T-Bar Removal - Any Downsides?
11-09-2012, 06:07 AM
#1
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T-Bar Removal - Any Downsides?
The consensus seems to be that removing the t-bars is the way to go & makes setting-up more straight forward & the results more predictable. For a caged racing car with reinforced turrets I can appreciate that there's limited impact on the torsional rigidity of the shell.
My question is this: For a road car or a dual purpose road/track car without a cage - will removing the t-bars & substituting with say 700 lb springs put excessive torsional loads through the shell via the turrets as opposed to distributing those forces through the structure at multiple points with a combination of springs & t-bars?
I suppose this comes back to the question why did Porsche go with t-bars in the first place? Why didn't they just go with coilover rear struts?
Was this just deliberately conservative engineering on Porsche's part?
It's a little like why did they put ceramic inserts in the exhaust ports? I always assumed that this was to maintain high exhaust gas temperatures for the cats in the US-market cars ( US-cars were over 50% of 951 production).
Tim
My question is this: For a road car or a dual purpose road/track car without a cage - will removing the t-bars & substituting with say 700 lb springs put excessive torsional loads through the shell via the turrets as opposed to distributing those forces through the structure at multiple points with a combination of springs & t-bars?
I suppose this comes back to the question why did Porsche go with t-bars in the first place? Why didn't they just go with coilover rear struts?
Was this just deliberately conservative engineering on Porsche's part?
It's a little like why did they put ceramic inserts in the exhaust ports? I always assumed that this was to maintain high exhaust gas temperatures for the cats in the US-market cars ( US-cars were over 50% of 951 production).
Tim
11-09-2012, 08:02 AM
#2
Race Car
the only problem I have a fear of is the lower bolt breaking and sending my wheel and tire up into the quarter panel at however fast I am going. This is because the torsion bars would have held the car up but they are gone
11-09-2012, 10:52 AM
#4
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So, I'll start by donning my flame suit, because I know there are a number of people who will disagree with me.
Most people do coilovers because it's easy. Like, stupid easy to install, especially if you don't touch the T-bars. Only problem is that now you're having to deal with two springs working in parallel with one another.
So, all else being equal, the best way to ensure proper handling is by removing the T-bars altogether.
BUT -- the rear suspension in stock form just ain't built for the load. With the T-bars in place (and no coilovers as well), the shock is a non-stressed member of the suspension. Add coilovers there and you've made it a stressed member. The trailing arm itself is plenty strong, but the bolt and mounting point are questionable. I've heard of plenty of instances of either broken bolts or broken mounting points. In fact, there was a RL'er who broke the lower bolt a few weeks ago.
That said, it's important to note that most racers do run coilovers in the back with minimal ill effects. Additionally, you can go to great lengths to spend money on super-strong bolts (Racers Edge sells some that are pretty good). Also, make sure the bolt is not bottoming out, as it is wont to do (due to casting/machining irregularities), else the shock will not be tight against the mounting point.
Also, don't get in an accident. Because your trailing arm is surely going to be the first thing that goes, being effectively the weakest link. I've seen this a few times.
As for your question "why did Porsche do T-bars in the first place", I would cite a couple of reasons:
-- Lots of previous experience with torsion bars: every 911 up until this point had t-bars in the rear. They were getting pretty good with 'em, and the Weissach trailing arm was a very cool piece of engineering at the time.
-- Torsion bars offer precisely the same spring rate through the entirety of its travel (assuming material properties are constant).
Most people do coilovers because it's easy. Like, stupid easy to install, especially if you don't touch the T-bars. Only problem is that now you're having to deal with two springs working in parallel with one another.
So, all else being equal, the best way to ensure proper handling is by removing the T-bars altogether.
BUT -- the rear suspension in stock form just ain't built for the load. With the T-bars in place (and no coilovers as well), the shock is a non-stressed member of the suspension. Add coilovers there and you've made it a stressed member. The trailing arm itself is plenty strong, but the bolt and mounting point are questionable. I've heard of plenty of instances of either broken bolts or broken mounting points. In fact, there was a RL'er who broke the lower bolt a few weeks ago.
That said, it's important to note that most racers do run coilovers in the back with minimal ill effects. Additionally, you can go to great lengths to spend money on super-strong bolts (Racers Edge sells some that are pretty good). Also, make sure the bolt is not bottoming out, as it is wont to do (due to casting/machining irregularities), else the shock will not be tight against the mounting point.
Also, don't get in an accident. Because your trailing arm is surely going to be the first thing that goes, being effectively the weakest link. I've seen this a few times.
