Sourcing TT Bearings and Sleeves
07-17-2009, 10:48 PM
#16
Rennlist Member
I'll just say this just once. There are many folks here that have used the VBX bearings (6006 ZZ, C3), several are tracked cars only, and not one complaint in well over two years. If people are concerned about rpm and heat suitability they are rated for 16,000 rpm and temps much higher that the TT area will ever see. Buying C4 bearings at $60/each compared $4/each in packs of 10. VBX are American made so I'm not too worried about QC. I've bought SKF wheel bearings made in Mexico and I returned them the quality and finish was so bad. I'm not saying not to use the C4 rating but the difference in radial clearance between a C3 isn't really an issue with manufacturing techniques today compared to 20 years ago. If it gives you piece of mind that Stuttgart specified C4 (C5 in early cars) then by all means spend the extra money. Using the same OE spec logic I'm not convinced the aluminum bushings are such a great idea either. Porsche used a non-metallic bushing for a reason and I doubt it was to save money seeing how aluminum isn't spared anywhere else on the car.
Just to follow up: Has any one had a C3 bearing fail in a TT yet?
Just to follow up: Has any one had a C3 bearing fail in a TT yet?
I agree about the aluminum bushings, it doesn't seem consistent with the general design. The difference in clearances between C3 and C4 is really, really small compared to the rest of the system, but the sloppier bearings will have somewhat lower stresses than tighter ones. C3 bearings in Aluminum bushings... now you've tightened up the system significantly IMO, and aided heat transfer into the bearing.
07-17-2009, 11:07 PM
#17
Hey Man
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My personal "engineering analysis" of the whole TT design is this. "Sloppy" bearings, plastic sleeves, rubber bushings all tell me the engineers were trying to avoid a condition of over-constraint. Four bearing points along a shaft cannot all be in the same line, it is just not possible, or necessary. By allowing some float, it assures none of the bearings are overloaded (more than the rest of them) and compensates for the different modal shapes the shaft will see at different RPM's. (Think of a sine wave with each bearing being a point where the wave crosses the X axis) Over-constraint can create very large stresses, whereas a little "give" can almost eliminate them with no real downside.
I agree about the aluminum bushings, it doesn't seem consistent with the general design. The difference in clearances between C3 and C4 is really, really small compared to the rest of the system, but the sloppier bearings will have somewhat lower stresses than tighter ones. C3 bearings in Aluminum bushings... now you've tightened up the system significantly IMO, and aided heat transfer into the bearing.
I agree about the aluminum bushings, it doesn't seem consistent with the general design. The difference in clearances between C3 and C4 is really, really small compared to the rest of the system, but the sloppier bearings will have somewhat lower stresses than tighter ones. C3 bearings in Aluminum bushings... now you've tightened up the system significantly IMO, and aided heat transfer into the bearing.
I agree about the aluminum bushings, I would avoid them personally.
07-18-2009, 02:53 AM
#18
Three Wheelin'
The only reason I went for C4 is because the car is tracked regularly and the tt can get quite hot from the exhaust and constant high rpm. The replacement bearings use standard grease and are only rated to about 110-130 degrees C, so the extra clearance gives a little extra safety margin. The OE bearings are rated to about 150 degrees C (Hi temp grease). I can't find C3 or C4 with high temp grease - so using C3 would be compromising both temperature and clearance. C4 with regular grease is the next best thing. If C3 is OK, why did Porsche go with C4? The technology of steel ball bearings and tolerance hasn't really changed in 20 years. C4 in 1989 is still C4 in 2009.
And, yes, C3 may still last quite some time - I just prefer the peace of mind of C4 - I don't want to have to go through this again.
Not convinced the rubber shielded versions are a good thing (bit like a rubber centered clutch!).
And, yes, C3 may still last quite some time - I just prefer the peace of mind of C4 - I don't want to have to go through this again.
Not convinced the rubber shielded versions are a good thing (bit like a rubber centered clutch!).
