Suspension Travel, MK4 with IRS rear

How much do the front & IRS rear suspensions move during driving?

I'd expect there will be a difference between street and track use, suspension set-up and spring rates will make a difference.

Mine car is a MK4 Challenge car, #8885, 2015 IRS rear and non-donor FFR front, both street and HPDE use. I think my ride height will be about 5" rear and 4-1/2" front. I'm in Michigan and our potholes swallow small cars, so ride height can't be too low.

I'll run 335 or maybe 315/17 rear tires and 275/17 fronts so there is some concern about rubbing. Track and street wheels will be selected for backspace/offset.

I've measured travel and can take wild guesses but would prefer info from those with more experience and first-hand knowledge. I've measured about 7" of total wheel travel but can't imagine that actual driving will get close to that with most shocks only having about 4" of travel.

I'm thinking and trying to learn.

Info greatly appreciated. Thanks.

Jim

On my 20th anviversary IRS with 18" wheels, I think while driving I'm getting a max of about 1 1/2" total travel. My body seems to be about 1/8-3/16 off center so if I go over a "dip" in the road, the travel allows for the left rear to just barely rub on the inside of the lip on the drivers side. Now, if I'm only trying to push down on either fenders, I can not really move the car up or down, perhaps 1/4" at the most.

Jim,
To a large extent the answer will depend on spring rates. I'll put some zip ties on my shock shafts and measure compression after some driving but I don't expect to have the car out until the 19th. Maybe Carl or someone else can do the same and give you the information sooner.

Jeff

Thanks Guys, info appreciated as will additional from other sources.

BadAsp427, I was thinking the movement would be in the 3" range but as Jeff mentioned spring rates will change things. Jeff, the wire ties are pretty standard for evaluating compression, any good way to measure extension?

I'm a long ways off from any driving concerns but I am trying to gauge travel and then tire widths and wheel offsets.

Thanks,

Jim

Quote:

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Originally Posted by Jim1855

How much do the front & IRS rear suspensions move during driving?

I'd expect there will be a difference between street and track use, suspension set-up and spring rates will make a difference.

Mine car is a MK4 Challenge car, #8885, 2015 IRS rear and non-donor FFR front, both street and HPDE use. I think my ride height will be about 5" rear and 4-1/2" front.

I'll run 335 or maybe 315/17 rear tires and 275/17 fronts so there is some concern about rubbing. Track and street wheels will be selected for backspace/offset.

I'm thinking and trying to learn.

Info greatly appreciated. Thanks.

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Quote:

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Originally Posted by Jeff Kleiner

Jim,
To a large extent the answer will depend on spring rates. I'll put some zip ties on my shock shafts and measure compression after some driving

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Quote:

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Originally Posted by Jim1855

info appreciated as will additional from other sources.

Jeff, the wire ties are pretty standard for evaluating compression, any good way to measure extension?

I'm a long ways off from any driving concerns but I am trying to gauge travel and then tire widths and wheel offsets.

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A lot of thoughts here - please try to appreciate my incomplete answers based on limited experience.

I think you can barely make it on 275/17s with 4.5" ride height in front - depending on how stiff it needs to be sprung to balance the 2015 IRS. I am Mk4 Tbird IRS and the rear has to be sprung "stiff" to keep from running out of travel on launch + shift to 2nd (750# springs).

This, in turn, requires 650 - 750# springs in front, or the car is, frankly, very imbalanced (tail happy).


So, my experience is that you can barely make it with the existing Mk4 body with 275/17s in front, and 315/17s in rear - it's going to require 4.5" ride height in front and 5" ride height in rear - and it's going to have to be "sprung like a go-kart" at that.

Or you're going to have to cut fenders + realign fiberglass work.

Because you're simply running out of room between the chassis and existing Mk4 fenders on both ends (with 315/17 + 275/17) + 4.5/5.0" ride height.


I wish I could tell you anything about the 2015 IRS spring rates, but I do not have that information or experience.


Zip ties on the shocks are a great source of " real world" info. (you're in trouble on compression, not extension).


Good luck,

Mike

Mike,

Thank you for the info. Yup, I'm evaluating and trying to figure out what will or may work. Ride height can change, nothing absolute with that.

I've seen 315 rears fit but would like 335s. We'll see what happens with the 275 fronts but have heard that it's close. I'd rather not modify glass, either with tire contact or a saw.

Lots to learn, I'm measuring and calculating. Kinda dangerous actually.

Jim

Quote:

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Originally Posted by Jim1855

Thanks Guys, info appreciated as will additional from other sources.

BadAsp427, I was thinking the movement would be in the 3" range but as Jeff mentioned spring rates will change things. Jeff, the wire ties are pretty standard for evaluating compression, any good way to measure extension?

I'm a long ways off from any driving concerns but I am trying to gauge travel and then tire widths and wheel offsets.

Thanks,

Jim

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Won't extension (droop) be limited by the fully extended shock? (unless you have droop limiters) Can't you just jack up the frame and measure?

davekp,

Yes, droop will be limited by shock travel but I don't currently have shocks and I'm trying to evaluate what will be needed and why. From the info in previous posts it seems that total wheel movement during driving is 1-1/2", I would have thought a more, maybe 3" total. This needs to be translated into shock travel which is about 60%F and 80%R of wheel travel.

I am leaning towards the Breeze QA-1 DA offerings but I have to know why they are the best reasonable selection. Fortunately I'm not in a hurry to buy and many options are being reviewed.

Please keep those cards & letters coming. All responses are greatly appreciated.

Jim

Quote:

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Originally Posted by Jim1855

Yes, droop will be limited by shock travel but I don't currently have shocks and I'm trying to evaluate what will be needed and why. From the info in previous posts it seems that total wheel movement during driving is 1-1/2", I would have thought a more, maybe 3" total. This needs to be translated into shock travel which is about 60%F and 80%R of wheel travel.

I am leaning towards the Breeze QA-1 DA offerings but I have to know why they are the best reasonable selection.

Please keep those cards & letters coming. All responses are greatly appreciated.

Jim

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To be clear - what I am saying is that 275/315 tires are barely going to fit between the chassis + Mk4 fenders at full compression (without massaging fiberglass) - droop just keeps giving you more room.

One nice thing I can see about the DA shocks is that you can adjust both actions without taking the shock out, but let's go back to this a minute:

Quote:

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Originally Posted by Jim1855

I'd expect there will be a difference between street and track use, suspension set-up and spring rates will make a difference.

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To me, the beauty (and challenge) of driving these powerful, light, short wheelbase, rear weight biased cars is how quickly you can change the handling of the car by what you're doing with the throttle.

So being able to adjust the shocks quick + easy is great - but it's a relatively minor adjustment compared to what you can (instantly) do using leading or trailing throttle to transfer weight to the end of the car that needs more traction (now).

That is - once you get the car sprung (front + back) to produce a relatively neutral handling characteristic in a neutral throttle corner.


I'm sure there are small gains to be made by being able to adjust shocks quickly - but once you get it sprung right "the big instant adjustment" is still (always) going to be strictly driver input (which is going to produce instant understeer or oversteer on demand, if you're doing it right).


Hope that makes sense,

Mike

I adjusted shocks exactly one time after putting them on. The only thing I change is tire pressure.

Jeff