Cold tear - can a worn shock linkage make it worse?

[TonyB]

Righto. BEARS round 3 yesterday- I was having some problems with cold tear on my mighty 62hp 1999 Ducati Monster. The tyre is a Pirrelli Supercorsa SC1. The damage you can see in the pic was done in 1 practice and 6 races of 6 laps on the Ruapuna B track. The right hand side of the tyre is mint- mainly because there are no right handers that you accelerate hard out of on the B Track.

I had some experienced guys advising me that the issue was that the tyre pressure was too high, so we tried lowering the pressure about 4 times and raised it again by 3 psi at the end. We started with 26 psi stone cold, then tried 27, 26, 25, 24, and 28psi with the tyre slightly warmer than body temp. It didn't seem to matter what we did, but then I get the feeling that once cold tear starts it kinda spreads like cancer....

I know the shock is not happy and I need to do something about it....money money money. On the last cool down lap of the day I rode around with my bum on the seat (I normally always hang off because sitting with my bum on the seat just feels wrong...) and it seriously felt like there was a couple of big chunks out of the tyre. I got in and had a look and the tyre was 'fine' except for the cold tear. When I got it home I had to lift the back end to park it where I wanted and I noticed that there is quite a bit of slop somewhere in the linkages- the back of the bike can move up and down maybe 10-20mm. I'll be pulling it apart ASAP to see whats up.

So my question is, can this slop in the linkage cause the accelerated cold tear that I'm getting? I'm picking that it can because the tyre won't stay in contact with the track properly, but would like to hear what people think.

[Robert Taylor]

Righto. BEARS round 3 yesterday- I was having some problems with cold tear on my mighty 62hp 1999 Ducati Monster. The tyre is a Pirrelli Supercorsa SC1. The damage you can see in the pic was done in 1 practice and 6 races of 6 laps on the Ruapuna B track. The right hand side of the tyre is mint- mainly because there are no right handers that you accelerate hard out of on the B Track.

I had some experienced guys advising me that the issue was that the tyre pressure was too high, so we tried lowering the pressure about 4 times and raised it again by 3 psi at the end. We started with 26 psi stone cold, then tried 27, 26, 25, 24, and 28psi with the tyre slightly warmer than body temp. It didn't seem to matter what we did, but then I get the feeling that once cold tear starts it kinda spreads like cancer....

I know the shock is not happy and I need to do something about it....money money money. On the last cool down lap of the day I rode around with my bum on the seat (I normally always hang off because sitting with my bum on the seat just feels wrong...) and it seriously felt like there was a couple of big chunks out of the tyre. I got in and had a look and the tyre was 'fine' except for the cold tear. When I got it home I had to lift the back end to park it where I wanted and I noticed that there is quite a bit of slop somewhere in the linkages- the back of the bike can move up and down maybe 10-20mm. I'll be pulling it apart ASAP to see whats up.

So my question is, can this slop in the linkage cause the accelerated cold tear that I'm getting? I'm picking that it can because the tyre won't stay in contact with the track properly, but would like to hear what people think.

The worn linkage will certainly exacerbate the problem further, I will send a detailed reply tonight, there are a lot of contributory factors.

[johnsv650]

maybe a sco is for a hot track and the track was a cold track , when robert answers could he tell us which compound to use on what temparture track, i think (limited knowledge) you should have been using a sc3 on that cold track, sco or sc1 for hot track tempartures......the tire didn't get hot enought for the compound to work , so cold shearing....

[TonyB]

Thanks to you both. I look forward to your reply Robert

[Robert Taylor]

Righto. BEARS round 3 yesterday- I was having some problems with cold tear on my mighty 62hp 1999 Ducati Monster. The tyre is a Pirrelli Supercorsa SC1. The damage you can see in the pic was done in 1 practice and 6 races of 6 laps on the Ruapuna B track. The right hand side of the tyre is mint- mainly because there are no right handers that you accelerate hard out of on the B Track.

I had some experienced guys advising me that the issue was that the tyre pressure was too high, so we tried lowering the pressure about 4 times and raised it again by 3 psi at the end. We started with 26 psi stone cold, then tried 27, 26, 25, 24, and 28psi with the tyre slightly warmer than body temp. It didn't seem to matter what we did, but then I get the feeling that once cold tear starts it kinda spreads like cancer....

