Calling Out Suspension Gurus!!!

[FrankDaTank]

Need Advice in Preparing my GT for an Open Road Race in Texas next April.

See Link for Past Race: Link

I plan to run an average speed of 120mph and attempt to run at a max of near 140mph. This will be done on a Texas Highway with some nice long straight ways and many long sweeping turns.

I need help in setting up my car, for High Speed Stability!!!

From the previous race I found three issues I want to fix:

#1 The Springs and Shocks seem SOFT and the car seamed to bounce up in two occasions were we encountered some asphalt patchwork.
#2 The Steering feels like I’m driving on a knife’s edge, and it is hard to keep on center.

And Non Suspension
#3 Build an Air Dam/Spoiler to limit the amount of air under the car

Here is the current Suspension Set up:

Front:
• Koni Red classic Shocks ( Installed with Factory settings)
• OGTS Lowered Sport Spring
• Rebuilt with PolyUrathane Bushings, new Bal Joints, and Tie rods.
• Steering Box was taken apart and cleaned and rebuilt with new Needle bearing
• Alignment Toe was set to Zero, But have changed it to About Half a turn of one Tie rod.
• OGTS Sway Bar (edit)
Rear:
• Koni Red classic Shocks ( Installed with Factory settings)
• Stock Springs were cut 2”
• Rebuilt with PolyUrathane Bushings, and Spring seats
• Adjustable Panhard Bar
• OGTS Sway Bar (edit)

Now for the Questions:

#1 What settings should I use on the adjustable Shock, Should I upgrade to Koni Yellows. I figure Firm is what I need but I’m sacred too Firm and the car might loose control.

#2 How can stiffen up the On Center feeling at speed, Alignmnet? Steering box changes? ….?

What do you guys think??

Any Advice would be helpful especially from you Gurus!!

[jordan]

At that speed on a road with patch work for that long may be fading the koni reds, but I doubt it. They could have the dampening rate changed towards the stiffer side to lessen the vertical momentum. Having the toe at 0 will definitely make the car feel unstable as the suspensions geometry is not forcing the car to track straight. A positive toe in and more (+) caster will cause the car to track more true in a straight.

[hrcollinsjr]

You don't mention the caster settings? More caster means more straight line stability.

The old used Koni's that I acquired are adjustable. I thought maybe they were worn when I worked them by hand. Me being the sporting person I am I thought I would adjust them to the stiffest setting - I could barely move them! I've not tried them on a car yet but you can bet it won't be on that setting.

The rest of the stuff looks pretty good, I'm sure others will weight in with more advice.

Harold

[Nathan Acree]

Did you cut your bump stops in the front?

You have a lowering spring so you may be bouncing off of the stops.

-Nathan
Albuquerque New Mexico

[RallyBob]

Yup, what everyone else here said before me holds true.

If you installed a sport spring and did not cut the bump stops at least in half, I guarantee you are riding on them. As in, they ARE now your springs. No Koni is going to dampen that, and the car will bounce like a go-kart.

A GT front a-arm has a narrow bushing spacing. This causes numerous issues on stock vehicles, like a propensity to bend the a-arms, deflection of the stock bushings which wears them like crazy, and a vagueness of the steering at high speeds. Essentially, your alignment is changing as you drive. I suggest either hard polyurethane and some track rods (strut rods), OR a switch to either a harder bushing (I like/use Delrin AF) or spherical bearings. Set the toe inward about 1/8", even with hard bushings there is enough deflection that you need this much toe-in to maintain zero toe at speed (forces are pushing the tires outward).

More caster. GT's don't have squat, Manta's have a lot, which is why the steering self-centers on them and one reason why they drive well at speed. Max the upper a-arms out with shims, but more importantly, make sure they are equal side to side. I don't mean just via shim thickness, I mean check caster angle. The crossmembers are notoriously inconsistant from side-to-side and from car-to-car, so even with equal shims the car might have a pull to one side thanks to a upper a-arm pivot in a mis-located position from the factory. I have corrected MANY of these things over the years!