As for your question "why did Porsche do T-bars in the first place", I would cite a couple of reasons:
-- Lots of previous experience with torsion bars: every 911 up until this point had t-bars in the rear. They were getting pretty good with 'em, and the Weissach trailing arm was a very cool piece of engineering at the time.
-- Torsion bars offer precisely the same spring rate through the entirety of its travel (assuming material properties are constant).
11-09-2012, 03:07 PM
#5
Rennlist Member
I thought the actual Weissach part was 928 specific Collin?
11-09-2012, 05:36 PM
#6
I removed my torsion bars this year and installed 500lb rear coils on Bilstein Escort Cup shocks (with the stock rubber lower shock eyes and new BMW grade 10.9 lower bolts).
Everything stayed together.
As an experiment, I did it first without changing the spring plate bushings from the original rubber ones. Apart from a lot of negative camber from the arms slumping in, it was pretty driveable in my case. It isn't a long-term or high-performance solution, though, so count on replacing the bushings.
If torsion bars had been as easy, I probably would have paid the higher price for them, since changing springs isn't really a high priority for me.
Everything stayed together.
As an experiment, I did it first without changing the spring plate bushings from the original rubber ones. Apart from a lot of negative camber from the arms slumping in, it was pretty driveable in my case. It isn't a long-term or high-performance solution, though, so count on replacing the bushings.
If torsion bars had been as easy, I probably would have paid the higher price for them, since changing springs isn't really a high priority for me.
11-09-2012, 07:11 PM
#7
Rennlist Junkie Forever
I've been running without torsion bars for my last 3 951's... going back to 2000. The last one (the most recent one) with extensive racing for the last 4+ years. None have ever had a problem. This with rear spring rates ranging from 550lbs to 1200lbs.
Additionally, I know a lot of people that race without torsion bars as well. And none of them has every had a problem.
But... I do know of one car that I personally saw the bolt break and he limped back into the pits. That, however, was not a race car. It was street car doing laps on a fun day at the track. Upon closer inspection, his bolt was loose and started to back out. Once that happens it's going to bend then break for sure. He was able to actually remove the portion of the broken bolt from the trailing arm by hand :-)
That said... I go to a bolt specialty shop and order the highest quality bolt I can get in the proper length and tighten the hell out of it.
Zero problems.
One thing to remember.... is that if you remove the torsion bars, you need to solidly mount the torsion bar carrier to the chassis with solid bushings.
TonyG
Additionally, I know a lot of people that race without torsion bars as well. And none of them has every had a problem.
But... I do know of one car that I personally saw the bolt break and he limped back into the pits. That, however, was not a race car. It was street car doing laps on a fun day at the track. Upon closer inspection, his bolt was loose and started to back out. Once that happens it's going to bend then break for sure. He was able to actually remove the portion of the broken bolt from the trailing arm by hand :-)
That said... I go to a bolt specialty shop and order the highest quality bolt I can get in the proper length and tighten the hell out of it.
Zero problems.
One thing to remember.... is that if you remove the torsion bars, you need to solidly mount the torsion bar carrier to the chassis with solid bushings.
TonyG
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11-09-2012, 07:52 PM
#8
Three Wheelin'
A lot of the race parts for the 1302 are interesting to look at.
Also, have a look at the Porsche 964 rear suspension. Coilovers, and a remarkably similar rear arm to the 944 - except for the addition of the Weissach axle.
Porsche engineering was particularly evolutionary at this stage of their career, and for marketing reasons as well as business and engineering ones.
Cheers,
Mike
11-09-2012, 09:26 PM
#9
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A nice feature of torsion bar suspensions is the spring is not part of the unsprung mass – in other words the moving parts are lighter!
11-10-2012, 11:21 AM
#10
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Certainly do not want to add a coilover helper spring without reindexing the torsion bars. If leaving the t-bars in, they need to be set at a much lower index height, so that the coil spring is compressed/pre-loaded when the car is sitting at normal ride height. This does take somewhat more screwing around to accomplish than just removing the t-bars all together.
And if you just dump the t-bars, that also reduces much of the need to screw around w/ the spring rate balance/effective spring rate "math" to determine combined rear spring rates, which seems to cause so much discourse in all these suspension threads...
With that said, by rule, PCA stock class club race cars are required to retain the original style spring. So there are a lot of track/race cars that are setup and successfully running the combined rear spring setups for many years, going back to the factory Turbo Cups and Clubsports.
The only concerns that may be legit w/ removing the t-bars are 1) more load on the shock mounts, upper and lower, and 2) that removing the t-bar will allow more deflection of the spring plate bushings.