I know the shock is not happy and I need to do something about it....money money money. On the last cool down lap of the day I rode around with my bum on the seat (I normally always hang off because sitting with my bum on the seat just feels wrong...) and it seriously felt like there was a couple of big chunks out of the tyre. I got in and had a look and the tyre was 'fine' except for the cold tear. When I got it home I had to lift the back end to park it where I wanted and I noticed that there is quite a bit of slop somewhere in the linkages- the back of the bike can move up and down maybe 10-20mm. I'll be pulling it apart ASAP to see whats up.

So my question is, can this slop in the linkage cause the accelerated cold tear that I'm getting? I'm picking that it can because the tyre won't stay in contact with the track properly, but would like to hear what people think.

1) We tend to race on cold tracks at low ambient temperatures much much more so than the Europeans and North Americans, so we get this problem much more frequently. As an oversimplification these Northern hemisphere riders may only have the issue for the first one or two early season summer meetings and then everything shuts down late in the summer as their winters are too harsh to race in. As it usually doesnt get so severe here in NZ we race all year round.

2) SC2 would have been the best choice, all else being equal and up to speed. ''Johns SV650'' has a handle on it, the softer compounds are usually not so good in cold conditions, which is counter-intuitive to what many believe. Simplistically, the chemistry of the compound is not designed to work so well at lower temps. Such tyres are produced to work at higher track temps for the mainstream Euro and North American seasons, and tyre companies are certainly not going to produce tyres specifically for a pin-prick in the ocean that orders minute quantities ( that is how they would see it )

3) I am guessing you are not using tyre warmers? If so the outside working surface of the tyre is getting a little temperature into it, albeit briefly and you are certainly getting no core temperature. There is also then effectively too big a differential between core temperature and the outside surface temperature. Efficient tyre warmers applied over a suitable time period will not only heat the tyre but also the rim so that the tyres will retain heat for longer. The rear tyre is running so cold that playing with tyre pressures was having very little effect.

4) If indeed you have 10-20mm of movement in your linkages than that is 10-20mm of undamped movement, movement where the suspension is imparting no pressure on the tyre to keep it interlocked with the pavement. Problem exacerbated. I would also question the smoothness of movement.
Moreover this really emphasises the need for pre race scrutineering because such an issue really is a danger to yourself and other racers.

5) Is the shock that is fitted a single tube Sachs with only a damping adjuster at the bottom and a progressive spring fitted? If so the single rebound adjuster has most effect on rebound control ( by approximately 80% ) and about 20% effect on compression control. Commonly I have found that many people overtighten this adjuster in a fruitless attempt to minimise squat, that results in way too much rebound damping which overstresses the tyre. Squat is caused by the start rate of that progressive spring being too light, especially in a road race situation ( which it clearly isnt intended for ) and also if your personal weight is beyond 70 kgs.

While still being well beyond what a proper racing shock can achieve we have been able to effect sizable improvements with these shocks simply by fitting constant rate Ohlins springs that have a higher start rate ( giving better squat control ) and a lesser end rate, so that in fact the shock will absorb the abrupt bumps a little better.

The condition of the shock also comes into question, if in its 9 years it has never been serviced then it is well beyond its best.

I know this all costs but how much is it costing in over frequent tyre usage and if you crash because of the suspension and tyre issues?

How many clicks out is the rebound adjuster and what is your personal weight in your everyday clothes? No point in asking what your sag setting correlation is until you sort the linkages out. Hope this helps.

[TonyB]

Thanks for giving such a detailed reply Robert. I always appreciate the expertise you bring to this forum.

1) Duly noted. Interestingly my last tyre, which was a Supercorsa Pro SC2, started to suffer the same symptoms on the last day of the 08 Sound of Thunder, when the back tyre was getting nice and hot- to the point it was uncomfortable to hold my hand on the tyre. Maybe this is when the slop started to develop in the linkage...or maybe the heat just cooked the shock...

2) Thats something I never would have considered, it seems backwards to my laymans way of thinking...which I guess is why I did it the wrong way around and had an SC2 in Summer and and SC1 in winter. Oh well, you live and learn! I have an SC0 on the front, which seems to work well. Is this a bad idea?

