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Thread: Discussion on Front Wheel Drive Drag Race Suspension

  1. #1
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    Discussion on Front Wheel Drive Drag Race Suspension

    Okay - First off, I've had WAY too much time on my hands, and secondly, this is an idea discussion group, so please keep the negativity out. If you have a different viewpoint, express it, but explain why.

    I was looking to upgrade some parts on my car in the future. I contacted a few companies, to ask some specific questions. One of them sent me this link:

    buildafastercar.com/tech/Sway-Bar-Rate-Calculator

    So I went out and did the calculations, and the stock front sway bar comes up around 180 lbs/in roughly, factoring in the motion ratio, with me not having a lift, and good measuring tools, nor patience enough to get more than a rough estimate.

    In my searching, I also was able to determine that the Sway Bar's do most of the "spring" work on our cars. This could be why some coil-over packages suck ass, is that they set them up for the sway bar on the car being tested initially. If that's stock, and you are stock, and you get them, they probably ride exceptional! If that's stock, and you're on a Hotchkis or Eibach bar, you're probably noticing the lack of control and the fact you're eating up struts quickly!

    But that has little to do with this discussion other than basics of what the sway rates are comparable to the springs. The springs are close enough to a 1:1 Motion ratio to take their rate directly.

    First point of discussion:
    - When the car is on a lift, and you remove the sway bar, there is about an inch of clearance under the cross member.
    - The suspension has an amount of "droop" available. I don't have a jack handy right now to measure that. If anyone here wants to volunteer to post up their setup, and the amount of clearance to the fender sitting, then with the wheel just clearing the ground, it would be appreciated.

    I'm going to use an estimate of 6" just to make the math easy here.

    The first inch is absorbed by the clearance. That leaves about 5 inches of pre-load. The motion ratio is about .4, so you have about 2 inches of pre-load on the sway bar. Because of the design of the front sway bar, the bar is actually pulling down on the suspension. This means it is pushing up on the body. When you try to accelerate, the body will want to rise. This is done to improve handling, as under braking and turn-in, the nose won't tend to lean as much, and the inside suspension will easily and compliantly drop, as the sway bar is assisting the spring to make that happen.

    The basic point is, we may be able to manipulate the pre-load on the sway bars to help control the body motion on drag launches, allowing for much quicker 60' times.

    Where the front swaybar is concerned, this would mean reducing the pre-load, which can be achieved with longer bolts on the outside, and spacers which move the mounting point lower. You now have reduced the pre-load on the body, and in effect softened the front suspension. This means that the springs in the rear will transfer more weight to the front. The interesting part is, that if you could get to ZERO pre-load on the front, you would have 180 lbs of force per inch (approximate) helping hold the nose of the car down! Theoretically, if you were to pre-load the opposite direction (providing there is room) you could effectively hold the nose of the car down.

    Main advantage of doing this - Higher percentage of weight on the nose.

    Let's assume you have a fairly stock car. It launches at around 2.10 seconds on your local drag strip, without losing traction. That's about .43 g's. If the car/driver weigh in at 3100 (A reasonable estimate for most SRT's) then assuming a CG height of an average compact around 20 inches, you're going to get a weight transfer of about 250 lbs front to rear. That's because at 1G, you'd have 590 lbs of weight transfer. Launch for a 2.0, and that goes to .47 g's. 1.9 = 52 g's. 1.8=.58 g's. 1.7 = .65 g's. Notice that this is increasing exponentially the number of g's to go faster on a launch. This means more and more weight transfer.

    Let's say you lose traction at 2.1. You have a 63/37 split, so you have about 1950 lbs on the nose. You lost traction at .43 g's average, or at about 1700 lbs on the nose.
    By lowering the pre-load on the sway, you've softened the front springs, and stiffened the rear in relation. This means that the car cannot transfer as much weight to the rear, and you will have improved traction.

    Now, let's take a look at the rear sway bar. One popular upgrade is a stiffer rear sway bar. Quite often, this is combined very simply with Lowering springs. More often than not, this is then done using stock hardware items.

