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Question re hot rims and effect on tires (and tube


mdk555
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Does hard long braking on a downhill have any negative effect on tires where they touch the rim?

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This question is related specifically to a situation where my boet and I where coming down from Table Mountain?s Cable car station and due to all the sharp hairpin corners had to slow the tandem down a lot (I think it is because our combined weight is 200kg). At the parking lot the rims where exceptionally hot and I was wondering what effect this could have on the tires (and tube pressure)

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Does hard long braking on a downhill have any negative effect on tires where they touch the rim?

<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

This question is related specifically to a situation where my boet and I where coming down from Table Mountain?s Cable car station and due to all the sharp hairpin corners had to slow the tandem down a lot (I think it is because our combined weight is 200kg). At the parking lot the rims where exceptionally hot and I was wondering what effect this could have on the tires (and tube pressure)

 

I don't believe so. Prolonged braking without giving the wheels time to cool down does easily heat the rims and tyres to way above the boiling point of water. However, butyl (the material black tubes are made of) and synthetic rubber (tyre compound) and rayon/cotton yarn (tyre bead tape and threads) doesn't burn or deteriorate at these temperatures. If it did, you'd see clear burn evidence at the tyre/rim interface i.e. at the bead.

 

Tyres blow off from descending tandems and solos not because the tube melts from the heat generated in the rim, but because the increase in temperature increases the tyre pressure to a point above what the tyre can safely handle. The tube expands, the tyre bead lifts off from the rim and the tube escapes and instantly explodes. This is evident from the loud bang, which indicates an over-inflation explosion. Were the tube to have burst inside the tyre, it would merely pop softly.

 

The fact that the tube, after exploding, immediately withdraws to its under-tyre position, is seen as proof to many people that the tube melted and exploded inside the tyre. This is not so. The truth is counter-intuitive in this case.

 

If you have ever ridden down a mountain in the rain, you would have seen water boiling out of your spoke holes, indicating that the temperatures inside there are in excess of boiling point - 90 degrees and above for a reasonably low altitude.

 

If you doubt that tyrews and tubes are not affected by these temperatures, prove it to yourself by first boiling a piece of tyre and tube and then applying a low temperature flame to it - say from a candle.

 

You don't need much speed to blow a tyre off - just dragging the rear brake at walking pace down a mountain can do the trick.

 

It helps a bit if you underinflate the tyres and thereby giving them some space for expansion. Unfortunately a tandem tyre is wider than a solo tyre and therefore cannot take as much maximum pressure. Pressure being expressed in force per area and wider/bigger tyres having more area and therefore more overall force on them than a narrow tyre.

 

The solution also doesn't lie with hub brakes. I don't believe there is a disk brake on the market that can handle the forces required to stop a speeding tandem going down a mountain. They overheat and fold up.

 

You can install a drag brake from Arai or Shimano on the rear hub if it has a screw thread on the left hand side.

 

Having a wheel pop on you completely destroys your confidence and many people give up tandeming for this reason.

 

One solution is to corner faster and brake less. I once had the privilege of following Andrew McLean and his then tandem lady partner down a mountain outside Nelspruit and their technique, speed and courage was awsome. They didn't touch the brakes, corned at what I believe was 90 kph and from my rapidly-diminishing view at the back, never even flinched. Admittedly I did not check their saddles afterwards for pinch marks.
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 Just a story: A friend of mine with his daughter (on a tandem, all up weight probably 150) had their back tire fail in 2 consecutive Argur's as a result of rim/tire/tube overheating as they entered HoutBay at the end of the Chapmans decent.. The following year he says he just braked less and it was all ok!!!! He went with the "the increase in temperature increases the tyre pressure to a point above what the tyre can safely handle. The tube expands, the tyre bead lifts off from the rim and the tube escapes and instantly explodes" theory....

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I disagree I have a road tandem with avid juicy 5`s with 203mm rotors and I can stop on a ticky even endo the tandem. I have never had brake fade or a tyre explode on me and im a speed freak. Combined weights are above 150kg sometimes so I still believe its the rim that rises the temps of the tube.

