I recently came across some good deals on a slightly used BMR adjustable and a used UMI torque arm relocation trans cross member.
Question: the relocation cross member has instant center adjustment, any suggestions as to where a good starting point would be?
The occasional drag racing but setting the car more for AutoX and some local road racing events
Currently running 200 lbs. springs in the rear.

https://www.umiperformance.com/home/product...gid=1&pid=2

 

Best place for the IC is usually along the line between the CG and the rear tire patch. You can consider the CG to be at the shifter ****, for most such purposes. For autocross the IC will want to be relatively close to the CG; for strictly drag race only it often works better farther rearward and/or higher. If you locate it too far to the rear or too high in trying to optimize for drag racing then you risk creating excessive wheel hop under braking. Stock, the IC is at the intersection of the line through the LCA bolt holes and the line vertically through the front torque arm mount: AT BEST somewhere under the car and below the rear of the transmission. Both too low and too far forward.

 

Is there any benefit to lowering the front torque arm position?
So lowering the car also lowers the CG, I know there are to many variables to hash out here I’m just wondering if setting the front of the torque arm in the middle or slightly elevated would be a better starting point.

 

Like you said, it depends on several variables. Now that the ride height has changed by lowering the car, the rear LCA angle has changed. Are you running aftermarket LCA's and brackets? Dragzine had an article for plotting instant center and anti-squat with BMR on a S197 mustang. Even though the mustang utilizes a different suspension design, it's really good to visualize the CG, Wheel Centerlines, ground line, and neutral line. If you draw this out with your car, you might be able to visualize the static changes that occurred with moving the torque front mount.

 

Torque arm length is a very strong factor. Torque arm height is a very weak factor. Control arm angle is another strong factor.

I might be wrong but I think the height is pretty much inconsequential with a stock style bushing mount (within the small range of adjustability). The torque arm can only pull up, so the line of action is the same whether the mount is 1-in higher or not.

Swing plate mount is different because it can apply a side load at the mount depending on how it's aligned. So people might witness a slight difference but really is probably just a misalignment of the swing plate.

 

I am using LCARB's, I'm finding a lot of info on instant center referring to drag racing but I'm not sure How much of that relates to AutoX and road racing.

 

So then I should be safe with mounting in the middle and making sure my pinion angle is good?

 

Pinion angle is not related to any of this except by accident. It has no effect on traction etc; only on U-joint matters.

Lowering the car lowers the IC DRASTICALLY. Traction suffers accordingly. LCARBs help restore it to somewhere more reasonable.

 

The instant center on a sliding link torque arm has nothing to do with the front of the TA's height- the IC is where a vertical line drawn through the front link crosses a line drawn through the LCA pivot points. (somewhere, way back in the day, I posted a drawing about all this).

if the front height is adjustable and it has some for/aft motion built into it set it to something that clears things well and you can set a reasonable pinion angle.

 

While the website I referenced refers to a website focuses on drag racing, the article refers to how Instant Center can be adjusted based on the requirements, such as AutoX or daily cruising. The article also overlays the measurements on a vehicle. I'm more of a visual learner, so illustrations on a car are appreciated. It's worth a look. https://www.dragzine.com/tech-storie...er-anti-squat/