GT's bump-steer at stock ride height. Lowered they are even more horrendous. The car will wander like crazy due to this. I suggest plotting the bump steer and correcting this with appropriate machining. This is not a bolt-on modification, but it will make the biggest difference in stability.

Install the polyurethane rack-and-pinion mounts that OGTS sells, if you have not already. They are a god-send, and worth it at any price (but they are fortunately very cheap). Stock rubber mounts move all over the place, as much as 1/4" laterally. Tolerance stack is a bee-yotch!

If you are okay with the trade-off of straight line stability in lieu of faster steering, then install Opel Kadett B steering arms. They are a direct fit, and they are slightly longer, so they will reduce the steering ratio a bit. They also have NO ackermann built in, unlike the GT.
Conversely, if you want to retain the stock steering ratio, but still want a bit more stability, then you can shim the stock GT steering arms towards the center of the car. Essentially you want the tie-rod ends to line up with the lower ball joints. You will need longer bolts to do this, and they MUST be up to the task in terms of hardness. This will reduce/eliminate the ackermann. The car won't turn-in as sharply with the reduced ackermann and the toe-in, but it will be stable.

HTH,
Bob

[RallyBob]

I'd lean towards the Koni yellows. They are designed for the lower ride height without bottoming out, they are valved for higher spring rates, higher speeds, and the potential for higher ambient temps (all can cause shock fade).
Konis are also nice in the regard that they have minimal compression damping, which allows the springs to be more compliant, which in turn helps to maintain contact patch and therefore traction. They have a lot of rebound damping, which prevents the 'porpoising' we all hate so much at high speeds as well.

Probably about 3/4 stiff on the front, and 1/4 to 1/3 on the rear.

You did not mention sway bars at all, but if I were building the car for high speed stability and handling, I'd use OGTS's big-ass 1" front sway bar, and a stock Opel Kadett rear sway bar (smaller than the OGTS bar, I think 13 or 14 mm?). Huge difference. A bigger rear bar will help to reduce understeer, but oversteer at high speeds is generally a bad thing, so I'd err on the smaller side. For Bonneville, I am running a huge front bar but NO rear bar for this very reason. But I don't have to turn at all...

[FrankDaTank]

This is awesome info, Just what I need,

I Forgot to list that I had the OGTS Front and Rear Sway Bars...upss.

Ok so
#1 Cut the Front Bump Stops
#2 Add Toe (at least 1/8")
#3 Add Caster (Shims and Measure)
#4 Install Koni Yellows to sugested Settings
#5 Change the Steering rack-pinion mounts to OGTS Poly's
#6 Install Opel Kadett B steering arms or shim GT Steering arms.
#7 Stiffer A-arm Bushings and Track Rods (Will need more research/Info)
#8 Bump-steer machining (Will need more research/Info)

... Did I miss anything??

Thanks a bunch to every one especially Rally Bob, it took me a couple of reads to fully comprehend the your tips.

[jeff denton]

The Car of Tomorrow, as it was called last year in Nextel Cup, has the ultimate front chin treatment. Couldn't guess whether it works on a GT, you need wind tunnels and college degrees for that kind of success, but they sure are intimidating-looking! I'd like to see it tried on a GT.

[opelenvy]

For the bump stops it was brought up to use poly ones from Energy Suspension. http://www.energysuspension.com/pages/bsp2.html
9.9121.
Also, for racing, instead of cutting your rear springs, I would buy the lowered rear springs from OGTS. They have a set that lowers your car 2" if I am not mistaken.
Did you get your charging problem figured out? When you left the Meridian meet you were having issues.

[RallyBob]

#7 Stiffer A-arm Bushings and Track Rods (Will need more research/Info)

I finally just remembered the German word for this...zugstrebe. This thread mentions what it does and shows what it looks like. Very common upgrade for high speed GT's driven on the autobahn.

