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sopwith21

How 'bout this for a race setup?

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Greetings. A newbie here.

Got a '74 260z that races on a 2-mile road course. Average speeds are around 75 mph. Slows to 35 and tops out at 120 or so.

The suspension was stock and I'm upgrading all four corners to Pro Shock coil over kits with Tokico adjustable struts, Pro Shock springs (350 fr/250 rear) and ArizonaZcar monoball camber plates all around. I know we can debate specific spring rates and strut brands all day, but won't this get me in the ball park and build enough adjustability into the suspension to have a decent base to work from? Right now it handles like a dog. Many thanks.

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Stephen, There are as you know many things to address while setting up suspension for competition. One item you may want to investigate and do is to replace all your old rubber bushings with polyurethane. You will be quite surprised with this improvement. Also, what about your tire/wheel setup...another biggy! Brakes, coilovers, tubular adjustable transverse links, etc., etc., the list goes on and on. But depending on what class you run, rules will dictate what you can and can't do.....check them first. Anyway, hope this helps some and wecome to ONE GREAT Z SITE.

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What do you mean it handles like a dog? What is it doing or not doing? Be as specific as possible.

What's your alignment settings like? What tires? Their age? Tire pressures? LSD? What about sway bars? Are the tokicos blown? I'm not sure Tokicos can handle 250# springs very well. Most setups use the stiffer springs in the rear as well. I never tried it so I don't know how well it works in reverse.

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Has it got a tied in roll cage? Makes a hell of a difference, the stock 240 body is very flexy so diverting suspension settings and loads. 350f/250r sounds a very peculiar spring setup, they should be much closer tending to a bit more on the rear than front, but debatable :) Dump the Tokico's, Koni's or Bilstein's minimum.

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It runs in a GT class so we can do whatever we like to the car. I didn't build the car and can't answer all your questions, but here's my best shot:

Yes, it has a tied in roll cage. Hoosier R45 slicks on 23x9x15 wheels, bought this year with approx. 80 laps on them, 20 lbs all around. Stocks brakes... insufficient, but will have to do until the chassis is turning. Sway bars fr and rear, don't have bar sizes yet. Stock four speed tranny, welded and locked rear end.

Previous owner had been running stock suspension set up. Believe it or not, the car actually handled halfway decent until the fr springs collapsed, necessitating the current revamp. The car understeers horribly all the way through the corner now. Unhooking the rear sway bar made it push going in and through the center and come out loose. A disaster either way.

For financial reasons, I must fix one thing at a time... that means the rear end, brakes, etc., must wait for another conversation. Right now I have to make it turn. The bushings are a good suggestion and I'm open to more. Thanks for all yr thoughts.

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With a locked rear, you have way to much front spring, that I can say for sure. Locked rears will increase the understeer considerably, and with the 350 front springs, it's gotta be pretty bad.

With what you have right now, I'd suggest buying a pair of 275-300lb springs and put them in the rear, and put the 250lb springs up front, then tinker with sway bar sizes to fine tune the handling.

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It sounds like you need to soften the front. If your bushings are stock, do those. I've used both Gmachine and others. I really don't have a preference. They all work for me. Also, I would first try switching the rear springs to the front and if you got an extra $100, switch to a pair of 275# springs for the rear. See how that works for you and go from there. If the tokicos aren't blown, I would wait on the decision to replace them until you test the other changes. Also, Hoosiers start out very grippy but don't stay grippy long. At 80 laps, they could be gone. It is hard to judge that though if you haven't driven on a freshly scrubbed in set before. Check camber and toe in. That makes a big difference. You shouldn't have more than a couple of degrees of neg. camber to start with. Go from there to adjust for tire wear and grip. Also, I like 0 toe to a hair of toe in. It does affect turn-in significantly. I like a little toe out better, but the steering effort goes off the charts with grippy hoosiers. you can play with that to see what you like.

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With a dedicated race car like that with a strengthened body if the track is not too bumpy you could go up to around 500 lb springs, a bit lighter in the front than the back. With those sorts of rates you may find that you can do without a rear anti sway bar.

My car which has to be driven on the road has 275f/300r, its the Koni shocks which make such a difference, my teeth are in no danger of being jarred loose either. Tokico's, pfffttt :dead:

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HI Sopwith21,

I'll throw out one more combo....72 240z SCCA ITS

I've ran F/R spring rates 400/300, 375/300, 350/300 , Last year I settled down with 400/300 koni D/As, without R sway bar, front bar about 1", quaife diff. zero rear toe, 1/8 toe in front....... Handles fairly neutral.

I to have heard the reverse combo of heavier springs in rear, but haven't tried..."Mine works" for me. YMMV....

BTW...Keep a eye on your Link Mount Brace..."they help locate the rear of control arms". I just noticed mine were tearing, or breaking at bolt holes where they attach to the body .....lots of over steer... :surprised

Waiting for next race weekend.....

