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Suspension tuning - brake dive
I have been a member for a long time but I rarely post so I'll give a quick intro. I'll start by saying I'm not an engineer so please use smaller words with me if you are nice enough to reply with advise. I have been reading a lots of information about suspension setup and I think I should be focusing on anti-dive with my car but not really sure. I am trying to figure out how to effectively upgrade components on my car and not just add every bolt on that is sold even though I think I have bought most of them....
The car is a 72 Camaro weighing 3500lbs with an aluminum headed sbc, 6spd, Pro touring F-body upper control arms, boxed factory lower control arms, QA1 coil over front, factory front sway bar, C5 front brakes, G-braces, DSE subframe connectors, Bolt in roll bar, Hotchkiss leaf springs, QA1 staggered rear shocks with a custom mount to the axle to clear my LS1 calipers. I don't recall the alignment spec but it is close to what I hear most people run with the aftermarket control arms. The issue with my car is that I feel it's either diving very bad under hard braking or the rear end is lifting hard under hard braking. I have ad the car a Thunderhill and also autocrossed the car this weekend. Thunderhill the car felt fairly stable but some push on tight corners and slippery overall. At the autocross the tight corners really showed me the issues and specifically the dive/lift made the car very unstable braking to enter turns. I'm not the best driver either so it could be normal and I need to get used to it. It didn't help that the brakes are far too touchy and easily locked on me (I'm adjusting the linkage closer to the pivot first then swapping MC if it doesn't fix the brake issue). It was probably due to entering the corners too fast but I also had a very hard push that I would use power to kick the rear around and straighten the car. I was on radials and it was very slipper, lots of fun but the car also seemed very easy to anticipate. I assume making some changes would make me faster or do I just need to focus on driving the car better first? I have not added a rear bar because I heard some advice that it is not always needed. I didn't want to "go-cart" my car and make is stiff as I can. I wanted to put together a system that worked all around since I also drive the car a lot. Should I look into a rear bar to reduce the brake dive? I have the factory rear bar at home I could build some mounts for to clear my exhaust. I am also curious if the QA1 coilovers are part of the issue since the spring is so small. Would swapping to a full spring with the same or higher rate possibly help? I have a set of Hotchkiss front springs at home as well. They were too tall for my stance so I swapped to the QA1 for adjustability but I could cut the front springs as needed. I've heard about moving the front leaf mount up ~3/4" by redrilling to help with the anti-dive. Should I try this? Would spherical front leaf bushing help articulation and help any of the issues I have? Any other areas I should look into first? Thanks and sorry for the novel... Steve |
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What are your spring rates and shock compression settings?
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I agree, look to matching your spring rates up first. Being QA1's I'd assume the standard 350 lbs spring.
Most likely coming down on your bump stops and unloading the rear end. If you are serious in performance stiffen up the subframe and look into a different spring and shock set up. Rear should be fine. Support down bars for the front, cross bar right above the sway bar and weld up your subframe seams. I'd like to see a video of your runs. |
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Thanks for the replies. I extracted many of the questions and here are they details.