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I've seen a tandem popping a wheel on a long downhill before. And my tandem partner popped both his wheels coming down golden gate two days in a row.

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Referring to your photo, I am  convinced it is the latter - the rim cut into the tyre.  Your photo doesn't give a close-up view and only a special macro lens can do what's needed.

 

Why not cut off the piece of tyre and mail it to me? I'd love to take some macro pictures of it and study it. We can publish the findings here and on your website.

 

Also, now that the tyre is ruined, why not experiment with some heat and see what it does to the rubber and underlying cords? Your problem is to get a low heat source - 120 degrees or so, anything higher will skew the results.
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I disagree I have a road tandem with avid juicy 5`s with 203mm rotors and I can stop on a ticky even endo the tandem. I have never had brake fade or a tyre explode on me and im a speed freak. Combined weights are above 150kg sometimes so I still believe its the rim that rises the temps of the tube.

 

Saying you're a speed freak doesn't lend anything to the debate. Also, stopping quickly and doing an endo also doesn't prove the case you're trying to make. Firstly quick stopping doesn't lead to heat build-up. It is long slow stopping, even at very low (walking) speeds that blow off tyres. Disk brakes working at low speed without the benefit of airflow are vulnerable to overheating and folding. It has been documented before.

 

Saying you can endo your tandem makes me raise my eyebrows. Endo-ing a solo bike is easy - you have so much traction on the front wheel that you will endo before the wheel skids. In fact, you cannot skid a bike's front wheel on good clean surface. On a tandem, with two up, you have so much resistance to overturning momentum that the wheel will skid first and the rear wheel won't lift significantly. However, this does not relate to tandem tyres blowing off.

 

Any form of hub brake will prevent tyre blow-off - that's clearly established and no need to debate it. What is debatable is whether a disk brake is robust enough to handle the type of braking that Michael described and documented with photo evidence.

 

 

 

 
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Referring to your photo' date=' .....

 

Why not cut off the piece of tyre and mail it to me? I'd love to take some macro pictures of it and study it. We can publish the findings here and on your website.[/quote'] I would like to send you a piece of the tire. Please PM me your postal address.
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This question relates to the accident that I had on 9 August https://www.bikehub.co.za/forum_posts.asp?TID=15577 and we are trying to establish if the tire failed due to heat' date=' or is it was a blow out or if the damage to the tire was caused when the wheel deflated and the rim cut into tire.
[/quote']

 

Giving the matter some more thought, I would look for evidence of even deterioration all around the tyre. What your photo shows is one spot of damage, which I interpret as the tyre being cut by the rim and road. Heat buildup is even right around the tyre and it will show damage all around and on both sides. I feel strongly that the tyre was damaged because of the accident and not that you had an accident  because the tyre was damaged. 

 

Your biggest enemy is bead lift, not heat damage.

 

 
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As far as endoing 2 up you are correct I ment to say while being single. But my brakes are so effective as to make the rear passenger slip off their seat on many occasions.As far as slow braking I have done just that as not always can I speed as much as I would like due to cornering or other obstacles in the way. The whole point of using the bigger discs 203mm in this case is to disperse heat so no folding will occur. I still believe converting to discs is the solution. You are basically saying that there is no solution!

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Heat buildup is even right around the tyre and it will show damage all around and on both sides.  Your biggest enemy is bead lift' date=' not heat damage.[/quote'] Yep there is only damage on one side but I would still like to send a piece as there are some interresting marks where the tire butts against the rim which could look like heat damage.
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Heat buildup is even right around the tyre and it will show damage all around and on both sides.  Your biggest enemy is bead lift' date=' not heat damage.[/quote'] Yep there is only damage on one side but I would still like to send a piece as there are some interresting marks where the tire butts against the rim which could look like heat damage.

