Some questions for all you suspension gurus out there

Greetings everyone,
As P-man commented once on my 71 GTO with it's pretty radical 428, that the current 50 series H profile tires would be hard pressed to find any traction, I find that he was right... (Again)
So here's my needs in preparing to "tune" my suspension and I'm looking for help and input on it.

Front end- I shaved so much weight off it that the stock springs don't compress anywhere close to stock height so my front end has a tendency to be very squirrely above 120MPH. Moving the batteries to the trunk, aluminum heads, aluminum radiator, intake, headers and complete glass once piece front clip has shaved a few hundred pounds off the weight so should I be looking at stock pring rate, cut down springs? or a lighter spring rate stock height spring? Tubular upper and lower A arms? Or something else? Gas shocks are going in both front and back I think.

Rear end- Is a Strange Engineering built 12 bolt 3:55 Posi and the drive shaft is also Strange built 4130 steel and plenty hefty. I do have a shaft loop to install for it. Currently I have the extra heavy duty springs in the back with inside air bags to keep the rear end planted and all seems to work well (no "hop" at all). Should I consider replacing the factory four link style with an adjustable set up? Other than the added strength I don't think I would benefit from it too much. I don't plan on tuning it that much.. I won't be racing but every one in a great while at the track here.

rear tires.. Still looking.. I need something close to stock height with at least 12" of tread in contact with the ground.. the 50-H's only have about 9".

Only other weak point I have will require me to install a beefier trans.. Current aluminum case Borg Warner C type T-10 has a rather small out put shaft with only 10 splines. (numbers tracing says it originally came in a 63 vett w/409)so I'm going to go look at Richmond gear Super T-10's beefed up to handle 600+ HP with a bigger tail shaft and 32 spline. A T-56 won't work for me as shifter placement is all wrong in the ones I've looked at.

Biggest help needed here is on the suspension changes I need to make. Any input is greatly appreciated!  
Jim,

your probably gonna want to stuff some 275 60 15's in the back. I'm not sure on the 70' but a 5" backspace on the rim is whats needed on the 64-67's to get that much meat in there. For the front weight is probably the only thing that will keep it planted at 120. Lower spring rate my help, but torque and wind are trying to lift the car.  
http://s1098.photobucket.com/albums/g372/instg8ter/1966%20Tempest/

"learning as i go, and going as i learn"

Brian

hello


you should get a lot of useful info from some drag racers here but below is what i use. i have done a lot of the spring work and lowering mainly on 65-70 fords specially for high speed and road course racing track. i also engineer springs. the same theory applies to all cars. heres the problem you can't have your cake and it it too. high speed requires stiff suspension and a lowered car, so how much ride comfort and height are you willing to give up and what type of driving is it? Here's briefly how it works to still be streetable sort of. These are just approximate numbers for reference in case you get no other info from someone that has done it. i did this on my 67 gto.

1. you need stiff springs to increase stability. you need stiffer [not softer] but shorter springs front and rear. this will lower the car and stiffen the suspension. just like a 200 mph nascar. it can then be adjusted for height with rubber spacers on top of the springs. lower the approx 1 1/2" from stock with the springs.

2. front lowering spindles- buy approximately 1 1/2" front lowering spindles. using the spindles helps lower the car without loosing suspension travel by doing it with the springs alone.

3. lower the rear to the desired "look" using the spring and spacers.

4. springs are more important than a arms if your on a budget however if you have the money then buy them also,

5. a arms - if you are short on money do the springs however at 120 plus they are really a must for safety. they will reduce sway also.

6. front sway bar - you need a large diameter after market one about 3/8 bigger than stock. a stiffer bar will cause stiffer suspension but reduce cornering tilt and front end sway at high speed.

7. rear sway bar - if it still sways you will need a rear sway bar, if you already have ine you will need a larger one. this will reduce sway and corner much flatter

8. panhard bar - this welds to this attaches to the body and rear axle housing. this keeps the tires and axle housing centered under the car so the both move in unison not separately. welding required.