Ok, for one what kind of QA1 coilovers are you using? Did you just get them "off the shelf"? or were they built for YOUR car? What spring rate are you running on these coilovers? They are off the shelf parts. I used HAL-GMP3350-3 which is spec'd below. Suspension Height Adjustment Stock Coil-Over Kit Position Front Recommended Front-End Weight 1,701-1,900 lbs. Shocks Included Yes Adjustable Valving Yes Internal Design Twin-tube Gas Charged No Number of Valving Selections 12 Shock Body Material Aluminum Shock Body Finish Clear anodized Bushing Material Polyurethane Bushing Color Black Coil-Over Springs Included Yes Spring Rate (lbs/in) 350 lbs./in. Instead of changing your pedal ratio, why not use a proportioning valve? Or do you know what size the piston is in your MC? You know one of the issues you could be having is, the MC piston might be a little to small, and creating to much pressure when you step on the brakes, making your brakes to touchy. I have an adjustable proportioning valve for the rear but the issue is that the pedal is too stiff. The MC has a 1 1/4 bore off an astro van and I have an extra MC sitting around off an S10/Regal/Malibu that has a smaller bore. Based on what I was reading I beleive the smaller bore makes the brake less touching. For some reason I always get confused with the efffect of MC ratio on the brakes. No offense but, if you are not well verced in suspension set-up, you are just going to make you car not fun to dirve. I agree completely which is why I'm asking the questions now. Before I start buying all new stuff I'm hoping a little advise on my current mismatched parts will point me in a better direction. I agree, look to matching your spring rates up first. Being QA1's I'd assume the standard 350 lbs spring. Most likely coming down on your bump stops and unloading the rear end. If you are serious in performance stiffen up the subframe and look into a different spring and shock set up. Rear should be fine. Support down bars for the front, cross bar right above the sway bar and weld up your subframe seams. I'd like to see a video of your runs. I'm also thinking the light coilovers aren't helping me. I have the subframe fairly stiff but have not welded the seams as of yet. I did add some down bars and also welded in a supprt bar near the sway bar. I have some video but they didn't turn out well at all and you can't really see what's going on. I wish my Replay XD had a video screen so I could test in advance. |
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You front shocks, get those 350 lb springs, and use them to hold your hood up. lol.. < just some humor. Way to light for that car man. I would start with at least a 650-700 spring. Like was mentioned, with that small of a sring rate, you are more then likley diving pretty hard, and unloading the rear of teh car. |
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Thanks for the guidance. I'll be swapping the MC this week to the smaller bore. I had forgotten I had an extra one sitting around. I'll also swap my smal block (600lb) Hotchkis springs in this week for the QA1 coilovers. I figured since many other people were using them they would be ok but apparently not so much. For now I'll use the shocks alone with the other springs but will eventually sell the coilovers and get some good shocks
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With a small block and the QA1 coilovers, you can get them to work very well. You have the 350lb spring which is a little light. Up the spring on the coilover to 450 or 500 max and it will work great. You do not need 700lb springs on a coilover. If you had a separate spring and shock in the stock location, a 700lb spring would be a good option. I would keep the coilovers and up the spring rate to 450 and see how it does. The rear sounds fine. I also agree with goin down in size on the MC.
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Hmmm? Ok Ill go off a lil. Ok wait? Ill stay on track for a second. The 350lbs springs are a lil light for your car. I like the advice with the 450lbs springs. And like smoker said, 700lbs (I think yer 600lbs are too tight) is not needed.
I dont mind too much load transfer (front end dip). It loads the front end up so it can really work. Too light up front and the car will be loose. And YES!! Loose in racing can translate to fast track times. But ONLY if its a track only car. Race cars are NOT street cars. I personally like a loose car for the track if I know the car. But I never want a loose (read light on the rear and basically a level feeling car) for the street. For you I would stiffen up the front springs some and focus on the rear shocks a lil. If they are adjustable I would tighten them up for rebound. Make them hold the rear end compressed for just a second while the car is turning and braking. Something as simple as the shocks will help with an apparent dive issue. And I say apparent dive issue (with braking). I dont have a problem with it. And Im talking about a STREET car that likes to see the track sometimes. I have driven front heavy street cars to their maximum adhesion on a closed track. Big heavy (on the front cars). Driven them to the point of major understeer, basically trying to rip the front tires off the rims with turns and braking. LOL Tires never came off but Im sure alot of rubber did. At first during this training session I was slow. But after a few more laps and understanding the car I was able to increase the lap speeds and bring the times down. My point? Even supposed heavy front end cars can dig their way around the track. Dont worry to much about how it seems to dig in cause you might be actually hitting the track at a decent speed. A car can feel out of shape and look out of shape but the clock will ALWAYS tell the true story. JR |
I agree Tobin is the brake expert. Since I have the other MC already I'll give it a shot first. I used it before with the same brakes but not the hydroboost so I know it matches well with the calipers so it should work. I only have the current large bore becuase it came with the hydroboost.
