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Old 10-04-2010, 07:44 PM
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Rear suspension geometry help 53 wishbone

I am building a 53 chevy truck and need some help for my rear suspension setup. I have a wishbone set up that is 28" long and 32wide. The lowers are also 28 long. So pinion angle shouldn't be a problem, plugging in frame upper x 28 y 0 z 15.112 axle x 0 y 16 z 17.112 lower frame x 28 y 17.501 z 8.887 axle x 0 y 17.5 z 8.887. The results on a calc are, 38.92% antisquat, basically 0 roll understeer RC height 15.11" Ic x115.17 Ic z8.8854". Could somebody give me some opinion on this, this is my first suspension build. The truck will be on air. I would like for it to handle well. Oh yeah. Wheelbase 116"
Any help would be greatly appreciated.

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Old 10-04-2010, 10:52 PM
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May find your answers here >> http://www.racetec.cc/shope/ He's a member on this site also.
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Old 10-05-2010, 06:51 AM
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Quote:
Originally Posted by WvGearhead
...some help for my rear suspension setup.
Your coordinate system is essentially that of the SAE, but with the origin shifted. I've a hunch you already know that. I used the "x" value for link length in the spreadsheet on Page 13. It assumes link lengths of a competition 4link, so you'd have to use projected link lengths when the links are angled in plan view.

If you're going to be doing some autocrossing, the antisquat percentage is all right, though you could probably go to 50% before encountering wheel hop during braking.

You have your lower link horizontal, which is common practice, but certainly not necessary. You can have it at any reasonable angle. It's the percent antisquat which is important. You probably are concerned about roll steer. Keep in mind that roll steer has no effect on tire loading. It merely controls the relationship between steering wheel angle and car attitude. In other words, you can add as much roll understeer as you like to an oval track car that's loose and it will still go into the fence tail first. Excessive roll oversteer, however, forces the driver to make uncomfortable steering angle changes during cornering. What I'm attempting to say is: Don't be worried about a little roll oversteer if it isn't bothering the driver.

I had to input a CG height of 23 inches before I could get that antisquat number. Is it really that high? You need to get it "down in the weeds" if you want good handling.

Antisquat percentage is 100 times the ratio of IC up to IC out divided by the ratio of CG height to wheelbase.
http://www.racetec.cc/shope

Last edited by BillyShope; 10-05-2010 at 07:04 AM.
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Old 10-05-2010, 09:25 AM
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I am guessing the cg @ 23". Is there a good way to come up with cg. The truck should weigh close to 3265 ft 1845 rr 1425. Now that's factory specs, I have replaced the I-6 with a 350 w/ aluminum heads and traded the leafs and I-beam for a mustang ii with tubular a-arms and air springs. I will also be putting a fuel cell on the rear behind the axle. Ride height I'm guessing will be 9" to frame or 4 1/2" to running boards with 26" tires and the air springs aired up 4" out of the total of 7" of travel. I have heard the top bolt in the bellhousing is close for the cg ht. Does that get you close?
Thanks
Aaron
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Old 10-05-2010, 03:07 PM
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Quote:
Originally Posted by WvGearhead
I have heard the top bolt in the bellhousing is close for the cg ht. Does that get you close?
You can certainly get a whole lot closer with very little extra effort. Take a look at Page 39.
http://www.racetec.cc/shope
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Old 10-05-2010, 07:55 PM
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Thanks for you input billy,
I think the center of gravity is going to actually be closer to 19" after looking at some things this evening. Also my link brackets are a little taller than I guessed So the z value changed on the axle end. They are 17.862 and 8.1375. I plugged in upper x 28 y 0 z 15.862 x 0 y 16 z 17.862 and lower x 28 y 17.501 z 8.6375 x0 y 17.5 z 8.1375 and got 53% anti squat it moved the Ic in to 109 roll center at 15". BUT i got into somemore measuring and looks like the drive shaft is going to start getting in the way of the wishbone link. So I am going to have to either raze the frame end upper up a little, reverse the wishbone to top of pumkin or stick it on the bottom like a satchell link. Any advice on this?
Thanks again
Aaron
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Old 10-06-2010, 06:08 AM
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Quote:
Originally Posted by WvGearhead
...reverse the wishbone to top of pumkin....
This might be best...or go to a 3link.
http://www.racetec.cc/shope
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Old 10-06-2010, 08:39 AM
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I know I'm not building a race truck but would like to put a little extra effort to make it handle great. I read that if possible it was best to keep the single link of the wishbone on the frame end to keep the roll center constant with the chassis throughout suspension travel, however to do this I would have to place the wishbone level which would raise the Ic and shorten the ic. Handling wise , would this be better than placing wishbone sigle link over the axle, I would also have to build a bridge due to the cast center section. Bare with me, I'm still learning!
Aaron
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Old 10-06-2010, 01:19 PM
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Also, if I reverse the wishbone and have to mount over the center on a bridge, it will push the the roll center up close to the CoG. If I understand right this will cancel out a lot of body roll, I believe having some helps with weight transfer doesn't it? Choices, choices?
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Old 10-07-2010, 06:26 AM
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Quote:
Originally Posted by WvGearhead
...I believe having some helps with weight transfer doesn't it?
What kind of weight transfer? Lateral weight transfer is distributed, front-to-rear, in proportion to roll stiffness AND roll center height. Weight transfer to the rear wheels during forward acceleration is unaffected by roll center height.
http://www.racetec.cc/shope
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Old 10-07-2010, 08:02 PM
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Seems like body roll would help with shifting weight to the loaded tire when cornering? If I raise the upper bar up 2.5" front and back I think I can clear my driveshaft with the wishbone. First can you run a 5.75" axle tab? and run the 3.25 on the bottom that I have. If so, Raising the uppers Z=18.36 and z=20.36 with the lowers at 8.14 and 9.14 it results 62% anti squat, 20" roll center, 2* oversteer, Ic x=114 y=12.
Or does this is an option change the lowers to z=8.14 and z=8.64 on the lowers with the same uppers and above at 44% A.S., 20" roll center height, 1* oversteer, Ic x=137 y=11.
If I can run that tall of a bracket, which of these sound better. or any opinions. I hate to have to put the wishbone over the pumkin, for another reason now, It will force me to raise the floor of the bed more than I would like.
Thanks For letting me rod some of you knowledge billy
Aaron
I am starting to think I should have read more before I ordered my wishbone set up! lol
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Old 10-08-2010, 06:39 AM
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sorry those roll center heights are wrong, should be 17 and 18
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