3) No I'm afraid I'm not using tyre warmers. Its on the wish list.... The problem with not having enough money to sort the bike fully, is that it just costs you more money in running costs. Its a bit of a viscious cycle.....my monster is aptly named. Thank god I was sensible and went for a low powered bike instead of punting some old shitter in F1!

4) I measured the movement last night and its about 7 to 10mm at the axle. In fairness to the scrutineers, they would have to lift a fair amount of weight to get the back to unload so they could feel the slop.

5) Yes it is the Sachs as described, but I had you fit an Ohlins spring maybe a year ago. I was too tight to get it freshened up, let alone go for the full Ohlins, so I instructed you to fit the spring and gas it up, but leave the shock untouched. You described the shock as 'tired' at the time and commented that it had almost no gas pressure when you first got it. It made a massive difference at the time and greatly improved the handling. Initially everything worked 'well' by my standards, the tyres lasted well and the bike did nothing nasty. But I think the shock is now well past its use-by date- see point 1.

I currently weigh around 87 to 88 kg in street clothes, and just under 100kg in all my racing gear. I have been using the last two clicks on the rebound damping

After much discussion with my long suffering wife, and talking to some people in Chch, I have decided to go for a short term 'bandaid' solution. DAS in Chch are going to freshen the shock up with some new oil and gas. Hopefully the seals are good as I hear they are still impossible to come by. With any luck the thing will do a passable job for 6 to 8 months, which will give me enough time to save up for a nice shiney Ohlins from your good self.

[Robert Taylor]

Thanks for giving such a detailed reply Robert. I always appreciate the expertise you bring to this forum.

1) Duly noted. Interestingly my last tyre, which was a Supercorsa Pro SC2, started to suffer the same symptoms on the last day of the 08 Sound of Thunder, when the back tyre was getting nice and hot- to the point it was uncomfortable to hold my hand on the tyre. Maybe this is when the slop started to develop in the linkage...or maybe the heat just cooked the shock...

2) Thats something I never would have considered, it seems backwards to my laymans way of thinking...which I guess is why I did it the wrong way around and had an SC2 in Summer and and SC1 in winter. Oh well, you live and learn! I have an SC0 on the front, which seems to work well. Is this a bad idea?

3) No I'm afraid I'm not using tyre warmers. Its on the wish list.... The problem with not having enough money to sort the bike fully, is that it just costs you more money in running costs. Its a bit of a viscious cycle.....my monster is aptly named. Thank god I was sensible and went for a low powered bike instead of punting some old shitter in F1!

4) I measured the movement last night and its about 7 to 10mm at the axle. In fairness to the scrutineers, they would have to lift a fair amount of weight to get the back to unload so they could feel the slop.

5) Yes it is the Sachs as described, but I had you fit an Ohlins spring maybe a year ago. I was too tight to get it freshened up, let alone go for the full Ohlins, so I instructed you to fit the spring and gas it up, but leave the shock untouched. You described the shock as 'tired' at the time and commented that it had almost no gas pressure when you first got it. It made a massive difference at the time and greatly improved the handling. Initially everything worked 'well' by my standards, the tyres lasted well and the bike did nothing nasty. But I think the shock is now well past its use-by date- see point 1.

I currently weigh around 87 to 88 kg in street clothes, and just under 100kg in all my racing gear. I have been using the last two clicks on the rebound damping

After much discussion with my long suffering wife, and talking to some people in Chch, I have decided to go for a short term 'bandaid' solution. DAS in Chch are going to freshen the shock up with some new oil and gas. Hopefully the seals are good as I hear they are still impossible to come by. With any luck the thing will do a passable job for 6 to 8 months, which will give me enough time to save up for a nice shiney Ohlins from your good self.

Thanks for that, I enjoy helpng people but I also often get condemned for it. Ho hum.

The SC0 will work fine on the front until perhaps you get the rear sorted and you are able to push harder, thereby loading the front more. There is always a fine window of what works and what doesnt. At Superbike level with the top riders the setting window becomes very narrow, we are always chasing improvements in setup ( springing, geometry and internal valving ) to go faster for longer and to save the tyres. This cannot be done with external clickers and anyone who says that everything can be done externally and that there is such a shock is either seriously deluded or gullible. An external clicker is a fine tuning device and will not magically alter the base character of the shock if a reasonably substanial change is required. Whilst shocks are said to be ''fully adjustable'' this is in fact a big misnomer. There is always an ideal amount of internal free bleed and a clicker sweet spot of response range. On your shock if the valving is correct for your application I would expect the clicker to be within 10-20 clicks out, no less, no more.

