Weak syncros may not be the issue after all...

[CaptainCaveman]

Been doing some research into the Syncro failure on the Clio, as I've had many performance cars over the years with much more power than the little Clio, and never had any issues with syncros, or gearboxes to be honest. I was struggling to see why the Clio would be different, as normally any issue with changing gear are down to clutch issues, rather than the gearbox.

Anyway, after lots of reading, found this article from a trannie specialist in the US, and they suggest that syncros shouldn't be a failure item, if the clutch is ok. They suggesting that our apparent gearbox issues and Syncro failures are down to clutch drag, and all the symptoms that they apportion to this are mirrored in the symptoms that we commonly experience with our syncros when they're on their way out.

A few interesting quotes from their article:

"When a clutch drags, it is not letting go of the disk fully which overloads the synchros and burns up the friction surfaces quickly. Synchros work by speeding up or slowing down the input shaft and clutch disk by acting like a brake. If the clutch is dragging, the synchro friction surface is trying to speed up and slow down the engine’s mass, which is impossible. The synchro will burn up, fail and the transmission has just been ruined."

"We believe that the synchros in all manual transmissions are brilliantly engineered and should last the life of the vehicle. If your synchros went bad and the transmission needed to be rebuilt, ask yourself why."

"Most clutches will drag at high RPM if there is a problem. If you miss a shift or have a hard time going into gear at high RPM, the clutch is dragging and needs to be fixed."

Full article here:

https://jackstransmissions.com/blogs/pages/clutch-drag-kills-synchros

Interested to hear people's thoughts...

M.

 

[peter]

@Northloopcup ...

 

[kmxgoon]

also interested, my 200 crunches going down into 4th from 5th but is fine going up from 3rd.

 

[petercronin]

I’m not disagreeing here (and it would be brilliant if this turns out to be true) but if that was the case how has there so many incorrectly fitted or faulty clutches.

 

[CaptainCaveman]

I’m not disagreeing here (and it would be brilliant if this turns out to be true) but if that was the case how has there so many incorrectly fitted or faulty clutches.

The same reason why there were so many apparent faulty syncros I guess. Clearly there is something wrong, but the article suggest it may be a clutch issue, rather than an actual Syncromesh design fault, which causes the syncros to fail. Definitely something to explore further I'd say...

 

[lewishunter98]

When I went to ktec.

They said they're seeing more and more synchro failures due to clutch drag. And they believe that is a big contribution to all the crunchy boxes.

That and people changing too fast obviously when they're on it

Sent from my ONEPLUS A5010 using Tapatalk

 

[CaptainCaveman]

When I went to ktec.

They said they're seeing more and more synchro failures due to clutch drag. And they believe that is a big contribution to all the crunchy boxes.

Very interesting - that adds enormous weight to the verdict from the article above then. So it seems that a properly fitted, and potentially uprated clutch should prevent recurrence of any Syncromesh issues once the syncros have been refreshed.... Maybe...

 

[LOCKE]

So how do you solve the clutch issue?

 

[CaptainCaveman]

So how do you solve the clutch issue?

• Adjust the master cylinder under the dash. Video of how to do this HERE
• Bleed your hydraulic clutch system and make sure you have a factory master cylinder from the dealer ONLY! Do not use generic auto parts store/eBay masters.
• Check for improperly torqued block-to-bell-housing bolts, flywheel, or pressure plate.
• There should be 2 dowel pins to properly center the transmission with the engine. If they are missing or damaged, the clutch will be out of alignment and will not work properly.
• Check for missing block-to-bell-housing bolts. If any are missing the transmission can be pushed away from the engine which can crack the bell housing and make the clutch drag.
• Make sure the throw out bearing fork is not hitting the bell-housing when the clutch pedal is depressed. Proper pivot ball shimming is HERE.
• Check for incorrect flywheel step height. Most aftermarket flywheels are stepped too tight.
• Check for warped flywheel, floater or pressure plate surfaces.
• You may have a broken or bent clutch disk due to improper installation.

 

[CaptainCaveman]

Reading the article again, it actually explains why our cars in particular are prone to Syncromesh failure. It seems that unlike the vast majority of normal cars, as our engines are subject to much higher revs than a normal engine, they are therefore subjected to much higher engine harmonics and crankshaft torsional whip.

So the answer to the question, why are our cars so liable to clutch drag?

"Clutches can drag if they are subjected to excessive engine harmonics and crankshaft torsional whip. Most engines don’t develop severe harmonics through the clutch until 5k RPM or higher."

Obviously our cars sit above 5k rpm a lot of the time, hence why the issue only appear in RS Clios, and not other Clios and Renaults that share basically the same box. It seems to be the high revs that cause the clutch issues to reveal themselves. This is also corroborated by the fact that the crunching on our gearboxes doesn't happen at lower revs, at least initially.

I really feel we might be onto something here that finally explains the Syncromesh issue and ensures that once fixed we don't suffer from it again. Maybe?

 

[sevenfourate]

Reading the article again, it actually explains why our cars in particular are prone to Syncromesh failure. It seems that unlike the vast majority of normal cars, as our engines are subject to much higher revs than a normal engine, they are therefore subjected to much higher engine harmonics and crankshaft torsional whip.

So the answer to the question, why are our cars so liable to clutch drag?

"Clutches can drag if they are subjected to excessive engine harmonics and crankshaft torsional whip. Most engines don’t develop severe harmonics through the clutch until 5k RPM or higher."