Thanks a bunch to every one especially Rally Bob, it took me a couple of reads to fully comprehend the your tips.

No problem. Hey, if it makes you feel any better there are times when I don't comprehend myself either.

Bob

[FrankDaTank]

For the bump stops it was brought up to use poly ones from Energy Suspension. Bump Stops Page 2
9.9121.
Also, for racing, instead of cutting your rear springs, I would buy the lowered rear springs from OGTS. They have a set that lowers your car 2" if I am not mistaken.
Did you get your charging problem figured out? When you left the Meridian meet you were having issues.

Thanks for the Heads up on the Bump stop, Agree on the OGTS Springs, but they have been back ordered since I started on my car 3 years ago.

As for my Charging Problem the New (remanufactured) Alternator that I had installed the day before was faulty it was only producing about 9V. I had to stop in the middle of my trip and buy a battery to make it all the way to Dallas. Basically ran both batteries dry but made it to the Hotel.

I finally just remembered the German word for this...zugstrebe.
Bob

Wow... I remember seeing that thread a while back in one of my searches, now I understand what it is for... all I need to know is how to get a set. I doubt the guys at Autozone know what a "zugstrebe" is... Still would be fun to ask.

[opelwasp]

You will need to make a set of Z bars.

[GTJIM]

In the picture linked by Bob the bar is attached too far in towards the inner end of the lower 'A'-arm. It should be attached much further out to effectively triangulate the suspension. The other (body) end needs to be positioned in the same planes as the lower 'A'-arm pivot bushes to minimise binding as the suspension works.

The whole idea is to minimise the fore and aft movement of the outer end of the lower 'A'-arm.

Pic: http://www.opelgt.com/forums/attachm...-zugstrebe.jpg

[RallyBob]

Wow... I remember seeing that thread a while back in one of my searches, now I understand what it is for... all I need to know is how to get a set. I doubt the guys at Autozone know what a "zugstrebe" is... Still would be fun to ask.

You can fab up a set pretty easily using parts from circle track supply companies like Behrents or Speedway Motors. Even McMaster Carr probably has the components to build a set. You basically need threaded ID steel or alloy tubing, high strength chromoly rod ends, high strength hardware, and some brackets. Although 1/2" rod ends would probably be just fine, a stronger/stiffer solution would be to use 5/8" ID tubing and rod ends that have 5/8" thread but a 1/2" mounting hole.

As Jim mentioned, these would be far better if you mounted the trailing ends of the Z-bars closer to the lower ball joints. They will triangulate the lower a-arms far more effectively. The leading ends of the Z-bars need to be exactly in line with the inner pivots of the lower control arm, otherwise you induce a bind.

Here's a bit of info on bump steer, including how to plot it and how to remedy.
Rack and Pinion Setup and Service

HTH,
Bob

[jtb]

Hi Frank,

You'll find more stuff on strut bars if you do a search for zugstrebe.

Do you think the weight distribution of your car is similar to stock, given your engine swap?

cheers,

jtb

[opelenvy]

Do you think the weight distribution of your car is similar to stock, given your engine swap?

cheers,

jtb

I'd say it's close to a stock 1.1 GT now. The Rotary is light.

[Manta Rallier]

Orignal query:

#1 The Springs and Shocks seem SOFT and the car seamed to bounce up in two occasions were we encountered some asphalt patchwork.

I can think of 3 things here:
- The softness is a too soft shock. When you couple this with the stiff springs, the spring-shock combination will be what is called under-damped; it's a mismatch of spring rate and shock rate, and this will cause the car to float and bounce up and down a bit after any sizable bump or crest. If the yellow Konis are stiffer, then go with them. (This statement qualified below.)
- I have to ask: When you say 'bounce' on the asphalt patches, does this mean that the rear feels like it is kicking up in the air, or does the whole car bounce and float. If the former, then your spring on the rear is too stiff; put in a softer rear spring and keep the softer shock. This happens all the time in rally cars: stiffer springs are often needed all around to meet harsh road conditions, but under certain conditions, like hitting a ditch accross the road, this will always cause the rear to "boiiing!!" way up in the air relative to the front. A nose heavy rally car should wisely run consistently softer spring and shocks on the rear.
- I have looked at this race online before, and it is both exciting and scary! One thought I have consistently had about this event is that there have to be roads crests and undulations as well as rough patches like you mention. IMO, this event really begs the use of RALLY shocks, NOT road race shocks. Rally shocks are valved somewhat differently than road race shocks for the very purpose of maintaining car stabilty in these conditions. I would strongly suggest that you present the above question and background info on this site:

