Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: Jonny Hotnuts on November 21, 2009, 01:21:18 AM
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I came to the conclusion the other day that I may need higher spring rates in my rear.
For question sake lets say that if you have an everyday car that has 4 wheels with a like track width. Each wheel is sprung when in its 4 wheel configuration. If you were to jack the front up, with the jack in the middle of the front bumper (to simulate narrow track width) and removed the front tires to maintain level you would have exactly 1/2 the spring tension to keep the car level, as it would with the additional spring pressure from the front springs.
This I feel, may have contributed to the sway side to side when running last year.
This may sound like an idiot question but should I consider using adjustable coil overs (or higher PSI springs, but I like the adjustability of the COs) to increase the spring rate. While my front end is not fulcrumed 100%, my track width is considerably narrower than my Rx7 rear end.
ANy thoughts??
~JH
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Jonny:
You might want to look at the thread on rear end alignment. A housing that's slightly bent could be a cause for the problem.
The wide rear end combined with the narrow front may also be part of the reason for the problem. In most classes of racing where acceleration is a major part of the competition the rear track is no wider than the front and in most cases it is significantly narrower. Drag racing and formula cars are good examples. Often times you'll see that the outside width of the front and rear end are pretty close, but track is measured from the centerline of one tire to the centerline of the other. The much wider rear tires mean that the track at that end is significantly narrower. Just another thing to think about.
What sort of speed were you going when the problem arose. Could it relate to aerodynamics? Would a fin possibly help if it's legal in the class?
While I think springs may be a crutch to overcome a problem that you can't correct with the present design I think your approach in that area is a good one. Coilovers definitely give you more flexibility and more stiffness is probably the best way to go first.
Hopefully this reply may trigger more response.
Good luck!
Pete
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"The wider you make a racecar, the worse they handle, because it wants to go nine directions at once."
Ralph Moody, speaking about track width and the Holman-Moody/Ford drag racing program.
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Anti-roll bars might help and you could keep the lighter springs. With the narrow front thread the car will tend to roll over on it self, especially when turning out.
JL222
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Jnuts, is your sway induced by your steering? How much caster do you have? Does your front end set up cause bump steer, which in turn will cause body roll steering? Does your rear suspension promote body roll rear end steering? Look at what you have, set up a few tests to help determine the answers before you start bandaiding any problems.
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Jnuts,
I completely agree with Stainless, make sure what you have is correct before you start doing anything. Did you scale the car so you know the wheel weights, is the front axle parallel with the rear, are the front and rear axles exactly centered on the center line of the car? Most of these can be measured with nothing more than a snap line, plumb bob and tape measure. The scaling will need some good individual wheel scales so find a friend that races circle track and barrow his. Look for the obvious first and then start looking for the less obvious. "Just because you hear hoof beats don't start looking for zebras"!
Milwaukee Midget, Ralph Moody built some great cars but he was wrong. Just watch a World of Outlaws race and notice how wide sprint cars are getting plus the basic math of weight transfer says that wider is better , for road racing not for Landspeed racing.
Rex
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Does the car have a sway bar on either end ?
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. . . but should I consider using adjustable coil overs (or higher PSI springs, but I like the adjustability of the COs) to increase the spring rate.
Coilovers are adjustable for height, but not spring rate. To change the rate, you need to change the springs or the leverage they work through.
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I think what you are saying is that as the front track gets narrower, you become totally dependent on the rear springs for roll stiffness. Any side force can only be resisted by the rear springs. To get roll stiffness you need either to have stiff springs in the rear (so small deflections cause a lot of change in spring force) or a rear anti-roll bar (sway bar) that is twisted as the body rolls side to side.
If your suspension has any roll steer, a side gust could cause it to slightly turn each time the body rolls, which could get you into a side to side oscillation. As mentioned king pin angles and scrub radius etc. can also cause steering effects as the body rolls.
You need to set up some tests where you can measure what your car does as the body rolls (at its ride height at speed!!). For example place dial indicators on the front and rear of one of the front wheels and then slightly roll the body and see if the front wheels are turning slightly. As mentioned many simple measurements can be done with just some mason twine and plumb bobs to check how your tow in and tow out changes with body roll and if your rear axle turns slightly as the body rolls.
