ITC Rabbit suspension help

[veeman]

So... we're trying to sort out the suspension on the "new" ITC rabbit. I've been doing a bit of reading here in the archives and talking to some folks about spring rates / bars. There seems to be quite a variety of opinions...

The car was originally an ITC car but became a track day car for a while, so the setup as it came to us wasn't exactly "full-on". I know that a lot of suspension tuning has to do with a number of factors (tires, weight, driving style, track, etc), but I'll try to detail what we've got below...

Current setup on the car:
Shocks: Carrera coilovers, custom-valved fronts with mid-range rebound and more compression. Rear shocks are standard valved Carrera race units.
Bars: None in front, stock rear bar along with what appears to be a H&H straight bar on the beam
Springs: 350 front, 275 rear
Tires: Kumho 215/50-13 on 13 x 6 rims (most likely)
Diff: Open for now, possibly welded in the future
Weight of the car: Hopefully under 2075lbs with me in it.

Common wisdom from this forum: (gleaned from archives)
Front springs: #600-650
Rear: #350-400 rear
Bars: no front bar, most advocate some sort of rear bar configuration

Advice from well-known respected champion IT VW racer #1:
-Don't go above 450 -500# on either end. Anything more and the spring won't compress...the chassis will flex instead.
-The FRONT should be softer than the rear and having them be within about 50 or 100# of each other (rate). Something like #450 in the front, #500 in the rear with swaybars in the back only.

ITC rabbit guy #2
-He runs 650 front and 600 rear with a rear swaybar (solid Neuspeed). He says that the car was quite a bit faster like this over the other setup he had (600 / 400?).
-He says that the stiffness in the rear really helps the car rotate and push less. He was running same tires as I plan to use and an open diff. He also said he thought that 600 front and 650 rear would have worked a bit better as well.
-His car is noticeably lower in the rear than in the front...definite rake towards the back.

Shine Racing (SRS)
-They say to go for 500-550# fronts and around 400 rear with big rear bars. No front bars.
-Their recommendations focused more on replacing the bushings in the control arms and rear beam pivot point so that there's ZERO deflection in the suspension...keeping the shocks at the right angle and the tires on the road.
-They recommended sticking to close to stock tire sizes and keeping an eye on the rear beam in case it becomes too flexible / worn out due to age.

Does anyone have any thoughts on these? The rates are within the same general ballpark, but the philosophies seem to differ a bit. Mid-Ohio is my "home" track although I'll run at ORP, Nelson's (if it remains open), Putnam, etc...

I've got a bit of experience with rabbits...autocrossed one for 4 seasons and don't mind a bit of rotation...then again, I don't necessarily need to be fighting a tail-happy car in loose grip situations all the time either...

Can anyone make any suggestions? I know I'm in for some changes, I just want to buy smart and start from some sound experience.

Thanks!

 

[markw]

Mid Ohio, can be slick. I found to stiff a rear on my GT car was nerve racking. I hate push, but a car that wants to loop is no fun either. You are smart to ask, but as you have found, everyone has their own ideas. Here are two options. Take all the front spring rates and average them. Do the same for the rear. Start with these average rates. Second option would be to leave your fronts alone and start with the rear. You have a 275, rear now. Purchase some 400 rear springs and run the car. You will get an idea from this change how spring rates affect handling. In the end, it will come down to personal preference. If you can setup the suspension with the same free height springs front and rear, it makes it easier to swap springs front to rear to try different setups.

 

[Sandro]

I was in the same situation as you about a year ago. Million different setups all from reliable people and didnt know where to go. Everyone said no front bar(just like you also said) so that was easy.

Then someone mentioned start with the corner weights of the car as a starting point. That sounded like a good idea to me and it was kind of a good starting point between all the recomendations. I went with I beleive 600# front and 400# rear and started with the original GTI sway Bar in the rear. It was pushing quite alot so I added another sway bar(I think its a 1inch diameter) and the car is a tick loose in the rear but very predictable, and the inside rear lifts very slightly in tight turns.

 

[chewy8000]

I'm not sure about removing the front sway bar? I've heard from a ton of people to do this but a buddy of mine has been running an ITC rabbit VERY successfully for years and swears it's better to keep it on. I have never run mine without it and don't plan to. I run a massive bar in the rear and toyo shaved 205/55/14's, but I'm ITB. The camber/toe can make all the difference on these cars.