    Let's have a look at the effects that will happen from that: The CG of the car will drop about 1-1.5 inches normally. That reduces the weight transfer at 1g to 545 lbs or so. Statically, we have almost 50 lbs more nose weight. We end up about 1" lower in the front usually, which means we get about 70 lbs of increase in the front rates, plus the difference in the springs. Teins are a popular option, so we will use theirs, which are 170 front, 165 rear, approximate. Compare that to stock at 170/125, and we see no difference in front spring rate. We take a look at the rear, and we add a rear bar that usually is about 170% stiffer than stock. It's attachment point gives it a fairly high motion ratio compared with the front, and we drop it 1.5 inches in the rear. Call me crazy, but this car has a lower CG, less static weight transfer, and a much stiffer rear in relation to the front. It probably gains quite a bit in the launch capability, as the springs are better able to counteract the weight transfer.

    If anyone there has a lift, measuring tools, etc., and wants to provide me with some good information, I can much better calculate much of this information....
    Last edited by Low-Speed; 09-07-2009 at 12:44 PM.

  2. #2
    i like where your going with this but there are 2 things to consider when softening up the front. the first and most important being wheel hop. if you soften the front suspension too much you loose the force pushing down on the front wheels and as they grip they are going to want to jump off the track. this is the cause of a whole bunch of shit you don't want.

    My best 60's were on a coilover sleeve over stock struts with stock sways. rear suspension was totally stock. the front struts were blown.

    So, stock rear bars, springs, struts. stock front bar, struts, and coiloversleves with about a 3k spring rate. 26x8.5/15 slick. All this = 1.61 60's. Do all the math you want.

    now I have bc's with a custom 8k rear spring... we will see how it does.
    www.PREracing.com


    Quote Originally Posted by Random Customer
    My blowoff valve is too loud, I think the turbo is going to run out of air

  3. #3
    i forgot to mention it was stock 03 trans (open diff) car was gutted including passenger seat.
    www.PREracing.com


    Quote Originally Posted by Random Customer
    My blowoff valve is too loud, I think the turbo is going to run out of air

  4. #4
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    1.6 in a FWD with slicks..........that's pretty damn good! But how much of that is good driver launching vs hardware? You are probably one of the better srt4 drag racers on this forum. Kinda hard to tell when no one else has that kind of setup.

    We need Ryan or JR to make some launches in the SRT4 with that setup, then we'll kinda know.
    Last edited by EvoRonin; 09-07-2009 at 06:46 PM.

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    Quote Originally Posted by Checksum View Post
    i forgot to mention it was stock 03 trans (open diff) car was gutted including passenger seat.
    You bring up something more important in the launch characteristics of a FWD, that is weight. The lighter the car, the better the launch.

    But, 3K fronts are SOFT! 167 lbs/in soft! Stockers are 170 lbs/in. And blown shocks = good too, as they don't rebound, although they don't "control" the front as well.

    By your theory, you should have wheelhopped worse than a stocker!

    Wheel hop has nothing to do with "spring rate" directly. Indirectly, a softer suspension = more grip. More grip = faster suspension movement. Fast suspension movement, and no control over the geometry = wheel hop.

    Fact is, eliminating the pre-load should help keep the nose from rising so hard, which will eliminate a lot of the suspension movement, and should aid in eliminating wheelhop, all while improving traction. At least I'm fairly sure of that. It will also cause the nose to sit lower, setting the CG lower, and reducing dynamic weight transfer. That will also increase static weight on the front, soften the overall front spring rates..... It seems like a good option to test for some of the guys who are struggling. I'd prefer to see it tested on street tires, to be honest. It will tell you more than slicks will about the potential.

    Unfortunately, I don't have the $$ to go test. I just now started a new job, after 5 months without work. I don't even get paid until next month. And because I have had so much time on my hands, I've been doing some calculations on fairly inexpensive things to test to try and get our cars to launch harder.
    Last edited by Low-Speed; 09-08-2009 at 03:49 PM.

  6. #6
    Quote Originally Posted by Low-Speed View Post
    You bring up something more important in the launch characteristics of a FWD, that is weight. The lighter the car, the better the launch.

    But, 3K fronts are SOFT! 167 lbs/in soft! Stockers are 170 lbs/in. And blown shocks = good too, as they don't rebound, although they don't "control" the front as well.

    By your theory, you should have wheelhopped worse than a stocker!

    Wheel hop has nothing to do with "spring rate" directly. Indirectly, a softer suspension = more grip. More grip = faster suspension movement. Fast suspension movement, and no control over the geometry = wheel hop.

