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Discussion Starter · #1 ·
I am looking to upgrade my shocks for ITB competition. Is there any experience on the list with performance shocks? It looks like the selection for Manta A's is getting pretty slim. I was looking at SPAX, but have also considered custom racing units by Ground Control. or having a set of Bilsteins vevalved for my springs.
 

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What spring rates are you running on your Manta? Ride height compared to stock?

Bob
 

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Discussion Starter · #3 ·
Hi Bob,

As far as ride height I would have to measure to be exact (the car is at my shop) but I woud say it is about three inches lower than stock. I know I do not bottom out at all, and that the rockers are not lower than the 5 inches the SCCA allows in IT.

The spring rates are with my build sheets that dissapeared years ago. I do remember that I went through drivers school (1987)with a cut set of stock rally springs and then had a new set made that were 40% stiffer because I liked the balance. I do not remember how we came up with the rear rates because my progressive wound rally springs were trimmmed a bit top and bottom. Currently the car has a set of KYB gas- adjust. Because they are so stiff and are high pressure gas they raise the effective spring rate a bit. All bushings are delrin right now and are still in good shape however to start with I want to replace the lower control arm bushings and one bushing on each of the rear trailing arms to sperical bearings to eliminate any binding or metal fatigue as the suspension pieces go through their arc.

The bottom line though is that last year when I had the car back on track The handling was as balanced as ever. Not real stiff, but very forgiving.

I will start racing again with the current setup, but at some point if I want to be near the top of the ITB field I think I am going to start from scratch. I have learned a lot since I built the Opel and have set up other cars for track use.

Heck my daily driver Volvo 740 turbo has adjustable suspension all around with 450lb springs in the front and 250lb springs in the rear. Re- valved Bilsteins. The complete suspension is spherical bearings. Yata Yata Yata. (a cage away from ITE with 268 rwhp and 332ft/lb of torque on pump gas).
 

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Mantas have a stock front spring rate of 135 lbs, but the rear progressives start at 113 and go to 180 under full load. However, for racing, since the Manta's front motion ratio is near .6355, the front rates must be increased by at least 300%, and as high as 400% or so. I typically run front springs on the order of 375-380 for the road, and 500-700 for racing, depending on the track surfaces. Rears usually only need to be near 175-225, again depending on surfaces, racing type, and driver preference.

Pete, this is why I was asking if you knew the rates, as the damping values can vary greatly from a 375 lb spring to a 700 lb spring. And of course, if the car is lowered a lot, the shocks must be far shorter (about 5/8" stroke reduction for every 1" of vehicle lowering), so you can either use '74-'75 lower control arms and bolt the shocks through the lower mounting holes of the a-arms, and/or you can make a bolt-in bracket to lower the bottom of the shock mount (legally).

Also, do you like or dislike high pressure gas shocks? Bilstein, KYB, etc. I like using high pressure in the front, but low pressure or hydraulics in the rear, they seem to take the track irregularities better...but that may just be my preference.

Bob
 

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Discussion Starter · #5 ·
Mantas have a stock front spring rate of 135 lbs, but the rear progressives start at 113 and go to 180 under full load. However, for racing, since the Manta's front motion ratio is near .6355, the front rates must be increased by at least 300%, and as high as 400% or so. I typically run front springs on the order of 375-380 for the road, and 500-700 for racing, depending on the track surfaces. Rears usually only need to be near 175-225, again depending on surfaces, racing type, and driver preference.

>> I know the car is too soft by todays standards and too soft for the current racing tires. The numbers you suggest are in the realm I was imagining based on other cars of similar weight/design that are currently front runners. I will have to find the proper balance for my driving style. Off the top of your head do you know the ID of the stock front and rear springs? Will 5" racing springs fit or do you have your springs made for you. Are the rates you suggest for use with a welded diff?

Pete, this is why I was asking if you knew the rates, as the damping values can vary greatly from a 375 lb spring to a 700 lb spring. And of course, if the car is lowered a lot, the shocks must be far shorter (about 5/8" stroke reduction for every 1" of vehicle lowering), so you can either use '74-'75 lower control arms and bolt the shocks through the lower mounting holes of the a-arms, and/or you can make a bolt-in bracket to lower the bottom of the shock mount (legally).