07-18-2009, 10:30 AM
#19
Hey Man
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The only reason I went for C4 is because the car is tracked regularly and the tt can get quite hot from the exhaust and constant high rpm. The replacement bearings use standard grease and are only rated to about 110-130 degrees C, so the extra clearance gives a little extra safety margin. The OE bearings are rated to about 150 degrees C (Hi temp grease). I can't find C3 or C4 with high temp grease - so using C3 would be compromising both temperature and clearance. C4 with regular grease is the next best thing. If C3 is OK, why did Porsche go with C4? The technology of steel ball bearings and tolerance hasn't really changed in 20 years. C4 in 1989 is still C4 in 2009.
And, yes, C3 may still last quite some time - I just prefer the peace of mind of C4 - I don't want to have to go through this again.
Not convinced the rubber shielded versions are a good thing (bit like a rubber centered clutch!).
And, yes, C3 may still last quite some time - I just prefer the peace of mind of C4 - I don't want to have to go through this again.
Not convinced the rubber shielded versions are a good thing (bit like a rubber centered clutch!).
Makes good sense. I could never find anything built with Krytox hi-temp grease either. There was one company that said they could the flush the old grease out though and add Krytox but it sure seems like that would effect the shield integrity.
Last edited by KuHL 951; 07-18-2009 at 07:32 PM.
07-18-2009, 06:38 PM
#20
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The sine wave analogy is a good one. Although the clearance difference between C4 and C3 is tiny I guess the extra clearance of each bearing adds up over the total length and gives the shaft more room to move through 360 degrees. The RPM's that we're exposing the bearings to is so low but I think with the exhaust so close, heat is a more likely culprit. If I could have found C4's with the same ease as C3's I would have bought them for sure. I figure it's taken 20 years for my bearings to go bad, I'd be happy if the new ones last less than half that.
07-19-2009, 08:20 PM
#21
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Coming to this party a bit late but would like to make a few comments from the research we've done into 928 and 924/944/968 TTs and drivelines:
1. The Original Porsche spec'd TT bearings are a double steel shielded (ZZ) 6006 C5 rated bearing that was initially made by INA for Porsche. C5 rated bearings have a looser tolerance between the ball bearings and races. They are no longer available from INA and not easily found. People who offer rebuilt TTs are usually using C3 rated bearings for cost and they are easily found.
2. Misalignment of the drive shaft within the TT was not the worry for Porsche in using C5 rated bearings. They did not want the bearings seizing when they got hot and the ball bearings expanded a bit due to their unique application, to hold onto and support a drive shaft in a high heat environment inside a TT. Too tight of a tolerance will have the ball bearings start to deform by catching and sliding along the races, than rolling on the race surfaces as intended.
Kind regards,
1. The Original Porsche spec'd TT bearings are a double steel shielded (ZZ) 6006 C5 rated bearing that was initially made by INA for Porsche. C5 rated bearings have a looser tolerance between the ball bearings and races. They are no longer available from INA and not easily found. People who offer rebuilt TTs are usually using C3 rated bearings for cost and they are easily found.
2. Misalignment of the drive shaft within the TT was not the worry for Porsche in using C5 rated bearings. They did not want the bearings seizing when they got hot and the ball bearings expanded a bit due to their unique application, to hold onto and support a drive shaft in a high heat environment inside a TT. Too tight of a tolerance will have the ball bearings start to deform by catching and sliding along the races, than rolling on the race surfaces as intended.
Kind regards,
07-19-2009, 10:25 PM
#22
Rennlist Member
My overall question is, why doesn't Porsche support this critical part of our cars? Most everything else is readily available either thru Porsche or aftermarket. Sure, these are low volume pieces (probably once in each car's lifetime, maybe twice), but that's four of each (bearings, sleeves, etc) and total production of these cars was certainly high enough to warrant maintaining a stock of the proper pieces. They are obviously critical to keeping these cars on the road. We shouldn't have to scour the world for the proper bearings and make our own delrin sleeves...