Things as simple as soft suspension bushings could be the culprit.
Take some measurements of all your suspension links and draw a scale diagram looking at the suspension from the side and see if raising or lowering a rear wheel (ie body roll) causes the suspension links to move the rear axle hub forward or backward as the wheel rises or drops. Depending on how your front steering is set up your front wheels could be changing toe-in and toe-out as they hit bumps this can also cause steering effects.
If your front wheels are toeing out at speed that can also cause "darting" where the car gets unstable in direction.
http://www.ozebiz.com.au/racetech/theory/align.html
If you can find someone that has this software it will do a lot of calculations for you once you make the measurements and plot curves for changes in toe and that sort of thing.
http://performancetrends.com/SuspAnzr.htm
Ask around the local circle track racers and find out who the local suspension setup guru is. They spend a lot of time learning about how changes in suspension link locations and lengths changes a cars handling. It might be worth a six pack to invite the guy over to look over your suspension to see if he can see any obvious issues that could lead to directional instability.
Remember at speed if you are getting any front end lift, your suspension is not in your static parked position at speed and might be doing something completely different than it does when the car is parked in the garage.
Larry
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some how I managed to double post
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By the new/proposed Indy Racing League design some people must think narrow front tread width handles alright .
http://www.examiner.com/images/blog/EXID31218/images/DeltaWing3_-_EX.jpg
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JH,
which Rx7 rear end do you have in the car?
If the 2nd gen, you could be playing with rear steer geometry as the suspension moves.
Makes a great street car, sucks when you really need to put power down and steer with slip angle.
John
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I think what you are saying is that as the front track gets narrower, you become totally dependent on the rear springs for roll stiffness.
As the front gets narrower, the car acts more like a trike.
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JH;
Coil-over adjustability is for setting the chassis ride height; the spring rate determines the spring rate. Your problem may be due to your suspension geometry-- even a slight amount of toe-out will cause those problems. While you are at it, check your bump steer.
Regards, Neil Tucson, AZ
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. . . but should I consider using adjustable coil overs (or higher PSI springs, but I like the adjustability of the COs) to increase the spring rate.
Coilovers are adjustable for height, but not spring rate. To change the rate, you need to change the springs or the leverage they work through.
Agreed. Additionally, sway bars are nothing more than finer spring rate adjustments. I guess you could always try rising rate springs but that may be more difficult than just getting spring rates to hold whatever ride height you want at speed.
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Jonny, it may have been cross winds, wheel spin or other.
You obviously know how the rotary delivers its power?.
Very different to 4 strokes!!!.
Maybe the problem isn't as bad as you think.
Check the shocks, anti roll bar links and other related components too.
Hopefully it's a small easy to sort out problem.
Just my 2c but the other advice you got is good. :cheers:
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Did he mention this crashed at Bonneville.
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Jonny, it may have been cross winds, wheel spin or other.
You obviously know how the rotary delivers its power?.
Very different to 4 strokes!!!.
Maybe the problem isn't as bad as you think.
Check the shocks, anti roll bar links and other related components too.
Hopefully it's a small easy to sort out problem.
Just my 2c but the other advice you got is good. :cheers:
Mike, this is a world you are entering into with your liner. You have to eliminate body roll to expect a three point vehicle to accurately respond to steering input's at speed without lag that can evolve into over input from the driver.
I don't know if that was the case with his crash but it can be a real problem with any vehicle at speed & is exaggerated with a three pointer.
Sid.
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The basic problem with 3-wheel vehicles is that they have zero roll resistance on one end and non-zero at the other. Whether it is a 'liner or an old Morgan, handling on a 3-wheel vehicle is tricky in anything but a straight line.
Regards, Neil Tucson, AZ
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Thanks for that guys. :wink:
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When Higginbotham got his balanced "right"--- I could drive it with very light pressure of on my thumb and a finger tip as we were going up through the gears. His was solid in the front and rear but he could change the frt. slightly with shims.