Obviously our cars sit above 5k rpm a lot of the time, hence why the issue only appear in RS Clios, and not other Clios and Renaults that share basically the same box. It seems to be the high revs that cause the clutch issues to reveal themselves. This is also corroborated by the fact that the crunching on our gearboxes doesn't happen at lower revs, at least initially.

I really feel we might be onto something here that finally explains the Syncromesh issue and ensures that once fixed we don't suffer from it again. Maybe?

I think it is; and find this really interesting. And yes, agreed: it could explain a lot. I'll be really interested to see some more info / see some well educated car / Engineering guys in this type of field chip in and see how the thread progresses. But there's no doubt at all that there is an awful lot of merit /truth in your findings.

I'm imagining with a little Googling the info wasn't too difficult to find ? With the amount of failures in Clio's alone - i'm surprised no-one else has found this info before; as the / a potential reason behind everyone's issues. I'm an over-thinker and if my gearbox suffered - i'd like to think i'd have researched / done a bit of Googling etc and found this for myself.

As it is: Good Stuff ! Lets see what happens......

 

[sevenfourate]

PS; My 41k 200 had a Lady owner for quite some years before me [Not to say she didn't have heavy lead boots] and has lead a cosseted life by me (Engineer with max mechanical sympathy). This should be the prime example not to have suffered or going to suffer (?) with Syncro issues if the reasons of constant high RPM leading to Syncro death is true. And gearbox as yet is as silky smooth as anything i've owned. Touch wood, lucky Rabbits foot, 4-leafed clover etc

On the same front: High mileage cars that have been bounced of the rev-limiter at every opportunity - and we know the engines are built for this - should be failing in droves.

So anyone got a high miler that's been regularly thrashed over the last XXXXX thousand miles with the original box still feeling awesome ?

 

[Beany]

PS; My 41k 200 had a Lady owner for quite some years before me [Not to say she didn't have heavy lead boots] and has lead a cosseted life by me (Engineer with max mechanical sympathy). This should be the prime example not to have suffered or going to suffer (?) with Syncro issues if the reasons of constant high RPM leading to Syncro death is true. And gearbox as yet is as silky smooth as anything i've owned. Touch wood, lucky Rabbits foot, 4-leafed clover etc

On the same front: High mileage cars that have been bounced of the rev-limiter at every opportunity - and we know the engines are built for this - should be failing in droves.

So anyone got a high miler that's been regularly thrashed over the last XXXXX thousand miles with the original box still feeling awesome ?

86k 3 Trackdays last year and still strong

 

[CaptainCaveman]

High mileage cars that have been bounced of the rev-limiter at every opportunity - and we know the engines are built for this - should be failing in droves.

I don't think there is necessarily an inherent flaw in the design of the clutch or the syncros. The more probable explanation is that due to the high revving nature of the engine, tolerances and clutch set-up, clutch health or maintenance need to be far stricter to ensure that clutches don't drag. From the article it sounds like there are quite a few variables that can cause clutch drag, so it only takes one or a combination of these to be out of kilter to cause clutch drag, and subsequent Syncromesh failure.

Like you I'm surprised that none of the specialists have called this out as the issue, but then again the failures are a regular and easy income, so why would you find a cure?... </Cynical hat off>

 

[kmxgoon]

PS; My 41k 200 had a Lady owner for quite some years before me [Not to say she didn't have heavy lead boots] and has lead a cosseted life by me (Engineer with max mechanical sympathy). This should be the prime example not to have suffered or going to suffer (?) with Syncro issues if the reasons of constant high RPM leading to Syncro death is true. And gearbox as yet is as silky smooth as anything i've owned. Touch wood, lucky Rabbits foot, 4-leafed clover etc

On the same front: High mileage cars that have been bounced of the rev-limiter at every opportunity - and we know the engines are built for this - should be failing in droves.

So anyone got a high miler that's been regularly thrashed over the last XXXXX thousand miles with the original box still feeling awesome ?

Mine had the box replaced under warranty at 29k, cars now on 64k with the second box crunching. I did have one of those sprint boosters fitted for around 6months. I believe this has contributed to the second being damaged.

 

[sevenfourate]

Mine had the box replaced under warranty at 29k, cars now on 64k with the second box crunching. I did have one of those sprint boosters fitted for around 6months. I believe this has contributed to the second being damaged.

Why the connection between the Sprint Booster and g/box (syncro) failures ?

 

[DS197]

I remember talking to my Renault specialist about MK3 gearbox failures. From what I can remember, he said that the boxes were originally designed for a lower revving Renault. Plus the Clio has a shorter throw than the model the gearbox was originally designed for. Add people rushing gear changes into the mix and its a recipe for disaster.

 

[kmxgoon]

Why the connection between the Sprint Booster and g/box (syncro) failures ?

The gear changes were much quicker from what I remember, and from reading the above they were probably too quick for the box. There may be no connection. I believe the sprintbooster only sharpen the throttle response.

 

[Doogle]

Mines on 122k now and no sign of failure. Theres nothing in the service history about changing a gearbox or clutch so I would assume its not been changed. I am fairly mechanically sympathetic in that I tend to put my foot down at 3k plus and if I change I dont smash it through the box. I also have a Sprint Booster fitted which I have had fitted for over 6 months. The car is a daily but does have a few foot down moments every week.

 

[RSRowe]

So all being 'well' and you proving that the issue is engine harmonics causing the clutch to drag. Then the only solution is to get the crank, fly, and clutch assembly dynamically balanced?