David Spillman

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I might have misread your initial post sopwith21. I was thinking you had already made the suspension changes and it still handled badly. If you havent bought those springs and shocks yet, you consider many dedicated racers use quite high spring rates, and the tokicos aren't valved for those extreme spring rates. Something to consider. I didn't know about the koni 8610 option when I bought my tokicos. I don't know anybody that has gotten the tokico illuminas custom vavled for high spring rates.

I know there are 2 schools on which end to put the softer springs. Both schools seem to work so I imagine that once you have made your choice, you can tune either to work well. My suspension setup is in about it's 4th iteration since I started developing the car and if the past is any indicator of the future, it will continue to change as the development progesses. I have heard about 300# springs is about as high as you can go with them. I'm running tokicos with 300# springs on both ends (contrary to popular opinion it seems to work at the level I'm at. You can spend a small fortune on shocks and such, but I'm not sure I want to go there with this car) and I don't have any complaints so far but my experience with my setup is low speed autox and a little high speed time trial driving so ymmv. (I may have a little push with that setup but it has been so long since I had new sticky tires I can't really recall. :) ) I don't run a full cage and that makes a difference what springs you might use too.

I know a very experienced driver with 400# springs on both ends and a full cage and he is very happy with his setup too.

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John, the comment about 300# springs leads me to question if that is the wheel rate (including sway bar rate) or just the spring rate alone.

If it is just the spring alone then what rate are people using for a sway bar? Remember that every 1" front sway bar doesn't give the same rate (the length of "arm", material, rod end or polyurethean bushed drop links, flat ends on arm vs cross holes vs clamps, etc.).

The spring rate alone will affect brake dive, squat, and two wheel bumps. The wheel rate will affect cornering, turn in, and trail braking.

Pancho

(didn't get package sent until last Friday, sorry)

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Pancho,

The 300# springs on my car are the spring rate (Eibach ERS, 7" springs in the front, 8" springs in the back). I run a 1" sway bar in the front that mounts like a stock sway bar so the dimensions are similar and a 7/8" in the back that bolts to floor just behind the seats in the area that slopes up. I don't know the brand, I got the bars second hand from Dennis Thoney's GT car, but they are pretty stout solid forged material. All bushings on my car are the poly type. I run my 16" wheels and tires extremely close to the struts (you can slip a piece of paper between the wheel and strut) and that affects wheel rates somewhat, giving me a little more stiffness than the typical race tire'd car. The stance feels like it handles pretty level and flat in all situations.

Here's a pic of the car in an extreme braking/cornering situation at the end of a long straight with me braking way too late .....

3648JBSmall-med.jpg

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Alright guys, smoke-em if ya got em! (tires that is)

Ya never know how late you can brake until you wait to long.

I used to know how to figure the wheel rate, but since AARP started sending me membership forms I've forgotten.

Springs are rated according to how much weight is required to compress 1 inch of travel (constant rate springs). You must apply 300 lbs to compress your 300# spring 1 inch. A sway bar is basically a torsion bar (spring) without a fixed anchor. One end moves up as the other end moves down while the "spring" portion of bar tries to resist.

Seems to me that the easiest way to measure the spring rate of front sway bar would be to jack car up level, place jackstands at the back, place one front wheel on blocks, place jack or jackstand under frame at other front, disconnect T-C rod, drop link, and balljoint from A-arm. Let arm swing out of way (tie strut to wall in garage so it doesn't get in the way. Add a plate to bottom of drop link (simulates A-arm, but not anchored) where you can start hanging weights. Measure height of end of sway bar then add weight until you get 1" of deflection. The number of pounds needed to delect one end of bar would either be the bar rate or twice the bar rate (unsure about that point, Help from an engineer would be appreciated). This method would take into account all the variables encountered in actual installation (frame stiffness, style of drop links, play in bar mount, etc.).

The rear would be more difficult to get arm out of the way without pulling pivot pin (not fun), but could be done by dropping arm at inner pivot.

Your picture shows body roll so the rate of the bar is adding to outside wheel rate.

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Oh, I also forgot that the effective spring rate of the bar would be increased by a percentage relating to it being working on a shorter lever than the strut spring (sway bar mounts closer to inner A-arm pivot than the ball-joint), but would be the same from Z to Z.

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Hey! I'm an engineer, but all this engineering stuff makes my brain hurt. :) I don't think I have anything heavy enough to hang to deflect the bar by 1" (well maybe if I could hang the suburban from it), but I think that measurement would be the rate.

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Just borrow weights from the neighbor kids gym! If it take more than 300# who gives a $hit what rate springs you use as they are less important than the transient rate of the bar:)

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Here are some Z anti roll bar rates that were posted here or at HybridZ some time ago. Using the stock bar setup.

Front: 22mm - 268lb/inch, 23 - 321.