    Fact is, eliminating the pre-load should help keep the nose from rising so hard, which will eliminate a lot of the suspension movement, and should aid in eliminating wheelhop, all while improving traction. At least I'm fairly sure of that. It will also cause the nose to sit lower, setting the CG lower, and reducing dynamic weight transfer. That will also increase static weight on the front, soften the overall front spring rates..... It seems like a good option to test for some of the guys who are struggling. I'd prefer to see it tested on street tires, to be honest. It will tell you more than slicks will about the potential.

    Unfortunately, I don't have the $$ to go test. I just now started a new job, after 5 months without work. I don't even get paid until next month. And because I have had so much time on my hands, I've been doing some calculations on fairly inexpensive things to test to try and get our cars to launch harder.
    with street tires it wheel hoped so hard i broke a radiator mount. i never had wheel hop issues with slicks, dunno why.
    www.PREracing.com


    Quote Originally Posted by Random Customer
    My blowoff valve is too loud, I think the turbo is going to run out of air

  7. #7
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    Quote Originally Posted by Checksum View Post
    with street tires it wheel hoped so hard i broke a radiator mount. i never had wheel hop issues with slicks, dunno why.
    I can answer that - Slicks have a deflection level in the sidewall that allows the tire to absorb a lot of the up and down motion you were getting. The blown front struts although good in that they don't rebound, also don't control oscillation in the suspension, which means you could see prolonged wheel hop. The street tire snatched off the ground, then banged hard when the nose set back down, as it has no give.

    This is why your car is not necessarily a "good" example. Not from a scientific standpoint. Which is why I bring up this thread. What I'm trying to do here, is to give everyone a better understanding of the suspension, and perhaps find out something that will consistently help anyone in the SRT community.

    Fact is this - It's not unusual on a moderate racetrack for a Cobalt SS or even a Ford Focus to easily nail 1.9-2.0 60's on streets, and 1.6-1.7 60's on Slicks. Most SRT's I've seen at comparable power levels struggle to run 2.1 60's on streets. Start looking on the other site at the best times on streets, and only a few have ever seen 1.8's, mostly on 40mm wider than stock rubber. Interestingly, we have a wheelbase advantage, the CG is likely as low or lower than the other cars which have higher ground clearances, and in the case of the SS, Side Curtains, and the re-inforcements that come with that.... So there has to be another reason. Some of it is simply weight. But why not figure out how to minimize the disadvantages?

  8. #8
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    Quote Originally Posted by LittleTurboSRT View Post
    What does the addition of "traction bars" do?
    Primarily, traction bars act to hold the geometry steady. They don't allow the LCA bushings to move back, as they solidify the lower A-arm to the Chassis logitudinally. They also limit droop to an extent, as they cause a binding as the A-arm tries to move up and down. That can slow the travel to an extent, and can have some benefits there. The other main theoretical advantage is that they pre-load the suspension to an extent, from the limited travel. I haven't really seen cars gain a lot from traction bars if they have solid bushings, and a proper alignment to compensate for the standard movement.

  9. #9
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    In regards to slicks. If you are serious about stopping wheel hop then Slicks at the track should be your first mod. Not Solid Motor Mounts and Solid Tranny Mounts. Not Traction bars either. Slicks should be first. The reason slicks should be first is because wheel hop is caused by a lack of traction for various reasons. Traction issues range from simply being street tires, to the LCA flexing, to the wheels actually cambering in and out under load when launching and that changes the grip significantly.

    But Serious get slicks first. If the wheels don't break loose or lose traction, THEY WILL NOT HOP!!! All the vendors would be pissed with me I'm sure cause the MM and Traction bars sell well but are only worthwhile to eliminate wheel hop on the street.

  10. #10
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    On street tires I was able to consistently get down to 2.1 or 2.2 launches at what is actually a poor traction track. Every mod I've put on my car has helped. I've done the MM, the tranny mount, and the traction bars. They helped mostly but taking the variation out from pass to pass and run more consistent.

    I was having a hard time with the cheater slicks I was using. I was able to get down to a 2.0 or 1.9 rather consistently. I may have been trying too hard to get off the line and having some issues with that but I got 1.9 or 2.0 rather consistently. This is with MM, Tranny mount, Traction bars, Stock Sways, Stock springs all around. Nothing really fancy and only stock power levels.

    We then took the Car of PCM controlled boost recently. So it is a stock powered car but now getting full boost in 1st gear. It was wild and I only have 1 race day with that setup. I had a couple issues like the car breaking loose at the top of first (4500-6000rpm range) which I hadn't had problems with ever before. Had to take the Cheater slicks down to 13PSI to get traction where it had been 15PSI before. I did get a 1.8 60' time though. And that is just Cheater Slicks, MM, Tranny Mount and Traction bars...but we opened up the boost in first gear.