 

[markw]

Did any of you watch Nascar at Sonoma on the Road Course. There were some good camera angles of the the front inside tire being lifted off the track in the turns. This comes from too stiff a front roll bar. It is not as much of a problem with RWD. A stock gti bar is flimsy at best and probably has minimal affect on handling. Too much front bar and you will lift the inside front wheel aggravting wheel spin. Driving style plays a larger part. Are you slow in fast out? Fast in slow out? Do you trail brake in, or do you brake in a straight line and then turn in. Too many variables. It is not that anyone is not willing to share their setup, their preference plays a big part in to what they like. I can tell you this, if you are comfortable, the car will be faster than if you are not. FWD has an inherint push by design. There is a compromise that needs to be found. Most fast FWD's will be loose in the beginning of the race, neutral during the middle, and push towards the end. The more you overheat the front tires the worse the push becomes.

 

[veeman]

Thanks for the input so far. Mark:

If you can setup the suspension with the same free height springs front and rear, it makes it easier to swap springs front to rear to try different setups.

Fortunately, the car has 8" length (2.5" ID) springs at both front and rear so that'd be possible. I thought about buying a whole range of springs from 450 to 650, but then I realized they're about $65 each.
Seems like ebay doesn't have many auctions for this type of spring and the prices are the same in the roundy-round racer catalogs. I still may buy more than one set, but I want to narrow it down a bit.

Driving style plays a larger part. Are you slow in fast out? Fast in slow out? Do you trail brake in, or do you brake in a straight line and then turn in.

For me it depends on the corner, but in general, I try to get my braking done in a straight line so I can concentrate on doing the corner correctly without pushing in the middle. I try to find the "ideal" amount of speed to carry through the corner so that when the car is pointed in the right direction after the apex, I can start rolling on the throttle.
Someone told me once that I should imagine a string between the steering wheel and the gas pedal. With the wheel turned, there's not as much slack and the gas pedal needs to be more "up" however with the wheel unwound and straight, the pedal can be down.
I definitely don't like understeer though (who does?)...and hate the feeling of overcooking a corner and being pushed off my line by too much speed, greasy tires.

I went with I beleive 600# front and 400# rear and started with the original GTI sway Bar in the rear. It was pushing quite alot so I added another sway bar(I think its a 1inch diameter) and the car is a tick loose in the rear but very predictable

This is very close to my original plans for the car... I have a hollow 26mm Autotech bar at home that I could use in the rear for that. I then heard about keeping the front and rear close to each other in terms of rates and even the idea of having the front softer than the rear. This got me wondering...

I'm not sure about removing the front sway bar? I've heard from a ton of people to do this but a buddy of mine has been running an ITC rabbit VERY successfully for years and swears it's better to keep it on.

I've heard that as well, Chewy, but it was mainly during my autocrossing years. There was an ongoing discussion (read: p-ssing match) on the vw forums about NFB (no front bar) and various rear bar setups compared with those that ran a front bar. People argued about whether front "droop" was more important for traction (sans swaybar) rather than having the added roll stiffness / weight transfer control of the bars, etc. I think that's why most people don't run one in road racing. They control the car's roll with springs rather than the swaybars and then rely on the added suspension travel to keep the tires on the road...

 

[rbt510]

Veeman -

I would go with what Shine Racing reccommends as a starting point. They have been doing this for a long time. You did not mentions what you have for camber adjustment or what toe specs you use.
I ran a ITC rabbit for a few years, and once you are happy with ride, these adjustments had the most impact on lap times. Also, you also MUST get the car weighed. (with driver and fuel) If the corner weights are all wrong, you will never be happy with the handling of the car. And without this information, you won't be able to make the right adjustments. What seems like a good handling car at one track, will be all wrong for the next track just because of weight.

Make small changes, and compare your times to the leaders on any particular day. Your last weeks times are never the best indication of improvement by itself.

Good Luck.

 

[shwah]

I run a front bar. It is not a huge one, but it works for me. I am less worried about unloading the inside wheel because of the weld beads inside my differential. The car does stay noticably more flat - with maybe an inch or two of rear tire lift max. This helps keep the outside front camber more consistent when cornering. The same can be accomplished with real big springs, but at some point the chassis will just flex anyhow.