Rear: 20 - 86, 21 - 105, 22.5 - 139

One reason why I prefer heavier springs to anything larger than a 22mm front bar.

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260DET, thanks for the numbers. I feel much better about my 260# front and

240# rears now (nice numbers hey!). I'm using a 28.6mm adjustable front (five or six positions) and 22.2mm rear (with four positions and much shorter "arm" than stock).

So I'm just guessing that my front bar should be adjustable from 400# to 600# and my rear from 150# to 250#.

The much lower rate on the rear may be the reason why some racers use stiffer springs in the back.

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Okay, guys. Many thanks for the thoughts offered so far. I wish I had talked to you all earlier, but I didn't and that's life. Here's the parts I have now... they're paid for and its not negotiable, so the trick now is to do the best I have with what I've got. My next race is at the end of the month and I'm in the points battle.

Springs: 350 fr/250 rear

ProShock coilover kits (4)

Tokicko 5 way adjustable struts (4)

Sway bars fr and rear, don't know sizes yet. Stock with no pre-load adjustments.

Hoosier R55 slicks

8 of the 11 corners at the next track are left handers... so... if I end up with a push (the parts are going on this week), here are my rather limited options:

- increase rebound on LR strut to wedge car out of corners and increase fr grip

- take off two rounds of coilover on LR and RF tires to wedge car and increase fr grip

- put a bigger tire on the RF for same effect

- swap springs fr to back and hope for the best (but I don't know if we'll have time and ability to do this at the track, so I can't count on that)

In the unlikely event that we end up with an oversteer, we can:

- unhook the rear sway bar for more bite in the rear

- do the opposite from above on tire stagger and/or coilover adjustments to tighten car

YES! I realize that some of this is a band-aid, but as I mentioned, this is what I have to work with. Our money and time are nearly gone and the challenge now is to do the best we can with what we have.

The suggestions you guys post are helping more than you know and its appreciated. We may not have the perfect parts, but we've got to work with what we have and get close... and I think we should be able to at least do that much. Thanks again to all.

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BTW, air pressure adjustments on a tire this hard seem to have only a the most minor effect, so I left that possibility out. When you're tuning a car from absolute zero, your first adjustments have to be major to get your car in the general ball park.

What fr/rear weight ratio is best when we scale the car? Now that we have coilovers we should be able to make some adjustment there.

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Sopwith, I don't know what you mean by taking two rounds of coilover off RF and LR. That will only affect the ride height (lower) at those corners and increase weight at opposite corners and has no effect on wheel rate.

I hate the idea of jacking a road race car out of whack.

Try something less drastic like lower LF air pressure (slight stagger), more camber on RF (or both), more preload on RF of sway bar (stiffer wheel rate on left turns), more toe in, or a little more castor (shim T-C rod). Also try not to over cook your turn entry, and use throttle to balance out understeer (blip-blip).

I'd find a parking lot and run a skid pad circle trying some adjustments prior to track time.

Just my 2 cents.

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Sopwith, I don't know what you mean by taking two rounds of coilover off RF and LR. That will only affect the ride height (lower) at those corners and increase weight at opposite corners

Exactly. Its an old circle track trick called "wedge." That deliberate stacking of crossweight helps turn the car in (for instance) left hand corners. Of course, it hurts you in right handers, but if the car is pushing and you're getting creamed anyway, you might as well try to get better in 8 of the 11 corners on our road course.

I hate the idea of jacking a road race car out of whack.

Me too. But if the car is awful and I have no other options, we can do it temporarily to try and pick up some time. I'm just trying to plan for a worst case scenario.

Try something less drastic like lower LF air pressure (slight stagger), more camber on RF (or both), more preload on RF of sway bar (stiffer wheel rate on left turns), more toe in, or a little more castor (shim T-C rod).

Good idea. I've tried using air pressure adjustments to stop understeer, but I've not used it yet to adjust stagger or crossweight. Great thought. I wonder how much these tires will respond to staggering by air pressure?

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With 250f/350r springs you could try doing without the rear anti sway bar, remove or loosen one link right off if you don't have time to take the whole bar off. With both stock bars in place though the car should oversteer if anything.

Or, if the springs are interchangeable swap them front to rear. But I'd play with the rear bar as above first. Don't want to know about the speedway tweeks :)

If it still wants to understeer no matter what you do with setup, try some light left foot braking through the problem corners, keeping the power on though and assuming that the brakes can take the extra work.

And let us know how it all goes, fascinating stuff.

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Adjusting in more rear brake bias may also help free the rear on corner entry. Weight jacking on stockers works because of only left handers and because the chassis is STIFF. If you don't have a stiff car you will only get more lost.

Still recommend running on a skid pad. Get a tire pyrometer, the tires don't lie. If all else fails use some driving tricks to hustle the beast the best you can. Good luck, and remember not to throw every tweak at the problem all at once. One change at a time.

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