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Certified Opelholic
1,560 Posts
Discussion Starter · #1 ·
Bob and Bob maybe you guys can help me out

let me explain--- I am trying to find some race rims so i call Keizer they wanted $315 a wheel

wow i said so i figured if i have to spend that much money on rims lets get something that looks half way decent

so i call Truechoice wheels, I start explaining to the guy what i want
13x7, 4 x 100 mm bolt pattern next i mention to the guy the reasons i want to use 13" rims

i said one-- to lower the CG of the car
two-- 13" tires are cheaper

He says 13" rims will not lower the CG of the car

so i call another friend a few min ago (use to races karts)

he says unless you are lifting both inside tires off the ground
CG height is not important

i wont go into all the argument but i have been busting my butt trying to lower the CG

what do you guys say feel free to go into detail


Super Moderator
15,032 Posts
Well, since I'm tired I won't go into much detail. 13" wheels alone won't lower the CG, but the diameter of the tires will. I think what the guy on the phone was saying is if you have a 15" wheel with a 23" tire and a 13" wheel with a 23" tire, all the 13" wheel has accomplished is a reduction of a little bit of unsprung weight (at the risk of gaining it back from the tire) and maybe some inertia. But, a 20" tire on a 13" wheel will lower the car 1.5" compared to a 23" tire on a 15" wheel. So yes, this will lower the car's center of gravity.

Keizer also makes one-piece and three piece all aluminum wheels. I got a set of 13" x 9" 3-piece wheels for $178 each. You must have gotten a quote for the magnesium center wheels. They're even lighter than the aluminum centers (my 13 x 9 wheels weigh 9.25 lbs each), and much stiffer. But expensive.

I don't know racing karts, but since they have no suspension, and therefore have no relationship between the CG and roll centers, his comments have little to do with an automotive racing suspension. The smaller the distance between the CG and roll center, the less body roll you'll have. So raising the roll center(s), OR lowering the CG (or both), will reduce body roll for a given spring rate.



Senior Contributor
901 Posts
The magnesium centers are intended to help with brake cooling rather than to be lighter. As magnesium is ~35% lighter than aluminum they are able to use more material, and since it's a cast center they have more options in the shape of the part to provide the best strength and heat conduction....


302 Posts
Roll center is a funny thing. First, smaller diameter tires do in fact move the roll center closer to the ground... it moves the entire car closer to the ground which is a good thing for aerodynamic reasons. However, that's not the whole story.

For instance, in the rear, the diameter of the tires has nothing to do with the roll center. Rather the panhard bar or watts link determines the roll center no mater what the diameter of the tire. (This does not apply to double wishbone or McPherson type rear suspensions.)

Up front, the diameter does have some affect. However it is not what you think it might be. The dominant factors in determining the roll center for double wishbone cars is the angle of the A-Arms. For example, if you lower or raise the car 2" you will significantly change the angle of the A-Arms and the roll center will move a lot. However, change the diameter of the tire by 4" (equal to raising/lowering 2", and the roll center will only move a little.

Center of gravity... really center of mass may move closer to the ground when the car is lowered, but just moving it closer to the ground does not make the car handle better. Only lowering the center of mass in relation to the suspension (and roll center) will make a difference. To explain... imagine you had had 1000 pounds on top of the car. It would try to roll over in every corner. However, if you added the same 1000 pounds to the floor-board, it would still corner "flat". Hopefully, you can see why wheels won't affect center of mass.

Now to add to the confusion... it is the center of mass that "rolls" around the roll center. So, like all suspension set-up, it is a matter of front to rear ballance. You want the roll centers (front vs rear) set so that the weight transfer in the corner moves weight equally to both outside tires (this assumes a perfect world). Since the rear center of mass is at a different height than the front, the roll centers at each end need to be different.

One last piece of confusion... except for a watts link type rear suspension, as the suspension moves up and down, the roll center moves around. In the rear as the springs extend, the roll center moves toward the top of the car... so when you stomp on the brakes, the roll center moves up. Up front, as the suspension moves, the angles of the A-Arms change and the roll center moves. The front is even worse because in a corner one side is compressing, and the other side is extending... so not only is the roll center moving up and down, it is moving side-to-side.

By now I have probably got you completely completely confused by my trying to explain this in words only. In another post I recomended a software program that you can use to "see" what various changes to the front suspension do to the roll center. If you really... really want to know what's going on with the front end, the software is the best way I know of to get a good graphical - visual image of what's going on.

About the wheels... unless you are using aftermarket vented disk brakes don't use the Kieser "solid center"wheels (or wheels like are shown in the picture) The tiny holes in these wheels don't allow enough cooling. The magnesium center Kieser wheels indeed have spoke type centers and are far superior for brake cooling. (If you're not racing I have a set of the "solid center" 13x7 Kieser "solid center" wheels I'll sell you for $135 each... used once).

About the 20x9.5x13 tires... don't use those if you have stock suspension... or near stock front suspension parts. You'll find out no matter how you try to set up the suspension the car won't handle as well as 23x9x13. (I found this out at a double national... I flat spotted a set of 20x9.5x13 tires and all I could buy was a set 22x9x13 tires. I picked up over 1 second a lap. Before the next race I did a test day with back-to-back testing with a new set of 20x9.5x13 and the difference held up... For sale cheap... 4ea 20x9x13 Goodyear race tires... only test time on the tires.)

Certified Opelholic
1,560 Posts
Discussion Starter · #6 ·
thanks for your reply

i read it 3 times already maybe a few more times and i will get it

i did down load the demo roll center program from performance trends

my GT's front is not stock it had different lower ball joints
don't know where they came from or what car they are off of
(one of those mystery Opel items that you Here Opel people talk about like the 1 wire 65 amp Opel alt)

Anyway back to the front end the different lower ball joint raise the steering knuckle up about one inch.

Then Rally Bob made me some modified upper Opel A-Arms but other then that it was pretty much stock.

I have a set of the "solid center" 13x7 Keizer "solid center" wheels I'll sell you for $135 each ... used once)

For sale cheap ... 4ea 20x9x13 Goodyear race tires ... only test
time on the tires.)

sure thanks for the offer send me you phone number and i will call you and we can work out the details
[email protected]

looks like i am going back to Solo II the cost of road racing is just to high. solo II was costing over $100 per event
when i stopped (which is why i wanted to go road racing)
that's $100 for 4 runs of about 1 min each


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