9. shocks - ideally you need shocks with high rebound dampening to overcome the quick return of your now stiffer springs. koni has some called 90/10 shocks. hotchkiss has ones with adjustable dampening. you might get away with your high pressure gas shocks ok for street bad for road courses.

10. maximum street traction will be achieved using "cheater" street legal slicks. mickey thompson makes them also probably hoosier. they may not look great and are bad for road course's. had them on an 11 second 67 camaro with nitrous.

11. if you are drag racing you might consider stiffening the right rear spring. it will help keep the car level under drag conditions.

12. don't know about your adjustable four link or trans.

13. check eisert, hotchkiss, and global west for springs and a arms.

PS i see you have your rear end handled already but you might consider a sway bar and panhard bar. also check out summit racing for the parts.

thanks  

front air dam to redirect the air around not under.
sway bars front and rear. undo front when drag racing.
cheap way is to cut 1 coil out of stock springs that will also make it a little stiffer.
to do it right you will need to scale the car (all 4 corners).
new upper and lower control arm with dubble adjustble coilovers would be cool and you could adjust ride height.

urathane body mounts and bushings.

their is a new thread for suspention & handeling.  
76 455/4spd TRANS AM
69 GRAND PRIX 406/5SPD

hi

rowsley good thinking on adding the front spoiler along with the suspension mods.

rowsley is also 100% correct regarding cutting the spring increasing the spring rate however this is why cutting the spring in a car actually makes the suspension softer. it's late so it's a bit garbled sorry. i'll try to clean it up a bit tomorrow if i have time.

how cutting a spring increases the springs rate, lowers the car but causes it bottom more easily then before. what? you might say. the math is simple, sort of.

for example take a 100 lb per in spring, compress it 3" for preload [installed height], that's 300 lbs pressure installed, total spring travel is approx 8" so 8x100=800 lb's so add that to 300 lbs and you have 1100 lb's pressure with the suspension totally compressed.

now when you cut a spring in half it doubles the rate. [100 percent increase] in other words if you cut it in half you now have a 200 lb per 1" spring. however to get that rate the car is now bottomed out because the spring is so short.

here's the math and the math doesn't lie. for every 10% you shorten a straight wound spring it increases in spring rate by 20%. so now let's use a realistic number like 300 lb spring 20" long like some car springs, 3" preload=900 lb, travel 8"x300=2400 lb's add the two numbers=3300 lb's with suspension bottomed. now cut that spring by 1" [5]% the rate has now increased by 10% which means it is now 330 lb per inch but now you have 1" less preload so preload is now 2"x330=660 you have also reduced the total travel of the spring because you lowered the car by reducing the preload so if your suspension ratio is 2 to 1 [close enough] you shortened the spring 1" lowered the car 2" shortened the distance the spring travels by 1" so you are now compressing it by 7" instead of 8" so 7"x330=2310 add the two together and you now have a total of 2970 lb's with the suspension totally compressed. this is 330 lb's less than the lighter uncut spring above plus you also have 2" less distance for the wheel to travel before it bottoms out. not good.

it's actually a little more complicated than this because the spring compresses some with the weight of the car so this is actually the simple version.


this being said your car is lighter on the front which means it bottoms less so you could cut it and even though the spring will then be lighter your car will not bottom out anymore than before however at 120 mph plus this rate will still be too low to significantly reduce front sway but you can try it since you already have them. to me it's not worth it to save $150.

thanks  

Thanks to all for your input.. Lots of thoughts to consider..
One thing- I do have over sized sway bars, front and rear w/Poly-urethane bushings installed.
I think I like the idea of a stiffer stock height spring up front w/the drop spindles. That'll give me travel on the spring and lower the front end back closer to a stock height. Using an air dam that might bolt up to the bottom of the radiator support would help even more.
I've got to go find a scale around here somewhere and get it weighed so I can talk to the spring manufacturers about spring rate and height.
All the bushings were replaced in both upper and lower front control arms but I went w/Factory style there and not Poly-urethane.  
Jim,

hello my71

what type of driving do you do street strip or just local parking lot drags? just curious. the cheater slicks will cut around 1/2 second off your 1/4 mile et's but you can obviously rent slicks at the track too.


