tried 18mm as recommended by others but I had issues with the front tire rubbing on the front portion of the fender when compressed.

All I have so far. Any other thoughts or recommendations out there? Hope everyone has a safe and Happy New Year's Eve tonight.

You can go 225 on the front. no issues

Chris,

Great picture and nice looking brakes. Are you running 225/40/18 up front? Also what width of rim are you running?

Thanks.

Devin

225/35/18, the rim is 8 x 18

Thanks Chris. What size tires and rims are you running on the rear? If memory serves me correct you were running 18's in back also.

Devin

oowps!!they are 18s 295/35, I also run 285/35 the tire's are nitro on the back and michlen 2000 on the fronts

additional fit pic's

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