Rally Anarchy - Don't Panic

under the Construction Forum. (Don't ask the steering stability questions; the Opel experts on that area are here!) You will probably get the most knowledgeable answers on this shock-spring setup challenge of anywhere in the USA. Now, realize that rally cars typically use coilover combination, and making that conversion is not trivial for your car; I would not do that at all. (I AM doing the conversion on our Manta A rally car build and so know the structural problems that need solving for this conversion.) But you can also buy Bilstein rally shocks and not add the coilover part; their rate would be matched to your stock mounted spring rates to get the right combo. A fellow named John van Landingham in the Seattle area is one of the knowledge guys on that site. He is VERY crusty, and just has to insult your intelligence at least once before he gets comfortable with you, but listen anyway!

BTW Koni shocks are pretty well ignored by ralliers as shocks; Koni only seems to REALLY focus on road race shocks. Bilstein is a leader in mid-priced rally valved shocks; we are using their rally coilovers for our Manta rally car.

Good luck! I think saw a photo of your car from last year!
Mark B.

[FrankDaTank]

Orignal query:

#1 The Springs and Shocks seem SOFT and the car seamed to bounce up in two occasions were we encountered some asphalt patchwork.

I can think of 3 things here......

Good luck! I think saw a photo of your car from last year!
Mark B.

Mark thanks for the insight,

I Agree 100%, I have a mismatch in the front and in the rear. In the front I think the Koni Yellows and cutting the Bump stops will fix this.

In the Rear I have the Stock Springs Cut so they are definitely to stiff. I think I found a set of sport springs in Europe that I think will work better. And I think thats why Rally Bob is recomending the softer settings in the rear shocks.

I will look into the other shock options, but Im not sure if I want to run 100% competition shocks, price and comfort, I do drive my car around town.

Thanks

[opelenvy]

A buddy of mine used to run Koni's in the front and Bilsteins in the back on his street car.

[Manta Rallier]

I will look into the other shock options, but Im not sure if I want to run 100% competition shocks, price and comfort, I do drive my car around town.
Thanks

Understood on the street driving; I agree there. But, at a 120 mph average speed for 59 miles down and then 59 miles back, I think it is a bit unwise to not have at a minimum:
- properly matched springs and shocks in the suspension, with proper shock valving
- the best tires
- a very comprehensive cage
- good belts and racing seats

Having the motor let go won't kill you (or your codriver if you have one). But this speed is a huge stress on suspension things and if anything goes awry, you are in big, big trouble in a little car with minimal caging.
(BTW I have rallied 30 years and am a rally scrutineer and organizer too, so I think more about the 'what ifs' than I did when a competitor only.)

Anyway, it won't hurt to find out. See what it costs versus your safety-comfort level. We're not talking about A $3000 DMS or $5000-7000 Ohlens shock set, we are talking <$1000. I would try to get real rally shocks for safety, and then just take them out during the rest of the year. And I GUARANTEE, once you use them you will say 'I can't believe the difference' and 'The control improvement is beyond what I ever expected'. Honestly, in this type of race situation, having real shocks will be the biggest performence eye-opener you will ever expereince.

BTW, maybe Bob Legere can comment: Do the things like the tiny Opel spindles ever break? I have had a spindle break at low stage speed, and it opened my eyes to what will happen at high speeds if the littlest front steering or suspension thing breaks.

Regards,
Mark B.