front end drop

if you are going 120 plus it is better if the front of the car is lower than stock. it will be more stable as someone above mentioned. the drop spindles come around 2" max drop. make sure they are cast in the us not just machined in the us.


front end weight

now your thinking, truck stops have them and so do metal recycling yards


springs

try eibach also, any mfg can calculate what you need, call their tech lines. this exactly the right plan you have.


front suspension arms and urethane bushings.

these are actually very important. if you have the money. your front wiggle will never go completely away with the stockers at least unless you install urethane bushings. the stock rubber ones act like jello at high speed due to the forces exerted on them and the flex changes your steering angles. this is why nascar and indy doesn't even use either they use needle bearings. the metal of stock arms also flexes a little at high speed.they designed them this way to impprove the ride on a stock family car.


don't forget shocks

what about the konis or adjustable hotchkiss?  

Barnett,
This car only comes out on weekends in nice weather. I only live about 2-3 miles from the royal purple race way down here in Baytown (Houston area) so my racing is all 1/4 mile drag and other than that, it's cruising around locally.
I understand, the lower to the ground the better for drag but I have to find a happy medium between that and cruising locally with all the inherent rough pavement.

Some higher speed passes have pointed out I need to do some tweaking on the suspension, hence I thought about the tubular A-arms but I might look at Urethane bushings for the factory A arms also.
Shocks are an integral part of the plan.. Has to be a complete package.. Considering Hotchkiss as I have done business with them before.
Thanks for the input!  
Jim,

These are my personal opinions, addressed to My71 only. Criticism or attacks from current or past professional engineers or specialists will be ignored by me, but friendly banter is welcome.    Sucks to need a disclaimer to post on HPP now.

My71, my suspension know-how is nonexistent and I am not a suspension guru nor any other type of such. I can take stuff off and put it back on but I've never been an alignment tech. My junk has all squeaky poly bushings and a stock sway bar, and most would say it handles like a wet muddy pig on Moog replacement Cargo-Coils. But it rides nice and last time I checked, if you break the speed limit thats a crime plus you could kill someone. Its hard to really lean on a rear sway bar while driving legally, if you ask me.  

If you want it bad enough, you should be able to fit 275/60R15s on the back and 255/60R-15s up front. I do it for looks even though I know the most predictable (safest during emergency) bet is four same-size tires. At least if you run 15s thats about as big as you're gonna stuff in there at stock ride height (or with altered weight balance and corrected ride height) with no body mods. I have run those sizes on several 68-77 A bodies so I know that can work with a 4 1/2" or 5" offset on an 8" wheel all around. Its what I run now and it works great if the old school sidewall height is something you can bear to look at while the rubber band-loving kiddos laugh and point. Even a stock 7" wheel works. The next step down in size would be 245/60 and 255/60, if the big ones rub too much. But thats a pretty huge downgrade. Each car has different clearance due to normal manufacturing variations, so what works for one may rub on another.

Also consider this- every time my favorite alignment and transmission guy (Rest in peace, Griz) has aligned my wheels after a suspension or steering freshen-up, he asks if I want high-speed stability or ease of turning at low speeds. Then he fudges the settings to one outside edge or the other, of the "in spec" range. Research this if you do your own alignment, or discuss it with your alignment person. Its possible thats all you need now other than a ride height adjustment to correct the post-diet aerodynamics.

In my opinion, drop spindles are the way to go for that. Geometry is best preserved in this way, to the best of my knowledge.

Properly adjusted wheel bearings and fully greased suspension and steering components are essential at high speed, and the commonly worn-out steering box has everything to do with 100+ mph ease of handling.

Also, don't ignore speed ratings on tires. Easy to ruin freshys in minutes, with Pontiac power. BTW- nice car, man.    