1) If using stock rear shock mounts, like bilstein escort/cup coilovers or koni 8242 dbl adj coilovers, you retain the stock 14mm lower bolt. I have never heard of the stock rear bolt failing. If you are running a rear coilover w/ the 1/2" spherical bearings, and using an aftermarket adpater, there is some more known risk of those failing. I believe the Racers Edge adapter bolts have the best reputation, but those do not have a zero failure rate either.
I have never heard of a problem w/ the upper shock bolt, and have never seen or heard of deformation or failure of the body mounting, though I have heard people concerned that the mount was not designed to handle the loading of a full coilover.
2) I don't know if there is a way to easily prove or disprove if the amount of spring plate bushing distortion increases w/ t-bars removed or not. I know in talking w/ Jeff Stone at Kelly Moss probably 12 years ago about this - he very much felt it did increase the loading on the bushing itself, and strongly recommended getting rid of the spring plate rubber. KMR used to offer a really trick (and really expensive) tapered roller bearing conversion for the torsion bar carrier/spring plate pivot joint. I think the best solution available now is the elephant racing poly/bronze bushing.
And if you just dump the t-bars, that also reduces much of the need to screw around w/ the spring rate balance/effective spring rate "math" to determine combined rear spring rates, which seems to cause so much discourse in all these suspension threads...
With that said, by rule, PCA stock class club race cars are required to retain the original style spring. So there are a lot of track/race cars that are setup and successfully running the combined rear spring setups for many years, going back to the factory Turbo Cups and Clubsports.
The only concerns that may be legit w/ removing the t-bars are 1) more load on the shock mounts, upper and lower, and 2) that removing the t-bar will allow more deflection of the spring plate bushings.
1) If using stock rear shock mounts, like bilstein escort/cup coilovers or koni 8242 dbl adj coilovers, you retain the stock 14mm lower bolt. I have never heard of the stock rear bolt failing. If you are running a rear coilover w/ the 1/2" spherical bearings, and using an aftermarket adpater, there is some more known risk of those failing. I believe the Racers Edge adapter bolts have the best reputation, but those do not have a zero failure rate either.
I have never heard of a problem w/ the upper shock bolt, and have never seen or heard of deformation or failure of the body mounting, though I have heard people concerned that the mount was not designed to handle the loading of a full coilover.
2) I don't know if there is a way to easily prove or disprove if the amount of spring plate bushing distortion increases w/ t-bars removed or not. I know in talking w/ Jeff Stone at Kelly Moss probably 12 years ago about this - he very much felt it did increase the loading on the bushing itself, and strongly recommended getting rid of the spring plate rubber. KMR used to offer a really trick (and really expensive) tapered roller bearing conversion for the torsion bar carrier/spring plate pivot joint. I think the best solution available now is the elephant racing poly/bronze bushing.
11-10-2012, 11:52 AM
#11
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11-10-2012, 01:08 PM
#12
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With that said, by rule, PCA stock class club race cars are required to retain the original style spring. So there are a lot of track/race cars that are setup and successfully running the combined rear spring setups for many years, going back to the factory Turbo Cups and Clubsports.
If using stock rear shock mounts, like bilstein escort/cup coilovers or koni 8242 dbl adj coilovers, you retain the stock 14mm lower bolt. I have never heard of the stock rear bolt failing. If you are running a rear coilover w/ the 1/2" spherical bearings, and using an aftermarket adpater, there is some more known risk of those failing. I believe the Racers Edge adapter bolts have the best reputation, but those do not have a zero failure rate either.
11-10-2012, 01:10 PM
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11-10-2012, 02:42 PM
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...
But... I do know of one car that I personally saw the bolt break and he limped back into the pits. That, however, was not a race car. It was street car doing laps on a fun day at the track. Upon closer inspection, his bolt was loose and started to back out. Once that happens it's going to bend then break for sure. He was able to actually remove the portion of the broken bolt from the trailing arm by hand :-)
...
But... I do know of one car that I personally saw the bolt break and he limped back into the pits. That, however, was not a race car. It was street car doing laps on a fun day at the track. Upon closer inspection, his bolt was loose and started to back out. Once that happens it's going to bend then break for sure. He was able to actually remove the portion of the broken bolt from the trailing arm by hand :-)
...
And here is my final solution: https://rennlist.com/forums/attachme...t-p1010085.jpg
The car since “died” for other reasons and I am back to T-bars in my current 951.
Laust
11-11-2012, 01:23 AM
#15
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