Does wheelbase length affect oversteer/understeer?
When I stretch out the Fiero's frame ('88) the necessary 16" for the Four-Twelve body, I need to know if I'm going to have to build a new suspension to balance out the understeer/oversteer issues that I'm afraid will ensue.
Anybody know any physics or good math on this question?
------------------ Patrick Horne phorne_tca@yahoo.com
"Yep."
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10:45 PM
PFF
System Bot
Howard_Sacks Member
Posts: 1871 From: Cherry Hill, NJ Registered: Apr 2001
Any ideas as to a new spring weight? I'm trying to keep this thing to where if I want to race it, I can, but also practical enough to drive around backroads or through town.
I was thinking ~300 lb rear springs and dampers to match, but as I said earlier, I don't know what to expect from changing the suspension up that dramatically.
------------------ Patrick Horne phorne_tca@yahoo.com
"Yep."
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11:42 PM
Patrick Horne Member
Posts: 144 From: Ruston, LA, USA Registered: Nov 2001
Also, at what point on widening the rear suspension do I need to start watching the overall geometry of the suspension? I know that some company out there makes a 3" widening suspension for the Ferarri guys, but have no idea who it is or if it's even quality workmanship...
(I'm trying to work out the math on what the difference in track width is going to be on the widened rear cradle, so I'll have some numbers soon hopefully.)
Thanks again
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11:46 PM
Sep 14th, 2004
Will Member
Posts: 14280 From: Where you least expect me Registered: Jun 2000
It won't be purely the change in wheelbase... but the subsequent effects. By pushing the rear wheels backward and moving the engine forawrd relative to the rear axle center line, you'll be shifting weight forward, which will affect the handling balance. However, you won't need to do much do the geometry... just retune. If you REALLY want to match the geometry to the new layout, figure out where the CG of your chassis without powertrain is, then figure out where the CG of your power train is, then figure out where the CG of chassis with powertrain is... then you can get a handle on the centroid axis and design a suspension with the roll axis parallel to the centroid axis...
------------------ Turn the key and feel the engine shake the whole car with its lope; Plant the gas pedal and feel in your chest neither a shriek nor a wail but a bellowing roar; Lift and be pushed into the harness by compression braking that only comes from the biggest cylinders while listening to music of pops and gurgles. Know that you are driving and American V8. There are finer engines made, but none of them are this cool.
Luck, Fate and Destiny are words used by those who lack the courage to define their own future
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04:26 PM
Patrick Horne Member
Posts: 144 From: Ruston, LA, USA Registered: Nov 2001
I agree with everything Will has wrote, although I think changing roll centres might (would) be overkill on a kitcar.
I also have heard that the stock suspension's roll centre is out of control under migration. (I think this was from you Will!) It really doesn't matter how nice things look static(ie a nice parallel mass centroid line and the axis between the front and rear roll centre) if they get out of hand under a dynamic situation.
Will, I figured you'd chime in here, but figured it would be more on wheel-bearing loads from the track change or sticking some longer control arms in related to the change in track.
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08:48 PM
Patrick Horne Member
Posts: 144 From: Ruston, LA, USA Registered: Nov 2001
Also, Held Motorsports was who I had looked at for widening the suspension. According to their site, it normalizes the stress on the bearings (since there's no spacers and the hubs are more like what they should be) and it gives a little bit better performance as far as suspension geometry goes. (At least that's what I picked up out of their website. I just skimmed through it.)
I'm assuming that with longer control arms, there wouldn't be any extra bearing load. I may be wrong, but hopefully HMS knows their stuff a little bit better than I do.
Which brings to mind another interesting point: Since the cradle is getting the hell modified out of it anyway (virutally an entirely new rear end, actually) would it be better to move the control arms closer into the car (making the control arms longer, ultimately) or just to use the distance that's on the stock Fiero?
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10:41 PM
Howard_Sacks Member
Posts: 1871 From: Cherry Hill, NJ Registered: Apr 2001
Dynamic as in the car taking a turn or a set of them vs sitting still.
The roll center of most cars does not stay in the same spot when the car rolls. IF it does stay in the same spot, it means the suspension's designer has done his homework. In the case of the fiero, I have heard that it bobs up and down and left to right. I have not modeled it. Will has. I'm sure Will will throw in some words about it.
Also, keep in mind the CG of the car will shift as the car corners. Thus further complicating keeping those axis parallel. As a result of change of CG, when you design your suspension, you have to decide if you are going to optimize the geometry for trailbraking or traditional style driving or maybe even make it forgiving for when drivers jump off the throttle sudddenly in a corner.
Tweaking suspension opens a large can of worms. I'm not saying you can't do it. Getting it to a point where you have improved what you had just won't be simple.
If talk of roll centers is new to you, Competition Car Suspension and Tune to Win are good books to pick up. If you are still a college student, this is a senior design if I remember right, a $200 2 day course called Optimum G given by Motec would be money well spent.
quote
Originally posted by Patrick Horne:
Howard, when you say that the stock suspesion gets out of contorl under a "dynamic situation," how dynamic exactly are you talking?
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11:23 PM
Howard_Sacks Member
Posts: 1871 From: Cherry Hill, NJ Registered: Apr 2001
All other things being equal, widening your arms will improve camber curves. You will generate some more G in cornering. There wouldn't be much change in wheel bearing load, but that I believe that would be due to the track change increasing G and the favorable camber curve increasing G and not the a-arm change itself.
The way I could see loads changing dramatically would be from you installing spacers or odd offset wheels.
If I were fabricating a cradle pretty much from scratch, forget about just bringing the mounting points in, I would ditch the strut setup and go to double wishbone with push rods and order up some 8100 series Penskes, but of course, I am a maniac.
FYI do a couple searches on Held and decide if they are a company you want to do business with.
quote
Originally posted by Patrick Horne:
Also, Held Motorsports was who I had looked at for widening the suspension. According to their site, it normalizes the stress on the bearings (since there's no spacers and the hubs are more like what they should be) and it gives a little bit better performance as far as suspension geometry goes. (At least that's what I picked up out of their website. I just skimmed through it.)
I'm assuming that with longer control arms, there wouldn't be any extra bearing load. I may be wrong, but hopefully HMS knows their stuff a little bit better than I do.
Which brings to mind another interesting point: Since the cradle is getting the hell modified out of it anyway (virutally an entirely new rear end, actually) would it be better to move the control arms closer into the car (making the control arms longer, ultimately) or just to use the distance that's on the stock Fiero?
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11:35 PM
PFF
System Bot
Sep 15th, 2004
Patrick Horne Member
Posts: 144 From: Ruston, LA, USA Registered: Nov 2001
Looking at the 8100 series dampers and wow. Nice. I'm still not sure which configuration I'll end up going with as far as the suspension goes, but if I end up having to try to figure out all of the original Fiero's suspension setup/geometry and then try to work out a geometry that would work solely for the purpose of a kit car, then you're probably right in saying that it'd be overkill (not to mention a pain in the butt...) If I end up going for the gusto, I'll just build an entirely new space frame and work from there.
Oh, and I'd love to attend an Optimum G course...if they were offered in the US. I think it'd be VERY well worth the money and for a three-day seminar to learn everything they teach, VERY much worth the time. I just can't find a listing for one in the US.
Thanks again for the info.
------------------ Patrick Horne phorne_tca@yahoo.com
