High speed tyre fail?

[mossy1200]

Just getting back from Cliffhanger drags and bike felt very mud going home and also hairy on the last flying during the corner at the beginning.
S20 rear is flat.
Same tyre as I ran last year when tyre was newish and it did the same last year during the event.
Its been fine ever since holding pressure all year.

Top end high speed seems to deflate it. Not sure if its the bend at the start of the run. Sweeping left about 180-200kph or the top end speed just over 300. Maybe its leaking past bead?

Anyone any ideas?

[FJRider]

Same tyre as I ran last year when tyre was newish and it did the same last year during the event.

Anyone any ideas?

mmmmmmm ...

[scumdog]

Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?

Just an irrational thought....

[mossy1200]

Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?

Just an irrational thought....

VALVE CAPS

Centrifugal forces work on the valve stem. At high speeds they have the same effect as pushing on the valve with your finger. The tyre can deflate. The valve cap is the only part preventing it. Normally this phenomenon happens only at very high speeds. But an old or low quality valve stem can open at speeds lower than 200 km/h. Therefore the cap should always be tightly closed. PIRELLI suggests the use of airtight metal caps with rubber seal.

Yeah just did some looking and my back has no valve cap.
Could explain also why my times got slower all day if it was losing some each run.

[Madness]

Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?

Just an irrational thought....

Wouldn't the valve be moving outwards, away from the axzle?

Mossy, are the valve stems straight or angled? The first ZX12-R came out with angled stems which were soon changed and a recall was issued IIRC.

[scumdog]

Wouldn't the valve be moving outwards, away from the axzle?

Mossy, are the valve stems straight or angled? The first ZX12-R came out with angled stems which were soon changed and a recall was issued IIRC.

You're right - AWAY from the Axzle is wot I ment.

Time for another mind-clearing bourbon...

[mossy1200]

Wouldn't the valve be moving outwards, away from the axzle?

Mossy, are the valve stems straight or angled? The first ZX12-R came out with angled stems which were soon changed and a recall was issued IIRC.

They are straight but without a valve cap this will be the reason.
Same tyre same set up has been running a year with no issue between the two events then same result as last year.
Time to get some new caps.

[mossy1200]

Wouldn't the valve be moving outwards, away from the axzle?

Mossy, are the valve stems straight or angled? The first ZX12-R came out with angled stems which were soon changed and a recall was issued IIRC.

Yes. Away from axel is stem being forced down like being pushed with finger.

[swbarnett]

But an old or low quality valve stem can open at speeds lower than 200 km/h.

Back in the '80s the tyres on my CB750 would spontaneously deflate at just over 100kph (IIRC). Thanfully it never happened to both at the same time. It happened 3 or 4 times before someone suggested replacing the plastic valve caps with sealed metal ones. Never had a problem after that.

[caspernz]

If metal valve caps are good enough for that ghost rider fella...then it'll be a good solution for speedy Wellingtonians I reckon

[sidecar bob]

I'm always dismayed as an MNZ technical steward, how many race bikes bikes I have scrutineered that are missing the valve caps. It's the most simple race preparation anyone could possibly be required to do & it always makes me suspicious how much else is going straight over the competitors head.
Your local tyre shop will give you a hand full used of caps for free.
Basic shit like that kills people.

[jellywrestler]

VALVE CAPS

Centrifugal forces work on the valve stem. At high speeds they have the same effect as pushing on the valve with your finger. The tyre can deflate. The valve cap is the only part preventing it. Normally this phenomenon happens only at very high speeds. But an old or low quality valve stem can open at speeds lower than 200 km/h. Therefore the cap should always be tightly closed. PIRELLI suggests the use of airtight metal caps with rubber seal.

Yeah just did some looking and my back has no valve cap.
Could explain also why my times got slower all day if it was losing some each run.

poor scrutineering for a high speed run event, this is not Rocket Surgery....

[mossy1200]

I'm always dismayed as an MNZ technical steward, how many race bikes bikes I have scrutineered that are missing the valve caps. It's the most simple race preparation anyone could possibly be required to do & it always makes me suspicious how much else is going straight over the competitors head.
Your local tyre shop will give you a hand full used of caps for free.

I guess its my fault partly for not knowing it can happen. There is no cap front or rear on my bike and Im not sure if they have ever had any. I don't recall ever removing them to check pressure and I would have returned them if they were there.
I agree my excuse(not a good one) is ignorance but a scrutineer should have this knowledge?

[jellywrestler]

I guess its my fault partly for not knowing it can happen. There is no cap front or rear on my bike and Im not sure if they have ever had any. I don't recall ever removing them to check pressure and I would have returned them if they were there.
I agree my excuse(not a good one) is ignorance but a scrutineer should have this knowledge?

yip, isn't that what caused barry sheenes high speed crash at daytona many moons ago, daytona being banked so extra pressure on component, i think it took them a while to figure it out.
who was quickest over there today, had planned on getting over for a looksee but plans don't always work out.

[mossy1200]

yip, isn't that what caused barry sheenes high speed crash at daytona many moons ago, daytona being banked so extra pressure on component, i think it took them a while to figure it out.
who was quickest over there today, had planned on getting over for a looksee but plans don't always work out.

I left early at lunch after a wobble on the bend in the last run. Parking on the gravel I never noticed the tyre deflation issue and I think when the tyre was hot it wasn't as bad inflation wise. The trip home was mud.

At the time I was running top 5 in flying 4.94 seconds and I think fastest was 4.88secs. No times were converted to kph at that point. I think my 10.1 standing was second at the time. Nobody had gone under 10 at that stage.
Because I didn't stay to the end the times were just placed against bike numbers at that point.