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I have a Lotus 7 replica based off a Mazda RX-8. It uses the complete front and rear subframes, motor and trans. I have probably a unique suspension problem- The factory RX-8 suspension geometry has too much camber gain. With only 1.5 degrees of camber the outside of the tires don't hardly get worked at all even on track. My guess is because of the 1400lb reduction in weight and much lower CG is why I'm seeing this problem. I'm looking for a way to reduce the camber gain front and rear. Any of you have any ideas on how I might be able to accomplish that?
assuming you have everything set at the minimum camber position, you must have it lowered a lot compared to the RX8 chassis then
there are ways to correct this on the suspension, but it appears that you’re running fairly large diameter wheels/tires for that chassis. So going to a smaller OD wheel/tire profile would allow raising the suspension up in compensation to help correct the geometry some. Because running the OE suspension at an excessively lowered height will result in less than optimum potential handling performance.
Otherwise adjusting the front is fairly easy. There’s a metal bushing pressed into the lower front upright that the front LCA ball joint tapered stud fits through. Mazda Motorsports sells a replacement bushing with an offset tapered hole that you can install there to get maybe a 1° - 1.5°. Most RX8 racers use it to increase camber, but it’s just as easily installed with placing the offset position 180° to decrease camber as well.
In the back, the rear LCA is tubular steel and can be cut somewhere along the length of it, shortened some, then welded with the addition of an OD sleeve welded over the joint to add strength. This will help bring the camber range back into compliance. Alternatively, there are aftermarket LCAs that have a threaded adjuster in the length of it that can accomplish the same thing. There are other variations; cutting the OE LCA and weld an adjuster in it, etc. It just needs to be shortened to bring the camber back into an adjustable range is the gist of it.
Yet again, that’s going to change the geometry some from the original intention, but Mazda never intended for those subframes to be on that chassis either. Anyway, it can be done in the manner you so choose.
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I should have said I'm running the minimum camber I could get which was 1.25 degrees front and 1.5 rear.
Current rims are 16" and were the smallest I could fit over the RX-8 brakes. Actually had to go down from the sport brakes that were on my donor to the smaller automatic brakes.
I'll look into that bushing for the front and the the rear control arm. I think I saw SPL made something.
It appears the subframes have been located in the chassis way too low. The angle of those upper arms show them in the full compression position even at rest. This means you are almost on the steepest part of the camber curve already.
The optimal solution is to relocate the subframes higher so that they are in a similar position to that which mazda envisaged.
I think I would take that pain for a better result.. The front could be partly addressed by relocating the front upper arms upward. I would model it in susprog3d first.
My ride height is maybe only 1" lower than stock. The factory arms have a pretty good up angle to start with. You can see here from a press release picture for the NC chassis from Mazda.
If that is the case, what do you think has changed when the subframes were installed into the new chassis?
Can we get pics of the uprights and the inboard mounting points for both arms?
you’re assuming in that picture it’s at the OE ride height, but the top position of the front shocks relative to where they mount on the chassis is going to determine that. Except you have rocker arms with inboard shock/spring mounting showing on the front of your car.
two things are going against your claim;
1. The OE suspension can easily get to 0° because that’s more or less what the factory front camber spec is on the RX8. It also has to be lower than 1”, because that’s not only what takes the camber adjustment in that direction away, but is also where the camber gain starts increasing dramatically.
2. I can see in the one front picture of your car that the angles of the front control arms are positioned in that fashion as well. Been doing this a long time having specifically bought my RX8 in early 2005 for autox competition and using it well in that regard. So I’m fairly familiar with this aspect of the suspension.
Raising the chassis up is going to bring it back into compliance, but I’m certain you don’t want to do that. So your only real option is to counter the amount of camber in it now as per my first post.
You could measure from the floor to the bottom of the front subframe cross piece in the center and someone can get the same measurement on their RX8 to compare against. I’m going to approximate it as 2.5” lower than stock.
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To the subframe is 4.75". I had measured a stock car before this and that was 5.5-6" but was hard to get an exact measurement as climbing under the car was difficult. Having access to a four post lift would have made that much easier and accurate. Suspension also could have been sagged some(or alot) too as the car was an 04 and this was in 2018. Also keep in mind my tires are 3" smaller OD than an RX-8 tire so that sticks me down 1.5" closer with no geometry change.
Was just looking at the car in the garage and that upper arm angle looks much more severe in the picture than in real life.
The problem is much more pronounced in the front than the rear. This going off tire temps. I've chocked it up to the drastically change weight and location of that weight as compared to an RX-8. Like I said I've dropped 1400lbs total. The weight bias is much further back also. The engine is moved back 12" in comparison to the front wheel centerline and the driver is 2' closer to the rear wheels. On top of that the CG of the car is dramatically lower. There might be 150lbs above the top of the engine now. I'm not sure what that number is on an RX-8 but I'd guess it's 5x that.
I ran a 245/35-17 Toyo R1R tire that was 23.8” OD to get a gearing advantage for the 140 TW street tire class; you can adjust wheel offset to compensate for scrub. That’s not relative to the discussion.
You’re still overlooking the fact that it can’t be adjusted to 0° camber in the front at a static position. The weight loading and such isn’t going to influence this much at all.
even the Mazda front suspension camber spec indicates how ride height affects the camber value.
on the upper camber spec for the Standard suspension, a 1.5” ride height change is indicating about 1° of camber variance. Again, that to me indicates the subframe/chassis is low. Because at the factory ride height it can be adjusted to the positive side.
Even in the stock class with some things done to compress the Sport suspension to lower it and build static negative camber, it’s hard to get -2.0° front camber. You’re not far from that with the adjuster all the way in the 180° opposite positive direction.
The subframe is low, but to get around that you can use the upright camber bushings from Mazda Motorsports to offset the the upright into a positive camber direction (lower end of the upright inward)
^^includes two offset bushings for S1 2004-2008 model, the S2 2009+ version is a different part no.
Since this is a motorsports part the general public can order it without any race results or full membership. Ordering Mazda OE parts requires full membership with race results as proof.
11-26-2023, 11:19 AM