Where you have the rebound set is seriouisly minimising the ability of the shock to follow the bumps as there is not enough free bleed and a pressure imbalance problem. That alone is enough to screw the tyres very quickly.

When Dirt Action freshen that shock it also needs a incremental change to the compression valving to give it more squat resistance. It is easy however to introduce pressure imbalance issues in such a shock due to the lack of a base compression valve. Also, if it is able to be lengthened a little whilst apart that would be useful. Get Linton to ring me.

[TonyB]

Thanks again Robert. I will ask Linton to call you.

I had the rebound set there because the bike felt unsettled and felt like the back end was 'pogoing' a bit. Then again I freely admit that I have no idea whatsoever on how to set suspension up, or what it should even feel like. I tend to ride faster when it works, and slower when it doesn't, but it always feels about the same, so I blame myself rather than setup when I'm slower.

[TonyB]

Righto, time for an update.

The shock was whipped out and rushed to A&E- Linton at Dirt Action Services. With some input from specialist consultant Mr Robert Taylor, the shock was pulled to bits, heavier shims transplanted, new oil infused and lots more gas pressure injected. The shock was then remounted, with advice to gradually lower the rebound damping until the shock felt loose, then go back a bit. I had assumed the 'odd' socket screws it was mounted with might be causing the slop, but after trying different ones it became apparent that it was definitely supposed to be mounted as supplied. With everything re-torqued the slop vanished.

To the track! Raced it on Sunday in cold but sunny conditions. The track was cold.

Practice- tootled around trying to bring the tyre back a bit- ie reduce the damage. It worked a bit, but I only had six laps of the B-track. It felt FARKING TERRIBLE, like it was on marbles, which it was I guess.

Race 1- 26 psi cold. Tyre felt awful but I tried to ride through it. Was barely warm when I came back in. Johnsv650 appeared and asked if I would mind if he helped out and offered to lend some tyre warmers and scrounge a generator! The next race was too close to sort anything, so we tried dropping to 26psi 'barely warmish'

Race 2- 26psi 'barely warmish'. Tyre was diabolical. Was sure it was flat, but as it wasn't trying to spit me off I circulated for points.

Race 3- warmers on for maybe 20 mins. More pressure- about 29 from memory. Massive improvement. Tyre staring to clean up....pity I rode like a knob...

Race 4- Dropped rebound 5 clicks. Big improvement again. Race went well, with me finishing about where I normally would. The back end was still drifting a bit on left handers but it was predictable and the improvement was noticeable over the previous race. The tyre was cleaning up a bit.

Race 5- Dropped another click off rebound. Race was better again. Tyre starting to look almost respectable.

Race 6- Generator was no longer available so tyres were slightly warm at race start. Wnet well. more improvement on tyre, but it was clear that it was still wearing rapidly- tread was gone.

Race 7- Cold tyres. Went well.

So, below is a pic of the shock showing how much travel was used in the last race. A picture of the much improved but worn out tyre is included. I noted that the good F1 guys with warmers and Race Tech K2's also had a bit of cold tear on the left hand side of the tyre. Anyone got any thoughts? I'm thinking that Ruapuna is a pretty smooth track, and maybe a heavier spring is required- especially with greymouth coming up. Will check sags.

[TonyB]

oh yeah, BIG thanks to johnsv650 for adopting me for the day!

[Robert Taylor]

Righto, time for an update.

The shock was whipped out and rushed to A&E- Linton at Dirt Action Services. With some input from specialist consultant Mr Robert Taylor, the shock was pulled to bits, heavier shims transplanted, new oil infused and lots more gas pressure injected. The shock was then remounted, with advice to gradually lower the rebound damping until the shock felt loose, then go back a bit. I had assumed the 'odd' socket screws it was mounted with might be causing the slop, but after trying different ones it became apparent that it was definitely supposed to be mounted as supplied. With everything re-torqued the slop vanished.

To the track! Raced it on Sunday in cold but sunny conditions. The track was cold.

Practice- tootled around trying to bring the tyre back a bit- ie reduce the damage. It worked a bit, but I only had six laps of the B-track. It felt FARKING TERRIBLE, like it was on marbles, which it was I guess.

