View Full Version : High speed tyre fail?
mossy1200
9th November 2014, 16:48
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
9th November 2014, 16:56
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
9th November 2014, 17:01
Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?
Just an irrational thought....:crazy:
mossy1200
9th November 2014, 17:04
Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?
Just an irrational thought....:crazy:
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
9th November 2014, 17:06
Centrifugal force at speed causing the tyre-valve to move inwards (towards the axle) a fraction allowing air to excape?
Just an irrational thought....:crazy:
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
9th November 2014, 17:09
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...:drinknsin
mossy1200
9th November 2014, 17:16
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
9th November 2014, 17:18
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
9th November 2014, 17:19
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
9th November 2014, 17:26
If metal valve caps are good enough for that ghost rider fella...then it'll be a good solution for speedy Wellingtonians I reckon :banana:
sidecar bob
9th November 2014, 17:41
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
9th November 2014, 17:48
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
9th November 2014, 17:48
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
9th November 2014, 17:55
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
9th November 2014, 18:04
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.
nodrog
9th November 2014, 18:22
Nasa called
ducatilover
9th November 2014, 19:06
Centrifugal force doesn't exist though :sweatdrop
:corn:
Gremlin
9th November 2014, 19:14
Sounds like you're on to it.
I did have a tyre once that would deflate when I hit 110-120kph. Reproduced it 3-4 times over 160km (80km down the line, 80km back again to replace). However, it was deflating all the way, going from full pressure to (in one case) 1psi before I got it stopped on the side of the road.
Poor bead on the tyre was blamed, and the replacement tyre (same model) was fine.
swbarnett
9th November 2014, 22:13
Centrifugal force doesn't exist though :sweatdrop
:corn:
No, but centripetal force does and without it the valve inner will move as far as it can on a tangent to the rim.
mossy1200
10th November 2014, 05:19
No, but centripetal force does and without it the valve inner will move as far as it can on a tangent to the rim.
If this thread turns into a heated discussion including circles with objects leaving in straight lines at angles in relation to rotation speeds and ends up with arguments idd like to point out I only wanted to know why my tire went flat. Lets point out that after that im no longer responsible.
Kickaha
10th November 2014, 05:46
Centrifugal force doesn't exist though
Doesn't matter, everyone knows what is meant by the word regardless of whether it is correct or not
nzspokes
10th November 2014, 05:56
Well I learned me something. :niceone:
Thats why at one of my track days I got told off for not having valve caps. :confused:
sidecar bob
10th November 2014, 06:28
If this thread turns into a heated discussion including circles with objects leaving in straight lines at angles in relation to rotation speeds and ends up with arguments idd like to point out I only wanted to know why my tire went flat. Lets point out that after that im no longer responsible.
This is KB bro.:lol:
Grumph
10th November 2014, 08:32
This is KB bro.:lol:
True...
It's not a new problem, I remember the old man pointing out to Neville Landrebe that he had no valve caps fitted on his TD2 at a flying quarter shortly before Neville was killed. After thinking about it for a minute, Nev hunted out a couple and fitted them...
sidecar bob
10th November 2014, 13:56
What is lacking is a downloadable scrutineering form on the MNZ website that can be printed by scrutineers prior to an event. We need one for road race bikes, one for dirt bikes & one for sidecars. That would go a long way to addressing the variance in levels of scrutineering between individuals & create a common standard.
Reckless
10th November 2014, 14:13
Common problem on road race karts with the small diameter wheels turning at a huge rate.
We also used to grub screw the rims with a O ring under them to keep the beads on the rim as well.
Good times Mossy :niceone:
scumdog
10th November 2014, 17:22
No, but centripetal force does and without it the valve inner will move as far as it can on a tangent to the rim.
Yep, knew that - but there would have been a shit-load of ????? from some had I used that word!
Kickaha
10th November 2014, 17:25
Common problem on road race karts with the small diameter wheels turning at a huge rate.
We also used to grub screw the rims with a O ring under them to keep the beads on the rim as well.
Hence why I didn't ever fit anything but metal valve caps with a rubber seal on my 250 Superkart wheels
swbarnett
10th November 2014, 17:29
Yep, knew that - but there would have been a shit-load of ????? from some had I used that word!
Indeed. However, never let the ignorance of others stop you using the correct terminology. It won't hurt them to learn something.
mossy1200
10th November 2014, 18:18
Ordered myself some cnc ones with o rings.
Rear wheel wibbly at 300 was not very much fun.
Guess im lucky it wasn't the front.
Might take a some spares next time and see if anyone has some missing.
ducatilover
10th November 2014, 19:20
Would a tickle of thread tape help instead of an o-ring?
Big Dog
10th November 2014, 19:22
Just be cautious of heavy caps. Like thee light up ones. They have been known to pull a stem out.
Stupid phone / Tapatalk, apologies in advance.
bogan
10th November 2014, 19:24
Ordered myself some cnc ones with o rings.
Rear wheel wibbly at 300 was not very much fun.
Guess im lucky it wasn't the front.
Might take a some spares next time and see if anyone has some missing.
Sounds like you just need some CNC lightweight valvey doodads, resist the centrifictional forces, save not only weight, but unsprung weight too, and not only unsprung weight but rotating weight at that, shave literally infintesimal fractions of a quarter mile run off your time. No vin deisel monologues for you while you head down the strip. :wings:
mossy1200
10th November 2014, 19:39
Sounds like you just need some CNC lightweight valvey doodads, resist the centrifictional forces, save not only weight, but unsprung weight too, and not only unsprung weight but rotating weight at that, shave literally infintesimal fractions of a quarter mile run off your time. No vin deisel monologues for you while you head down the strip. :wings:
What I needed was for the rim to get damaged and talk the wife into a set of BST carbon race (safety) wheels.
Main reason is if I get nice ones im not likely to forget to put them on after checking tyre pressure.
F5 Dave
13th November 2014, 06:18
Yeah Google that shit and decide whether you want to run BST on the road
mossy1200
13th November 2014, 09:08
Yeah Google that shit and decide whether you want to run BST on the road
I couldn't afford BST anyway. Its just so much money that could be spent on other things.
R650R
13th November 2014, 17:33
I had this issue on the GSXR once after fitting a new tyre, was worried as mate had dented his ZXR front rim on that poxy BP entrance at Howick...
Shop pulled it off and remounted with some kind of bead sealant which cured it. Next tyre changer cried about the mess and didn't use it, haven't had problem again.
Maybe its a quality control issue with the odd tyre. Was only quick road use too, not track stuff.
Wonder though with how much the centre expands at speed that its plausible for the bead part to be doing similar, even a fraction would leak air.
How old are you valve stem outers? Mechanic replaced my rear one, few tyres ago. Not much room to get air line in so was always getting twisted a bit.
Something from trucking world, have had the longer valve stems (about two inch) get cracks. Even top of the line Alcoa allow wheels fatigue crack with age, had one replaced on steerer at about 800,000km.
mossy1200
13th November 2014, 20:05
I had this issue on the GSXR once after fitting a new tyre, was worried as mate had dented his ZXR front rim on that poxy BP entrance at Howick...
Shop pulled .................................................. ..........
Something from trucking world, have had the longer valve stems (about two inch) get cracks. Even top of the line Alcoa allow wheels fatigue crack with age, had one replaced on steerer at about 800,000km.
Bikes under 2 years old on its original tyres (rear is due to be replaced 4tho km now)
Tyre had about 1 pound once cooled down and was still beaded. Inflated and holding.
Powered by vBulletin® Version 4.2.5 Copyright © 2025 vBulletin Solutions Inc. All rights reserved.