My 71:  I have been following your thread for awhile now and if you don’t mind I have a couple of suggestions.  First let me say that the lighter car will certainly help performance a lot.  In effect by lower the weight on the front end you have already stiffened the springs.  If you contact one of the spring manufactures they can tell you about how much stiffness you have added.  There is such a thing as too stiff but I have run 300 pound springs with good results.  Cutting coils off springs is a very bad idea.  Yes, I know dozens of you will disagree about cutting the coils and even more of you have done it, .but for less than $200 you get correct spring compression and return rate and ride height are all engineered into the spring . When you cut off a coil you are overriding some serious engineering.  

Adding drop spindles does come closer to maintaining factory alignment and a steering geometry but it does change it and it is essential to take it in for alignment.  In fact, alignment is a very good idea after any suspension or steering mod, so make sure you go in for a new alignment (and I am not talking about Midus Muffler).  Matt is very often right and you would do well to pay attention when he posts. BTW  Hi Matt!

Besides controlling body roll, Sway Bars are there to balance your handling. Going to a stiffer bar up front reduces grip at the rear and this will induce understeer.  This is not bad for a street car since loosing control means that you will drift to the outside of the turn.  Letting off the gas will transfer some weight to the front making the car turn in, and keeping it on the road.  So, some understeer is safer and is more forgiving.  As a rough guess, ¾ of all say bars are incorrectly sized in incorrectly installed.  Normally everyone gets by fine because they never really push the car that hard.  An out of balance car will swap ends so quick all you will be able to do nothing about it except wonder why; you may destroy your ride; and maybe yourself, and others.   You will get your best advice on sway bars from people that road race or do autocross. If you can get a professional to set up the car with a little understeer that would be a good idea. You mentioned you drag race and some of those racers will disconnect the front say bar when racing. I am not sure what their reasoning is but the drag racers can explain that.  On the road course you need precise, and balanced, steering and handling with as close to neutral steer as possible. As I mentioned understeer is better in a street car.

Not long ago I posted two articles on performance handling here on the Forum.  The articles are pretty long so if you decide to read them pull up a comfortable chair and a cold Coke.  I think you will find some of the information you’re looking for in them.  There is also a thread on the site about springs posted 6 -8 months ago I think.  

Some of the ideas expressed in the replies you got is spot on.  Some needs further research.If your car is a racer then keep it on the track where racing belongs.  If you’re building a street car for a slightly better handling stick to legal driving and autocross or club track events.  

Milt

http://forums.highperformancepontiac.com/70/9378915/the-general-discussion/article-5-performance-handling-part-1-of-2/


http://forums.highperformancepontiac.com/70/9320747/the-general-discussion/article-5-performance-handling-part-2-of-2/  

Hello Mr Milt

I read the article you wrote for the magazine titled “Physics 101”. I thought it was good and it all made sense to me except for the part regarding sway bars. I am certainly no physics expert but some of your comments confused me and appear to me to be contradictatory. Could you please explain how a stiff front sway bar can reduce but at the same time increase traction on the inside wheels in a turn.


1. You say that without sway bars your rear wheel came off the ground in the turns and lost traction.

2. then you state you installed sway bars and the rear wheel stayed on the ground thereby increasing traction since it was now on the ground.

4. your last comment says that a large front sway bar will reduce rear traction. how can that be if keeping the tire on the ground by adding a sway bar increased traction?

4. You say that if you reduce roll with a stiffer front sway bar in the front that it will reduce the traction on the inside wheel.


Execerpt from your article.

“Physics 101

Once upon a time a very long time ago, we were racing a little Morris.  The Morris had a tendency to lift the inside rear wheel and when a rear wheel lifts off the pavement grip efficiency is reduced dramatically at the rear, and you get a sudden and usually unexpected transition from understeer to oversteer causing the tail end to swing wide and the car becomes impossible to control.

The sway bar controls body roll, improving steering response and makes the tires stand up straighter so you get a more consistent Contact Patch. Sway bars also allow changes to weight distribution from corner to corner. For example, making the front sway bar stiffer adds weight to the outside front wheel while reducing weight on the inside front wheel, and vice-a- versa at the rear. By adjusting stiffness of the sway bar you can adjust the relationship between front and rear lateral grip, thereby adjusting oversteer/understeer.  