    I temporarily have the swap bar disconnected as a Friend is straightening out my traction bar for me this week. I can certainly tell that there is more body roll when making sharper turns. Kinda interesting to see actually. I have 2 race days left this season that I will be going to. One is a Big Bucks Jackpot Race + Test and Tune. I will be practicing at that one. The points Series final is Sept 13th. And I'm very excited because I'm in the lead by just 11 points. And I want to win that Wally Trophy!!! http://www.racesir.com/standings/websitepoint2009h.pdf ...I am the only SRT4 in the points series. Sport Compact Class.

    With the Sway bar disconnected do you think I would be getting a better 60' time?

  11. #11
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    Traction bars to me prevent the Lower control arm from going forward to far. Mine bolts up to the bottom side of the Rad support where the H-Brace is connected to. H-Brace is connected back to the Frame so the brace and frame take all the pressure of the launch. The other side of the Traction bar bolts to the sway bar. So from what I can see it tries to keep that from going forward and thus more in place and thus better traction.

  12. #12
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    Quote Originally Posted by SilverBeast View Post
    In regards to slicks. If you are serious about stopping wheel hop then Slicks at the track should be your first mod. Not Solid Motor Mounts and Solid Tranny Mounts. Not Traction bars either. Slicks should be first. The reason slicks should be first is because wheel hop is caused by a lack of traction for various reasons. Traction issues range from simply being street tires, to the LCA flexing, to the wheels actually cambering in and out under load when launching and that changes the grip significantly.

    But Serious get slicks first. If the wheels don't break loose or lose traction, THEY WILL NOT HOP!!! All the vendors would be pissed with me I'm sure cause the MM and Traction bars sell well but are only worthwhile to eliminate wheel hop on the street.
    Bump for someone who has some understanding of the actual causes of "wheel hop".

    Traction Bars are a legitimate solution, in that they hold the geometry, and solid mounts, although they won't eliminate wheel hop, help keep the motor from rocking back and forth on a hard launch, causing an uneven traction condition. So both are very good mods, especially given that slicks/rims used = $350, and you can get new MM's and Traction bars for around that....

    But thank God for someone who actually understands that it's a lack of traction that causes the wheel hop. Slicks expand, keeping a contact patch, and if you try to spin them fast enough, you'll see just how much they expand. Even spinning, slicks maintain a fairly even grip, deform to the camber and toe changes, and should they lose grip, they try to get larger. It absorbs the up and down movement you normally will see.

    As stated before, street tires will NOT do this. They will not expand much when spinning, causing the weight to shift forward suddenly, then do not give on the downward movement, causing the car to bottom hard, and if it doesn't break anything, and you hold the throttle, it will have found traction due to the heavy loading of the front suddenly, and be attempting to snatch the nose in the air again, causing the same result.....

    The reason for this post, is to analyze some of the reasons that the cars lose traction on launch, and to try to improve the situation with small budgets, and common sense engineering.

    Think of it this way - A set of slicks = $350-600+. A set of traction bars can be found for about $200. A set of shortened rear sway bar end links bolted to the front will run about $20. If this removes the pre-load from the sway bar, and the car is actually trying to hold the nose down on launch (that's right, sway bars, although affecting spring weight by their nature, at a rate much higher than the springs normally, are used primarily to control body motion), it may never try to jerk the wheels off the ground in the first place. It may have traction in 2nd gear, and the only thing you will have to get used to, is the "soft" feeling in the front when cornering. The body will attempt to roll a bit, as it has a lot less spring rate initially. This could shorten launch times on streets to the 1.8's even, if it works well, and as such make for one hell of a stoplight racer, when you aren't giving up a crap ton to an EVO/WRX on the launch.....

  13. #13
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    Quote Originally Posted by SilverBeast View Post
    On street tires I was able to consistently get down to 2.1 or 2.2 launches at what is actually a poor traction track.
    We were pulling 2.11 60's on a track where a big turbo EVO was only pulling 2.04 60's. I don't feel too bad. But I definitely need to figure out how to make use of 2nd gear!

    Quote Originally Posted by SilverBeast View Post
    With the Sway bar disconnected do you think I would be getting a better 60' time?
    See PM

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