Race Tech pistons in stock cartridges

**SixPackBack** · 31st May 2008, 17:47

Originally Posted by FROSTY

Robert my mental image is that the entire shim stack flexes up and down with every suspension stroke.
Isn't that a concern from the point of metal fatigue?

Generally speaking 'metal' can be bent up to a certain point almost limitlessly without fear of fatigue. Deformation is the term, and is explained in more detail here. If you have ever seen an aircraft's wing flapping around on take-off or under storm conditions you will have some appreciation how reliable the science behind this is.

**Drew** · 31st May 2008, 18:26

Originally Posted by Crasherfromwayback

Yes.

I can help you there though Bruva.

They're bending.

Bending under preasure even.

**White trash** · 31st May 2008, 18:43

Originally Posted by Crasherfromwayback

Yes.

I can help you there though Bruva.

They're bending.

Pete! Thank christ you're here

**Robert Taylor** · 31st May 2008, 20:15

Originally Posted by FROSTY

Robert my mental image is that the entire shim stack flexes up and down with every suspension stroke.
Isn't that a concern from the point of metal fatigue?

Hiya Frosty! And in an MX rear shock there are approximately 17,000 of those movements EVERY lap, some of them quite violent. Given what they go through they last surprisingly well but it is something we check everytime we pull a shock or pair of forks apart. The hardest worked shims are thin ( usually mid-valve ) .1 to .15mm thick that act as the sealing shims right next to the piston port face. We pay particular attention to those. The bigger the ports and the smaller the clamp washer the less support they have. In an MX situation like that we replace shims often.
Your road race suspension should be serviced once yearly in a typical NZ usage scenario. During the Road Race Nationals our contracted riders may have their dampers pulled apart up to 5-10 times over the course of that short season, but ostensibly to change settings from track to track and to continue in a never ending quest to improve lap times.

**Crasherfromwayback** · 1st June 2008, 09:54

Originally Posted by White trash

Pete! Thank christ you're here

I reckon. I actually tried to call him to let him know, but his line is always engaged.

**Crasherfromwayback** · 1st June 2008, 10:03

Originally Posted by Robert Taylor

Hiya Frosty! And in an MX rear shock there are approximately 17,000 of those movements EVERY lap, some of them quite violent.

And it's amazing the amount of heat generated by the fast guys!

Years ago when I was working at Boyle Kawasaki, Mark Boyle was looking after Darryl Kings bike. He was having a lot of trouble with the rear shock fading (a stock unit), and we tried all sorts of different fluids to try and reduce the fade. In the end we used the hydraulic fluid they use in aircraft susp units, and it helped. Fucking expensive it was too!

After each race, the shock reservoir was that hot it'd sizzle if you spat on it, way too hot to touch.

**Drew** · 1st June 2008, 10:08

Originally Posted by Crasherfromwayback

And it's amazing the amount of heat generated by the fast guys!

Years ago when I was working at Boyle Kawasaki, Mark Boyle was looking after Darryl Kings bike. He was having a lot of trouble with the rear shock fading (a stock unit), and we tried all sorts of different fluids to try and reduce the fade. In the end we used the hydraulic fluid they use in aircraft susp units, and it helped. Fucking expensive it was too!

After each race, the shock reservoir was that hot it'd sizzle if you spat on it, way too hot to touch.

Wonder how that feels on a crosser, I have ony ever noticed shock fade as less grip and running wide on exit, must be fuckin bone jarring on a dirt bike.

**Sensei** · 1st June 2008, 10:09

My back header on the RSVR runs right beside the TT-X shock & have wondered surely that must become hot while riding . I think I may have asked Robert about this but if not , How hot is To hot Robert ???

**Crasherfromwayback** · 1st June 2008, 10:11

Originally Posted by Drew

Wonder how that feels on a crosser, I have ony ever noticed shock fade as less grip and running wide on exit, must be fuckin bone jarring on a dirt bike.

Couldn't tell ya. I've never been able to ride one hard enough. I was always just flapping behind them like a limp flag!

**Drew** · 1st June 2008, 10:16

Originally Posted by Sensei

My back header on the RSVR runs right beside the TT-X shock & have wondered surely that must become hot while riding . I think I may have asked Robert about this but if not , How hot is To hot Robert ???

Same on the 749, pipes sit right beside the shock, and riding a 999 on the road the heat eminating from under the seat is really noticable, even at 100k's.