 

[racer_tim]

Don't forget, if you decide to go with a spool or a welded front diff, that all the rest of your adjustments will go out the window.

Your driving style (brake in a straight line) and then hit all of your marks, is EXACTLY what you need to do with a locked front diff. In order for it to turn, you have to be on the gas.

I've been running a locked front diff for years, and have adjusted my setup and driving style to utilize the added traction that you get from both front wheels bitting all the time.

Like others have said, corner weights and alignment have a lot to do with you setup, and how you like the car to feel.

 

[Beran]

I'd go with the shine setup to start with then work from there. No bar up front gave me the opportunitity to set current track record at Road Atlanta. IMHO no bar up front is faster.
I found that it depends upon how new (sticky) your rear tires are going to be... You will need to find what you like for how loose you can run the car... this is a balance between how fresh your rear tires are and how comfortable you are with a loose car. Fresh tires will allow you to run stiffer springs in the rear but when the tires go away then you might end up with a car that is too loose. Mid-Ohio does have a couple off camber light in the rear (elevation changes) turns that might prevent you from going too stiff in rear. Run the rear ride lower then the front.
The spherical bearings are a must especially in rear which ends up with lots of slop without them. If you run new tires all the time setup for that... if you run 1 set of tires for many races then setup for this.
again... just my .02 and mostly from MK2 & MK3 data.

 

[flaboy]

I run an itc rabbit.I run bilstein coil over front and rear with 550lbs front and rear with no front bar and the biggest bar i could find for the rear.The car is pretty easy to drive abd pretty quick.

Tim

 

[Eric Parham]

My ITC Scirocco had 650 lb/in front springs, no front bar, 425 rear springs, Shine rear bar. That was after many iterations with fronts between 400 and 750 and rears between 250 and 500. I found that different brands of springs were off up to 10%. I ended up with Hypercoils to reduce variables. My setup was on the hairy end of loose (even with new Hoosiers), and a real handful in the wet (400 rears or some sort of adjustability was wished for when the heavens let loose), but was definitely fastest that way. I almost always ran a VWM clutch-pack LSD or a Quaife TBD. As others have mentioned, a spool or welded changes everything (have one, only ran it once -- not my preferred style).

EDIT: I also had a very stiff cage, and Shine's spherical bearings front and rear.

 

[veeman]

Thanks for all the input!

Don't forget, if you decide to go with a spool or a welded front diff, that all the rest of your adjustments will go out the window.

Tim... Out of curiosity, how would the setup change with a welded diff? Would the car have to be softer in the front (that's what I'd heard) so that the car can grip / slip appropriately? Sounds like I need a "spring library" so I can check out different rates to try them out / swap with others!

Eric... your setup is what I had on paper initially to run, but now I'm considering going with something more "moderate" like Shine is recommending while keeping the rear bar.

Speaking of which, I looked at my rear bar again and it's pretty stout. According to the PO, it's a custom piece made by Walt Puckett...very similar to what he ran on his scirocco. It's a straight bar (probably around 23-25 mm) with square ends that's held in place by a block on either end end of the beam.

As for corner weights, I'll be figuring that out soon (we're working on redoing the cage now). I'd like to get the cross weights within about 3-5% of each other.

The spherical bearings are a must especially in rear which ends up with lots of slop without them. If you run new tires all the time setup for that... if you run 1 set of tires for many races then setup for this.

Beran... are you talking about the spherical cartridges Shine sells for the rear upper mounts? Right now the car just has urethane on them. The carrera's have a monoball style bottom mounting already...

You did not mentions what you have for camber adjustment or what toe specs you use.

Shooting from the hip here, I'd say that the Kumho's need about 2 degrees of camber in the front (need to verify) and I'm looking at running probably about 1/8 inch of toe out.
In the rear, I'm still debating. I've never really messed around with shims, but I'd like some camber in the rear (1.5 degrees?) along with a touch of toe in (1/16" inch?). Not sure I'll have time to experiment with the rear alignment before running the car. That may be too many variables right now.

Feel free to chime in with other ideas on alignment settings if you'd like.