2012 RCR #19) Lower control arm length and tolerance

[MHISSTC]

I didn't submit this RCR, but I thought more about it when Cody was trying to accurately measure the distance between bolt centers on our MM Extreme Duty non-adjustable lower rear control arm while it was still on the car at our last event. It was a pain in the butt.

 

I told Cody I have a bunch of OEM lower rear control arms in my shop and that I'd get some OEM measurements that might help with this issue.

 

Here's what I did this morning.

 

I took one pair of "very good" condition FORD OEM lower rear control arms bolts and placed them in the bushing sleeves of 6 different OEM Ford fox lower rear control arms and measured both a "max" length and a "min" length to the nearest 1/32" for each arm while holding the bolts at each extreme in the sleeves. My error in each measurement is probably something between 1/64" to 1/32". I also did the same for a set of generic tubular non-adjustable aftermarket arms that are similar to the Summit or UPR brand arms. I didn't measure the distance between bolt centers on either the J&M or the MM lower rear control arms we have because they are currently on the cars.

 

Here are my results.

 

OEM #1 max = 17 24/32" min = 17 20/32" diff = 4/32"

OEM #2 max = 17 21/32" min = 17 18/32" diff = 3/32"

OEM #3 max = 17 22/32" min = 17 21/32" diff = 1/32"

OEM #4 max = 17 23/32" min = 17 21/32" diff = 2/32"

OEM #5 max = 17 22/32" min = 17 21/32" diff = 1/32"

OEM #6 max = 17 25/32" min = 17 11/32" diff = 14/32"

 

Aftermarket #1 max = 17 37/64" min = 17 17/32" diff = 3/64"

Aftermarket #2 max = 17 19/32" min = 17 17/32" diff = 2/32"

 

A few things stand out. First I had to measure #6 multiple times to see if I was getting it right. I was. I think one of the sleeves on it is worn out, so we can probably toss out any data from that arm. Second, the Aftermarket arm is supposed to be a direct replacement to the stock arm, but the longest max length measurement on those arms is only longer than two of the shortest stock arm measurements, and one of those is from the #6 arm with the worn out sleeve. And third, the difference between the max/min lengths of the aftermarket arms falls within the range of the stock arms pretty well.

 

If you were to come up with a mean length with a range of lengths based on the OEM data above, you would have:

 

OEM arm average length = 17 21/32"

OEM arm min shortest length = 17 9/16" (17 18/32")

OEM arm max longest length = 17 3/4" (17 24/32")

 

That's 17 21/32" +/- 3/32", or a total range of allowance of 3/16".

The Aftermarket arm would barely fit within the short side of this tolerance.

 

Using the average shortest and longest lengths only gives you:

17 21/32" +/- 1/32", or a total range of allowance of 1/16".

The Aftermarket arm would be outside of this tolerance by 1/16".

 

Therefore, based on the data above, if you are looking for a specific lower control arm length and range of values that would be legal for the Mustangs, I would recommend the following:

 

Mustang lower rear control arm length from attachment bolt center to attachment bolt center = 17 5/8" +/- 1/8". That would allow a bolt center spacing difference of 1/4" and allow measured values to range from 17 1/2" (17.5") to 17 3/4" (17.75").

 

I'll let you know if we get around to measuring the bolt center distance on the J&M or the MM lower rear control arms also.

Edited October 27, 2012 by Guest

[Al F.]

Thanks for the data Scott.

[MHISSTC]

UPDATE: I figured out the generic looking aftermarket lower rear control arms I have appear to be JEGS brand arms. JEGS advertises their arms to have a center-to-center length of 17.562" which is 17 9/16". That agrees with what I measured and makes their tolerance +/- 1/32".

[BADVENM]

Bringing this up from the dead for our new director Supercharged111. Our lower control arms were teched at impound yesterday. I mentioned the dimensions in the current rules were via MHISSTC measuring the various sets he has laying around.

[Dustin M.]

I observed the same as Cody, these things were a pain in the ass to measure. With that said, the Pumpkin's arms were, from my point of view, on the high side of the current allowances (but legal is legal so who gives a shit). I can't imagine stock arms being any easier to measure though. Just odd that what I observed seemed to contradict what Scott saw, though Scott had the arms on a work bench when he did his checks.