Going to a stiffer bar up front reduces grip at the rear and this will induce understeer. “


Thanks  

Matt and Milt,
Your replies are much appreciated!
Milt, I've read your articles with great interest. I knew i needed "more meat" on the ground in the back but once I do that, then I had to start thinking about other factors and possible weak links that may come into play (hence the trans replacement) and your articles were the "trigger" that got me to thinking about the suspension as a "complete entity". I've got most of it covered as far as weight distribution but wanted to lower my CG a bit and given the specifics of my car, I need to address ALL of it. The Royal Purple raceway has friday night "test and tune" for street rodders. Pay 20$ and make all the runs you want. 1st couple of runs were easy runs, gettingthe feel of the light and what the car would do. Next couple of runs I started to run it harder w/faster passes. the 2 things i noticed were;
1. Tire spin was killing me. Didn't quit leaving rubber behind until 3rd gear
2. 120MPH + was making the "feel" and steering a bit skittish.
So I quit there and brought it home.
Matt,
Both you and Brian mentioned the 275/60-15's. I've looked at those, but they're 28" tall(factory hieght is 26.8") and they have a little less "foot print" than what I have now (H50x15's).
I think I'm looking at something in the 345-355 width series.. Or possible one of the streetable drag tires from M&H or Hoosier, or one of the other manufacturers.
My steering box is one of the newer 2.5 turns lock to lock type.  
Jim,

My 71 I would look into some Mickey Thompson and similar tires as they will hook up much better than any street tire.
So you may not need to go wider just a tire that grips better.

What tires are on it now?  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

My last opening statement applies to this post also-

My71-
I'll be anxious to see a foot wide tire in the factory wheelwells! Show us how you do it please, I won't say it can't be done since you have bags but you might need a custom wheel to make it just right. I do know sidewall height is a big clearance factor with those stock 'wells and that clearance isn't the same from car-to-car. You might be suprised to find that the taller sidewall of a 60 series better buffers the shock that breaks traction initially, and that the taller height may slow down the tire spin as you ease into the pedal and climb the gears. Top fuelers use giant soft sidewalls, not short little 50s, right? A taller tire also tilts the weight of the car back towards the front, which might help your stance and high speed stability but undo a small amount of your weight transfer efforts.

Your tires are only 7/10 of an inch wider than a 275/60 and if you have too much spin now, the shorter tire is just running your engine longer in the same spot and not helping you get to the other end. Thats my trouble when I'm at the tree, I want to dump it all in when I see green. Once you have your stance like you want, lots more practice passes is what I would try. A buddy of mine loves doing grandstand burnouts so much, at test n tune, the starter tapped on his window and told him "Son if you lift a little, you'll get to the other end quicker." Ha.

If you improve enough to get sent home by tech for no rollbar or loop on that last pass, you know you've got a fast street car and the technique to use it. I wish I was there, I'd definitely let you try my 60s just to see. Even if I'm way off base it would still be fun times. I chopped off four tenths once, just changing from 275/60s to 255/60s. Yes it made the 1-2 shift a handful and the launch more challenging. But I had the opposite problems: low power, tight converter, too much cam and plenty of bite. Those test and tune sessions are the bomb. You might want to pick up a pair of cheap wheels the right size to try used tires and do some experimenting for size before you commit to any brand new meats. Thats what I would do.

A little instability at the finish line is to be expected from an overpowered antique vehicle like yours, and as long as you can deal with it and everything is in good shape, no worries. Theres not much weight on the front tires when the loud pedal is down, so it makes sense that the car feels squirrely. Dang I wish I had your dilemma, your car sounds pretty evolved as-is!

All just food for thought and discussion here. Let us know what you come up with and how it works.  

Matt  

Maybe I should be shot for thinking of dragstrip physics on a Sunday morning, but consider this: I would whole lot rather you ran over my foot with a fat slick than with a pizza cutter. The smaller contact patch is heavier on my foot because the same weight is on less square inches. I know when I drive on ice, wider tires are less desireable due to this.