Race 1- 26 psi cold. Tyre felt awful but I tried to ride through it. Was barely warm when I came back in. Johnsv650 appeared and asked if I would mind if he helped out and offered to lend some tyre warmers and scrounge a generator! The next race was too close to sort anything, so we tried dropping to 26psi 'barely warmish'

Race 2- 26psi 'barely warmish'. Tyre was diabolical. Was sure it was flat, but as it wasn't trying to spit me off I circulated for points.

Race 3- warmers on for maybe 20 mins. More pressure- about 29 from memory. Massive improvement. Tyre staring to clean up....pity I rode like a knob...

Race 4- Dropped rebound 5 clicks. Big improvement again. Race went well, with me finishing about where I normally would. The back end was still drifting a bit on left handers but it was predictable and the improvement was noticeable over the previous race. The tyre was cleaning up a bit.

Race 5- Dropped another click off rebound. Race was better again. Tyre starting to look almost respectable.

Race 6- Generator was no longer available so tyres were slightly warm at race start. Wnet well. more improvement on tyre, but it was clear that it was still wearing rapidly- tread was gone.

Race 7- Cold tyres. Went well.

So, below is a pic of the shock showing how much travel was used in the last race. A picture of the much improved but worn out tyre is included. I noted that the good F1 guys with warmers and Race Tech K2's also had a bit of cold tear on the left hand side of the tyre. Anyone got any thoughts? I'm thinking that Ruapuna is a pretty smooth track, and maybe a heavier spring is required- especially with greymouth coming up. Will check sags.

Frankly, I doubt that any of the tyres we race with in NZ are designed to run in such cold conditions. In Europe it would very much be the exception rather than the rule that they race frequently in conditions such as we race in in the winter, their winters being so relatively cold that racing basically shuts down. No tyre manufacturer is going to produce tyres specifically for ( cynically ) a ''pindrop in the ocean''
Cold tear has been a particular problem this year because its the coldest sustained winter we have had in a while. Getting heat into the tyres and getting them to retain that heat with a workable core and surface temperature is a near impossibility. The ''cure'' to get by is often counterintuitive to what many think....increase spring rate ( and often internal damping ) so that the suspension moves a little less and therefore loads the tyre more and generates more heat. In turn giving more grip and saving the tyre a little longer. This is not a blanket cure but often works.
When Linton faxed the shim spec to me my immediate thought was ''this ( monotube) shock is designed for a commuter / touring bike and is never intended to go anywhere near a race track'' Sad but true.
The monotube design doesnt incorporate a base valve so inherently doesnt ''pressure balance'' anywhere near as well as a reservoir type shock. Because of that very fact it has to run a much higher gas pressure to keep away from cavitation and therefore loss of damping. The valving respec was a guess sight unseen and far less than the ideal of being able to dyno the shock, revalve, redyno, pull apart again, reset and keep on redynoing until some sort of half reasonable damping curve and response range was obtained. But by that stage you would be thinking ''I should have purchased a high quality aftermarket shock anyway!'' I wish I could find the batteries for my magic wand!!!
The very fact you have kept coming out and out on rebound doesnt surprise me as all too often we come across rear shocks that we press down on and its ''is it going to return anytime soon?'' An oversimplistic logic is to run the rebound as fast as you can get away with without Cadillac style aftercycles...the wheel must also react downwards fast enough to keep the tyre interlocked with the pavement when accelerating over hollows. ALSO, a more open rebound bleed allows the shock to pressure balance more readily. It really gets to diminishing returns given the real serious design and function limitations of that shock, as you firm up the compression valving there is much more risk of hysterisis.
Yes, a firmer spring will most probably give you more tyre heat and grip, for the reasons stated above, not so much because of the smoothness of the track. During the summer months you will be able to run the current spring.
As an example an R6 racing in the Nationals here in summer with the best Pirellis we can get will be sprung around 10 to 10.5 newtons. The R6s in WSS on their special Pirellis run 8.5 newton springs typically. Having worked directly with revalving shocks for Parkes and Foret I can report that the shocks they run are EXACTLY the same as we do except they have more closely defined spring options and works style valve bodies designed only to faciltate faster valving changes, same function.
The window of setting related to tyres, shock calibration and track surface and temps is very narrow.