>>That is why an adjustable shock is appealing. It is a pain in the butt to re-valve shocks every time you make a spring change. I know SPAX makes a shorter shock for lowered applications. Currently I do have the '74 lower control arms and the rear shocks are actually spec'ed for a BWM 2002 and are shorter that the Opel rears.

Also, do you like or dislike high pressure gas shocks? Bilstein, KYB, etc. I like using high pressure in the front, but low pressure or hydraulics in the rear, they seem to take the track irregularities better...but that may just be my preference.

<< Since I have been tuning Volvo's I have almost exlusively used Bilstein shocks and struts. I have not tried anything but the KYB's on the racing Opel. My Opel street experience was that the high pressure gas in the rear helped control the weight of the live axle better. What shocks do you run on your race car?
 

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>> I know the car is too soft by todays standards and too soft for the current racing tires. The numbers you suggest are in the realm I was imagining based on other cars of similar weight/design that are currently front runners. I will have to find the proper balance for my driving style. Off the top of your head do you know the ID of the stock front and rear springs? Will 5" racing springs fit or do you have your springs made for you. Are the rates you suggest for use with a welded diff?

The Opel springs are roughly 4 5/8"-4 3/4" OD. ID will vary with coil thickness. I have my springs custom wound, but I know of people 'hammering' the spring seats to fit off-the-shelf 5" OD racing springs. For a welded diff, you'll want to err on the high side for rear rates, about 225 lbs, with 500-550 front.

>>That is why an adjustable shock is appealing. It is a pain in the butt to re-valve shocks every time you make a spring change. I know SPAX makes a shorter shock for lowered applications. Currently I do have the '74 lower control arms and the rear shocks are actually spec'ed for a BWM 2002 and are shorter that the Opel rears.

I ask about the spring rates because even an adjustable shock does not have the range of adjustment to work with both a 135 lb spring and a 500 spring. You still need a baseline valving. But for a 500-550 lb spring, I like a split-valving shock, usually 260-370 lb. Bilstein ratings are different however.

<< Since I have been tuning Volvo's I have almost exlusively used Bilstein shocks and struts. I have not tried anything but the KYB's on the racing Opel. My Opel street experience was that the high pressure gas in the rear helped control the weight of the live axle better. What shocks do you run on your race car?

Don't you mean what shocks haven't I run?:) I've tried nearly everything. I've used KYB, Koni, Bilstein, Pro-Shock, Rancho, Spax, Carrera, Doetsch...haven't had the funds for Ohlins or Penske though....
My favorite combo has been the Doetsch for the front (high pressure) and either the Rancho for the rear or the Pro-Shocks. The Ranchos (yes, truck shocks) are at least adjustable, but they take the bumps a lot better than anything else I've tried.


Bob
 

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Peter,

I had custom springs made from Coil Spring Specalities, www.coilsprings.com. I believe they got all (or at least most) of their spring info from RallyBob, so they should have info on 500 lb by ? inch drop springs.

Do a search on this site. You should find 1 or 2 threads that discussed spring rate and ride height combinations.

Their prices seemed reasonable at $138 (plus shipping) for a pair.

I had my Bilsteins revalved for the new springs. Since I havn't put the car back on the road yet, I don't know how well the valving we selected will work. My target to get it driveable is October.

GL
Paul
 

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Paul said:
Peter,
I had custom springs made from Coil Spring Specalities, www.coilsprings.com. I believe they got all (or at least most) of their spring info from RallyBob, so they should have info on 500 lb by ? inch drop springs.
Coil Spring Specialties has info on some 500 front/ 200 rear springs that are 3" drop front and 2.5" drop rear. You can always modify that spec of course. I used to sell that set for ITB or GT-4 Opels.