Time to fashion my one off GP style exhaust system and leave one poking out at the bottom to give the shock more cool air space methinks.

**Crasherfromwayback** · 1st June 2008, 10:19

Originally Posted by Drew

noticable, even at 100k's.

.

Fuck off Noddy...when did you last do 100kph?

**mashman** · 1st June 2008, 10:41

Originally Posted by Sensei

My back header on the RSVR runs right beside the TT-X shock & have wondered surely that must become hot while riding . I think I may have asked Robert about this but if not , How hot is To hot Robert ???

Sorry for the thread jack as it's off topic... You could try getting the link pipe and headers ceramic coated... apparantly that shifts a lot of the heat...

On thread though. I'm slowly getting to grips with some of the concepts and do enjoy reading Roberts articles as they're pretty unambiguous... Long may they continue...

On a side note to JD... You may well be an excellent suspension professional, but you may want to work on your professional courtesy skills. Robert said he was more than happy to discuss things further through email, yet you persist in an open forum... Nothing against you, but your posts read as a big FU...

**Robert Taylor** · 1st June 2008, 10:41

Originally Posted by Crasherfromwayback

And it's amazing the amount of heat generated by the fast guys!

Years ago when I was working at Boyle Kawasaki, Mark Boyle was looking after Darryl Kings bike. He was having a lot of trouble with the rear shock fading (a stock unit), and we tried all sorts of different fluids to try and reduce the fade. In the end we used the hydraulic fluid they use in aircraft susp units, and it helped. Fucking expensive it was too!

After each race, the shock reservoir was that hot it'd sizzle if you spat on it, way too hot to touch.

Without in any way wishing to sound a little smug might that have been at approximately the same time his brother Shayne was enjoying the fade free / consistent race performance of Ohlins?
Shock temperature and fade is always a problem in MX and the oem manufacturers always insist on a one piece piston ring with very little ''corrective /compensating'' expansion. As the alloy body heats up it expands at a greater rate and there is bypass past the piston ring = fade
Another cause of fade is if the locknut for the piston and shimstack assembly is overtorqued, as the sintered piston heats up the shim stacks ''curl'' and no longer seal properly on the sealing faces of the piston. Also the rebound rod that is through the middle of the main shaft is made of a different material that expands at a faster rate to compensate for the thinning out of the oil, the choice of materials is a key factor.
Cheap oils and cheap shocks with imprecise tolerancing and cheap material choices will fade faster. Excessively high gas pressure will increase friction which builds heat faster. Gas pressure in a shock should not be used as a tuning tool and only needs to be just high enough to preclude cavitation. That depends on how well the shock internally pressure balances, for example an Ohlins TTX36 only requires 6 bar gas pressure whereas more conventional designs most often require 10 - 12 bar.
If the internal valving spec is very firm and there are long stroke movements fade will occur faster. An oem shock in a sportbike will require firmer valving to hold it up in its stroke, but especially on a bumpy track fade will initiate much sooner than a high quality aftermarket shock.
If the bleed job on a shock is careless there will be a much higher percentage of air bubbles homogenised into the oil and that will also initiate fade sooner. We have an Ohlins vacuum bleeding machine pending on arrival within the next month, that is another subject.
ETC...
A high quality shock with excellent material choices, precise tolerances and high quality oil will have stable performance at temperatures of 80-90 degrees celsius. Remember that as you raise pressure you raise boiling point. The right hand rear shock in the Toyota Race Series cars is especially close to the exhaust muffler, we measured temps of 115-120 degrees, some 20 degrees higher than the left side. That gave me cause for concern as the dampers are also so small and hold only about 200mls of oil. But they will go a full season between rebuilds as it has turned out. According to Ohlins such temps are quite normal in formula cars.

**Crasherfromwayback** · 1st June 2008, 13:57

Originally Posted by Robert Taylor

Without in any way wishing to sound a little smug might that have been at approximately the same time his brother Shayne was enjoying the fade free / consistent race performance of Ohlins?

It doesn't sound in the least smug, and you make a lot of valid points and obviously know your business incredibly well.

I don't wish to sound smug either, but Darryl with his stock shock was faster than Shayne. That's not to say obviously that Darryl wouldn't have been able to go faster longer with an Ohlins unit, but it reminds me of the one thing that never changes with fast racers. They'll win on/with anything, because they're that good.