[TonyB]

Thanks Robert. My reasoning for a stiffer spring was a whole lot more simple than what you mentioned. In my laymans thinking, if I am using up nealy all the available travel on a smooth track, I figured a bumpy street circuit would see it bottoming out regularly.

As I mentioned above, the intention is for this to be a bandaid type fix- something to get me back out on the track until I can get the funds together for a more suitable shock. I'm quite encouraged by the results from Sunday, but am aware that there will be no miracle fix for this shock.

[Robert Taylor]

Thanks Robert. My reasoning for a stiffer spring was a whole lot more simple than what you mentioned. In my laymans thinking, if I am using up nealy all the available travel on a smooth track, I figured a bumpy street circuit would see it bottoming out regularly.

As I mentioned above, the intention is for this to be a bandaid type fix- something to get me back out on the track until I can get the funds together for a more suitable shock. I'm quite encouraged by the results from Sunday, but am aware that there will be no miracle fix for this shock.

Yes indeed that is also an indicator, its also to keep it way from the rising rate ''kick'' in the link, which can also screw tyres rather quickly, dependent on the ratio.

[TonyB]

Righto- a bit of an update, and a question.

After several meetings, the tyre wear is way better. However the shock needs a bit of a tweak as I suspect it has too much rebound damping and not enough compression damping. Initially it was spinning up the back tyre while leant over- a bit of a feat on a 62hp bike me thinks. I thought the clutch was slipping, but when a former F1 rider had a go on it he said that the shock was "farking awful" and that it was definitely spinning the rear. The shock was also bottoming out. The theory was the the shock wasn't rebounding fast enough, causing it to pack down and eventually bottom out. We kept taking rebound out until it stopped spinning- and ended up on the last click. The shock still bottoms out. I have tried various things- increasing rear preload, decreasing it etc, and eventually got a bit lost.

After a run up a private test road, and at the last race day at Ruapuna, I found the shock was bottoming out with a rear sag of 36mm, front of 40mm (rider on board). Today I thought it would be a good idea to reset the sags. Couldn't remember the recommended sags so I went to the Ohlins site and used the sags given there. They give a pretty big range and its obviously not specific to my bike. I went for roughly the middle of the ranges they give- Front 41mm, and rear 32mm. Both with rider on board. Do these sound about right?

I'm currently about 95kg with all my gear on.
The bike is running stock 1995 900SS forks with 35mm showing above the top tripple clamp. The rear end geometry is all standard, except I have the ride height adjusted to the maximum possible with the adjusters on the ends of the linkage yoke.
The shock spring is an Ohlins 01093-31/98 L406. The length of the spring in order to achieve the 32mm sag with me on board is 130mm

[Robert Taylor]

Righto- a bit of an update, and a question.

After several meetings, the tyre wear is way better. However the shock needs a bit of a tweak as I suspect it has too much rebound damping and not enough compression damping. Initially it was spinning up the back tyre while leant over- a bit of a feat on a 62hp bike me thinks. I thought the clutch was slipping, but when a former F1 rider had a go on it he said that the shock was "farking awful" and that it was definitely spinning the rear. The shock was also bottoming out. The theory was the the shock wasn't rebounding fast enough, causing it to pack down and eventually bottom out. We kept taking rebound out until it stopped spinning- and ended up on the last click. The shock still bottoms out. I have tried various things- increasing rear preload, decreasing it etc, and eventually got a bit lost.

After a run up a private test road, and at the last race day at Ruapuna, I found the shock was bottoming out with a rear sag of 36mm, front of 40mm (rider on board). Today I thought it would be a good idea to reset the sags. Couldn't remember the recommended sags so I went to the Ohlins site and used the sags given there. They give a pretty big range and its obviously not specific to my bike. I went for roughly the middle of the ranges they give- Front 41mm, and rear 32mm. Both with rider on board. Do these sound about right?

I'm currently about 95kg with all my gear on.
The bike is running stock 1995 900SS forks with 35mm showing above the top tripple clamp. The rear end geometry is all standard, except I have the ride height adjusted to the maximum possible with the adjusters on the ends of the linkage yoke.
The shock spring is an Ohlins 01093-31/98 L406. The length of the spring in order to achieve the 32mm sag with me on board is 130mm

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