Bob
 

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Discussion Starter · #9 ·
Thank you Paul and Bob,

500/200 would be a good place to start. With the 3''drop in the front and the 2.5" drop in the rear will the car have a rake or will the rockers be flat? Any idea what the rocker height will be?
 

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The car will have a very slight rake, but it will be legal ride height. Unless you run 175/50-13 tires or something else awfully short....

Bob
 

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Before talking about shocks, you must consider what they do. Going straight down the race track they only keep the car from bouncing on the springs like a pogo stick when you hit bumps and ripples. Once that is accomplished they do nothing else in a straight line. Almost any shock will do this.

In STEADY STATE cornering, shocks have no effect on handling. In the middle of a sweeping corner it is only the springs and sway bars that can change the understeer/oversteer characteristics of the car. So, before doing anything with shocks you have to have the right springs and sway bars. What this really means is having different springs and adjustable sway bars for various tracks and/or track conditions.

So, where do shocks come in for handling? They have a MAJOR effect on transitional characteristics. This means when you first turn in to a corner... accelerate and/or straighten the steering wheel on corner exit. You may have a car that is perfect through the middle of the corner, but as soon as you accelerate it understeers or oversteers. Transitional characteristics are modified by changing the compression and/or rebound settings on the shocks.

There is no such thing as one combination that works everywhere. Watch the NASCAR folks during "happy hour". They are changing springs and shocks for the whole session. Then during the race they are adjusting something at every pit stop.

So, what shocks? I use Ground Control double adjustables in everything I race. (I not only use Ground Control shocks, but whatever else they make when I can... they make really superior stuff for McPherson strut cars, but nothing unique for the Opel. Like everything I like, I become a dealer for Ground Control).

All double adjustable shocks are expensive... $399 each for the Ground Controls and well worth it. However, for any adjustable shock to be worth the money you pay, you have to use the adjustments. I can't believe how many people buy double or triple adjustables ($1000+ for "triples")... then set the shocks once and the adjustments never get changed.

So, for an alternative I have another recommendation. For a lot of years I used the Koni single adjustable "yellow racing" from the Opel GT Source and found them to my liking... and the price is good. They only have rebound adjustments, but if you are not going to be changing springs and/or sway bar settings then you can set them to give a reasonable "transitional balance".
 

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thanks Bob for the info
that would explain why when i set my koni Reds to full stiff i flew off course in my GT

Damn car would not turn in. i also did not like the fact that they where only rebound adj only and i felt i needed more jounce to help prop up the springs (make up for not stiff enough springs)


I called Ground control on the phone and got absolutely no help talking to them, so i have some JIC coilovers coming from Japan for the MR2

You never seem to here anything about how to select springs and shocks

How do the big boys do it?

the only thing i can come up with is to calculate the weight transfer of the car and use either motion ratio or corner weights (pick your poison) to come up with a starting point

Never read that any where just a guess on my part.

David
 

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I also have never found a 'formula' for figuring spring rates, at least not for road racing. A lot of circle track books hint at front/rear percentages for wheel rates, based on vehicle weight, and based on track banking. Using some of these formulae, I had figured my friend's circle track Ascona to 'want' about a 250 lb. wheel rate for the front wheels. Calculating the motion ratio, then the wheel offset's effects on leverage, I ended up with a 715 lb right front spring, and a 620 lb left front, but with only 185 lb rear springs. That car also used a 1" front sway bar, and no rear bar.

When I raced that particular car at a club road racing event, I had to change a LOT to make it turn left and right. I ended up with 525 lb front springs, 225 lb rear springs (had a locked rear), and added a 3/4" rear sway bar. With the circle track springs, it wouldn't have turned at all at the road course. With road racing springs/bars, it would have gone tail-first into every wall at the circle track. Different conditions, different springs.

In order for me to find the 'proper' wheel ratios for road racing, it was not so simple. It relied more on trial and error, and also a lot on the track conditions (smooth or bumpy, grippy or slick, flat or cambered). There's no such thing as a perfect spring rate, only perfect rates for a certain driver, at a certain track.

Bob
 

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What do the "big boys" do? Lots of testing. Even the F1 guys with their millon dollar data aquisition and telemetry systems do tons of testing.