 

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As far as endoing 2 up you are correct I ment to say while being single. But my brakes are so effective as to make the rear passenger slip off their seat on many occasions.As far as slow braking I have done just that as not always can I speed as much as I would like due to cornering or other obstacles in the way. The whole point of using the bigger discs 203mm in this case is to disperse heat so no folding will occur. I still believe converting to discs is the solution. You are basically saying that there is no solution!

[/quote']

Even V-brakes are effective enough to throw an unbraced stoker into the captain's back. That doesn't prove the point of heat build-up.

 

Your disks are made of stainless steel and they quite easily heat up to a dull red colour, just about the point where they go soft and fold. Their size helps a bit but just a bit. The amount of heat they can take is a factor of a:) their material and this is defined as the material's specific heat capacity and expressed as kilojoules required to heat a given amount of the material by one degree.

b) Their size, which will affect their general heat capacity or, the amount of heat required to heat that particular disk by one degree and

c) Their heat dissipation ability. For stainless steel this is lilmited to airflow. Stainless is such a poor conductor of heat that virtually none of the heat will go into the hub. The spokes once again, are stainless and little of the heat will escape that way. Therefore the heat that builds up either gets removed by the airflow or stays there.

 

If you think about the above three points, you'll see that a larger disk is only better due to its slightly increased surface area and slightly bigger general heat capacity. Once the latter is saturated, the size of the disk has no effect on that anymore and you're relying on cooling capacity. This is not that much greater, the bike easily saturating that with high speed and prolonged braking.

 

There is a point at which those disks will heat up too much. Yes it is better than a small disk but still not big enough. Disk also suffer from boiling fluid and do so quite readily.

 

In my view the solution is good V-brakes fitted with Koostop salmon pads, aluminium deep section rims (not available in 48-hole drilling), lots of spokes and, a drag brake not affected by heat. The only solution here we have is a drum brake.

 

This drum brake can be dangerous if operated by a stoker and should be given to the captain to control via an old-fashioned friction ger shifter. This way he or she has absolute control over the bike without the stoker doing something to affect the captain's decisions.

 

A hub brake will not stop the tandem, but it will scrub of speed safely and can be used for prolonged distances. It wiull give the pair that extra bit of confidence which they quickly loose after an incident like Michael describes.

 

Both Arai and Shimano makes good drum brakes, but nou need a threaded hub for that. Most tandems sold in SA are sold with under-specc'd wheels and a rebuild is required. Most frames don't even have hub brake mounts on the left chainstay.

 

So yes, there is a solution but not a silver bullet.
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Im still yet to see my discs glowing red!I tell you what, I would be interested in you joining me on a ride with prolong disc usage providing you replace the disc and pads due to anywear and then we do a writeup.Let me know

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A lot has been said in the answers above. I agree with all. The damage caused by a rim is much greater than the damege caused by a tyre "popping". A disk brake should never be used as a drag brake. Disks does not have the heat dissapating abilities to be used as drag brakes. Disks can and will warp when subjected to "abuse" ie overheating. Fit an Arai drumbrake as Johan suggested. The Yanks use V-brakes with the Arai drum brake. This means an extra brake handle and they mostly leave the job to the stoker. Do this ONLY if you REALLY trust your stoker!!!! They also use drogue chutes used to bring speeds down or control speeds on long decents. We do not have the type of decents in South Africa to warrant the use of a drogue chute. Judicious use of V brakes is more than sufficient. I alternatively brake in fornt and then at the back to bleed speeds on long descents.

 

Rad - on what type of bike do you use the 203 mm rotor, MTB or Road bike and is that both on the fork and rear? If you use a 203mm rotor on the front I really hope you have a fork that is designed for it. Some carbon forks has been known to fail with the use of disks on a road bike. If it is a MTB you and you ride aggressively you should use a 20mm front axle or maxle or even the 23 mm axle as used on the Maveric Duc32 forks. There s been documented cases of normal skewers snapping under load and cases of forks collapsing. So beware!!!!!!!!
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