Check your rear trailing arms!
07-10-2007, 10:54 PM
#1
Three Wheelin'
Thread Starter
Check your rear trailing arms!
In the thread about camber, the subject of bent rear trailing or "banana" arms came up. Pete Z mentioned that a special fixture, apparently pretty rare, can be used to check if the arm is straight and true. We actually happen to have one of these at the shop, and it's funny that a couple days after it is mentioned I get to use the tool!
The reason I was using the tool is because I noticed something unusual on a customer's 73 911T Targa when I was servicing it . . . . the right rear axle was basically touching the shock absorber instead of being a couple inches in front of it, and the body of the shock was dented in to accomodate the axle. Something was obviously amiss! Nothing seemed obviously bent, so I took some quick measurements, and I initially thought that somebody had replaced the original trailing arm with an early SWB one! But, I measured the wheelbase side to side and it was the same. But, I then found out that Porsche had redesigned the trailing arm for the '72-73 cars to accomodate the change of the rear shock angle. Up to '71, the shock was angled slightly back when viewed from the side. In '72, the shock was made vertical when viewed from the side, but angled more inwards when viewed from the front or back of the car, to improve ride and handling. The mounting hole for the bottom of the shock was thus extended towards the rear of the car. So one can see what was wrong w/ the customer's car: some smart person had replaced the original '73 arm with a '69-71 arm. What makes this worse is the person had to have seen that the axle now rubbed on the shock, so they dented the shock inwards on purpose to make clearance!
Now I am wondering if Pete Z or Steve W or anybody has ever seen anything like that? Nobody at my shop had! Another trailing arm change happened in '74, when Porsche switched to lighter aluminum arms. Also, it was now possible to remove the trailing arms without dropping the engine and tranny!
Now I hope nobody here ever runs into this, but it is something to consider of you have a bent rear control arm and need to replace it, get the right one! I'll leave you with pics of the special tool P295B!
---Chris A.
The reason I was using the tool is because I noticed something unusual on a customer's 73 911T Targa when I was servicing it . . . . the right rear axle was basically touching the shock absorber instead of being a couple inches in front of it, and the body of the shock was dented in to accomodate the axle. Something was obviously amiss! Nothing seemed obviously bent, so I took some quick measurements, and I initially thought that somebody had replaced the original trailing arm with an early SWB one! But, I measured the wheelbase side to side and it was the same. But, I then found out that Porsche had redesigned the trailing arm for the '72-73 cars to accomodate the change of the rear shock angle. Up to '71, the shock was angled slightly back when viewed from the side. In '72, the shock was made vertical when viewed from the side, but angled more inwards when viewed from the front or back of the car, to improve ride and handling. The mounting hole for the bottom of the shock was thus extended towards the rear of the car. So one can see what was wrong w/ the customer's car: some smart person had replaced the original '73 arm with a '69-71 arm. What makes this worse is the person had to have seen that the axle now rubbed on the shock, so they dented the shock inwards on purpose to make clearance!
Now I am wondering if Pete Z or Steve W or anybody has ever seen anything like that? Nobody at my shop had! Another trailing arm change happened in '74, when Porsche switched to lighter aluminum arms. Also, it was now possible to remove the trailing arms without dropping the engine and tranny!
Now I hope nobody here ever runs into this, but it is something to consider of you have a bent rear control arm and need to replace it, get the right one! I'll leave you with pics of the special tool P295B!
---Chris A.
07-11-2007, 12:59 AM
#2
RL Technical Advisor
Hi Chris:
I've certainly seen my fair share of bent trailing arms in both steel & aluminum but I've never seen anyone swap an early one for a late to correct that problem.
Nice tool, BTW. We use our alignment machine to see whether they are bent or not, but that go/no-go tool would REALLY be great to have before the car goes on the machine and the heads are all attached and compensated.
I'd bet you that many shops who work on these car would love to have one, should you have some of these made.
I've certainly seen my fair share of bent trailing arms in both steel & aluminum but I've never seen anyone swap an early one for a late to correct that problem.
Nice tool, BTW. We use our alignment machine to see whether they are bent or not, but that go/no-go tool would REALLY be great to have before the car goes on the machine and the heads are all attached and compensated.
I'd bet you that many shops who work on these car would love to have one, should you have some of these made.
07-11-2007, 08:30 AM
#3
Racer
Join Date: Feb 2005
Location: Camden, SC
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We use our alignment machine to see whether they are bent or not,...
...I suspect I want to read more into this statement than is there.
07-11-2007, 10:02 AM
#4
Rennlist Member
Originally Posted by Steve Weiner-Rennsport Systems
Hi Chris:
I've certainly seen my fair share of bent trailing arms in both steel & aluminum
I've certainly seen my fair share of bent trailing arms in both steel & aluminum
It's possible to bend the aluminum TA on an SC? Seems like it would crack/break pretty quick. I ask because I was considering using the ones off my smacked 78 (with monoballs already installed) on my new 78. The side in question hit hard enough to shatter the CV joint bearing cage (or it could have chewed up driving afterwards but I doubt it). Sounds like for the extra cost and trouble I may as well rebuild the "new" cars TA's with new monoballs.
07-11-2007, 12:09 PM
#5
Rennlist Member
I've seen/replaced a number of bent, aluminum trailing arms with nothing more to go by than our neighbor's alignment shop (they did a lot of Porsche work and we trusted them) telling us that they couldn't get a rear corner into spec. Gary, I've also seen them hit so hard that they break all the way through near where the brake hose connects to the line that comes from the front of the car. Chris, I have tool ENVY! That's one of those great tools that if I had one I would have been happy to use it only once! No matter how much we knew that we were right, there was always some anxiety until the car that we just put a new arm on went back onto the machine, and was found to be perfect. For liability reasons we never installed used suspension parts, only new, but even then it would have been nice to check the new part before putting it on the car. By the way, great eyes! I've never seen that done either, but as these car go from old to really old you're going to see a few pretty weird things.