When I go to a race with our "Z" car, I take two boxes full of springs. Before we went to the Phoenix nationals this year, we did two full days of testing at Buttonwillow. Then when we got to Phoenix we spent the whole three days changing springs and adjusting shocks. Finally after that we got "the" set-up for the race... a half shaft on the second lap.:mad:

For the GT the problem is a little more difficult... or easier depending on how you look at it. There's not a lot you can do about the front leaf spring. Rally Bob did some development work with fiberglass springs, but with that exception, you're basically stuck. So, you end up adjusting/changing everything around the leaf spring that you have. The approach I used is to go with adjustable/changeable sway bars.

Colin Chapman... a very smart fellow... said to use the softest springs that will barely keep the suspension from bottoming and use sway bars for the rest. So, on a modest budget a modified variant of this works for the GT or Manta. Get the race front and rear springs from the Opel GT Source for the GT or custom springs from the links above for the Manta. Then go with tubular sway bars with adjustable end links.

Tubular sway bars because you can buy them in a variety of wall sizes off the shelf. Adjustable end links to fine tune everything. Then instead of changing springs, change sway bars and/or end link settings to adjust the front/rear ballance of the car. Don't forget the adjustable panhard bar to change the roll center in the rear.

This may sound a little wierd... changing sway bars instead of springs, but the cornering effect is almost the same. Changing springs changes roll stiffness... changing sway bars changes roll stiffness. I know that all the books say that sway bars should only be used to fine tune... but for our Opels we can't carry around boxes of readily available, relatively cheap springs. So, the sway bars are an adequate substitute.

All this seems like we've forgotten about the original question... which shocks? I think this points out that until you get everything else right, the shocks are not the most important part.
 

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RallyBob said:
The Ranchos (yes, truck shocks) are at least adjustable, but they take the bumps a lot better than anything else I've tried.[/I]
I'm not sure exactly how to interpret this. Damping is damping until you start getting away from linear valving. Do you think these shocks have some type of digressive properties?

-Travis
 

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No, but they have a higher rebound/compression damping ratio than most other shocks, and since they are not a high pressure shock, they seem to react faster to compression inputs, as there is no added 'spring rate' from the high pressure gas. That's my perception anyway. I did find, in back-to-back use, the rear axle followed bumpy roads FAR better with the Ranchos than with a high pressure gas shock. At least, that's my experience.

Bob
 

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Discussion Starter · #17 · (Edited)
Ok guys,

I have ordered the springs from Coil Spring Specialties, and have decided to order the shocks from Ground Control (sorry Bob D. but we also have a wholesale account with GC).

I need some help with some measurements that I cannot take at this time. I need the distance between the lower eyelet centerline and the base of the upper stud on a stock front and rear shocks both fully extended and compressed. Any takers?

GC also need the front and rear weights of the Manta. Can I use the weights from a stock vehicle? My car is in too many pieces to weigh. Does anyone already have the weights?
 

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Shock Dimensions

Are the dimensions you need for a stock, non-lowered, shock? Or one that has been lowered? How much did you lower the car? How accurate do the weights need to be?

I help if I can.

Paul
 

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I'm going off memory here....

54% front weight, early Manta, 2170 lbs. So about 1172 front weight, 998 rear.

Shocks, if I recall, are 15.5" extended in front, 10.5" compressed. Rears would be about 25" extended, and about 16" compressed.

Front motion ratio is .6355, can't recall the rear, maybe .82? I don't know where my 'books' are with all these specs, I usually keep good records but haven't seen them in years.

Bob
 

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Discussion Starter · #20 ·
Re: Shock Dimensions

Paul said:
Are the dimensions you need for a stock, non-lowered, shock? Or one that has been lowered? How much did you lower the car? How accurate do the weights need to be?

I help if I can.

Paul
I will adjust for the amount the car is lowered, so I just need stock measurements. I may just be able to use the listed weight of the vehicle (57R) and the listed F/R %, but if someone has actually weighed a car those numbers can be verified.
 
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