I'll work on adjusting the rear shocks a little. They are only single adjustables but I'll play with them a click at a time and see what happens. The 600lbs springs I have are not coilovers, they are standard full sized springs. Since I already have them I was thinking to give them a shot but I just noticed the cost of new QA1 springs is not too bad. At this point I am actually waiting on my new trans to be delivered and the car is inoperable for a while. I'll focus on the brakes and the shock adjustments first then after the new trans is installed will decide if I'll use the 600lbs or purchase some new coil over springs. |
Gerno,
A couple of us from Chassisworks were out at Mather the same day as you. We noticed your car porpoising around the course and commented that you definiately needed either a better set of shocks or some serious adjustment. I agree that the 350# front springs are too light, assuming they are 9"-10" free length. Also, most of the time on a car like yours we would recommend either a 1" or the 7/8" master so you're going in the right direction. If we see you out there next time we'll make sure to introduce ourselves and maybe we can help you out a little. |
Chassisworks - I've never heard a car referred to as porpoising but I would have to agree that's what it felt like. Can you explain a little more about what you saw for my reference? I assume my driving skills also had something so do with it since it was my first autocross. Do you have any quick tips on areas I could possibly improve my driving skills?
Both my front and rear shocks are single adjustable QA1's and were set on setting 7 of 12. In addition to the stiffer spring, do you think simply adjusting them to a stiffer setting would help from what you saw? I'll be around more often, feel free to come say anytime or let me know what car is yours and I'll come by. |
Gerno - One way to help you better understand the valving is to adjust the shocks full soft and make a run, then adjust to full firm and make a run. Then you'll know the extremes and could do another at 75% firm and you'll have a first hand idea of what your shocks are capable of and if your springs are in the ballpark.
The spring and shock work in harmony so if one isn't complimenting the other you're in a compromise situation. If the spring isn't holding up it's end of the deal, that little shock is working overtime. |
Porpoising is a term we use all the time with the drag race guys but it applies in autocross too. In simple terms what happens is the car hooks then loses traction in quick succession. The most common reason is because the shocks do not have enough extension/rebound dampening. The suspension extends too quickly so it's not applying the power smoothly in the rear which upsets the front. The opposite holds true under braking in regard to the front. That's an oversimplification but hopefully it makes a little bit of sense.
Lino (Psydwaze) will probably be at the 6/9 event in Stockton. I'll have him keep an eye out for you. I have to go to a birthday in SoCal. Feel free to swing by the factory m-f if you have questions. I know you don't have our parts but we're always happy to help out a local guy. ~Carl |
Sieg - good point with the adjustment. Funny how I forget the simple things first. I just wish the front shocks were a little easier to adjsut. The rear can be adjusted with ease but the front know is stuck deep in the spring pocket. Either way I'll get it figured out.
Chassisworks - I can't make the 6/9 event but will aim for the next one. I sold the trans from the car this weekend and am waiting on the new one to arrive. The car will most likely not be ready until the mid to end of the month. I might take you up on the offer to stop by when it's back together. I talked to some of your guys at Autorama and wanted to come see the shop. This first time out I figured I needed to focus on the driving aspect first since even with a completely wrong setup the car is still better than me. Considering I knocked 6 seconds off my time I think I did ok learning. With a little intellegence behind the setup I'm excited to see how much better I can do. |
I took some advice from the comments here and made some changes to the car. I changed the front spring rate from 350lbs to 500lbs. I also fixed an issue I had with the plumbing and pedal ratio of the brakes. Yesterday I went and did some testing at T-Hill. The car feels much better with the stiffer springs and corrected brakes. I'm amazed that the car is actually smoother with the springs when the shocks are on a light setting. I expected a harsher ride
That said, I also had my shocks on a mid range setting and decided to go full stiff to see how the car would react. The car was incredibly stiff but did handle well. I noted much less noise from the tires. Only issues I felt was a harsh feel under braking, ouside tire wear on the driver side front and the rear feeling loose in the turn. I decided to take 2 clicks of adjsutment out of the front shocks and to increase the tire pressure from 38 to 41. After these adjustments the car was more stable and the rear felt more solid. I can't tell about the tire wear but there was much less tire noise in the corners. The car was still a bit harsh (for lack of better words) under braking but seemed better. Hard to say if I was faster or not becasue I am too much of a novice for timing with NCRC but compared to my first time at T-Hill the car was 100% better. The car was not diving near as much under braking and seemed to be pushing much less in turn 11. I'm very interested to see how the car reacts with these settings on a tighted autocross course. |
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