Wheel Problem
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fossyant
Ride It Like You Stole It!
- Location
- South Manchester
Having gone through rims on a regular basis with commuting, the minute they start to 'dent' or 'bulge' when they are getting concave, it's time to replace. I had one rim explode as above, but others would dent on a pot hole - time to go.
[QUOTE 4536879, member: 9609"]No, it will have to keep going until I find a solution, it has 30 to do the morn and another 30 before the end of the week, it is back together with only 70psi in it, I will keep the speed down on the hills and only ride roads with mobile reception where the wife can rescue me - I will wear the lid as well 
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The last time it was replaced the local bike shop said that he couldn't buy a wheel for me and rebuilt it with a new rim. I know he will be awkward about it this time as he just wants to sell me a new bike. So what measurements etc do I need for finding a wheel online?
it takes 700 28C tyres
it is a 7 speed
and the gap between the rear frame is 125mm[/QUOTE]
That'll be 126mm, not 125mm, but what's 1mm between friends? You've pretty much nailed the dimensions there. One more will be to measure the internal width of the rim. Is it a freewheel or a cassette (although I'm not really sure this makes a difference to your prospective purchase).
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The last time it was replaced the local bike shop said that he couldn't buy a wheel for me and rebuilt it with a new rim. I know he will be awkward about it this time as he just wants to sell me a new bike. So what measurements etc do I need for finding a wheel online?
it takes 700 28C tyres
it is a 7 speed
and the gap between the rear frame is 125mm[/QUOTE]
That'll be 126mm, not 125mm, but what's 1mm between friends? You've pretty much nailed the dimensions there. One more will be to measure the internal width of the rim. Is it a freewheel or a cassette (although I'm not really sure this makes a difference to your prospective purchase).
- Location
- Glasgow
@User9609 you badly need N+1 till somebody (maybe @Spoked Wheels ) can build you a new wheel 
Or drive to the campsies, Big Al will build you a wheel for around 80 pounds
Or drive to the campsies, Big Al will build you a wheel for around 80 pounds
Yellow Saddle
Guru
- Location
- Loch side.
That reminds me....
fossyant
Ride It Like You Stole It!
- Location
- South Manchester
I know a good dentist, just in case ! 
Yellow Saddle
Guru
- Location
- Loch side.
[QUOTE 4585813, member: 9609"]Finally got round to getting a new wheel, and the LBS who built the new wheel for me suggested the rim wall thickness was not too bad but asked what pressure I was using. Interestingly I had only started using 110 on some new Marathon plus (I don't know why I went up to 110 as I have always used 90) anyhows, He suggested that 110 was too much for the rim with 28mm tyres. And I learned two things one of which I don't entirely understand, so if anyone could explain it to me, please do so.
1) I never knew rims were pressure rated (I know everything will have a limit but have always preesumed the limiting factor would always be the tyre)
2) And this is the bit I'm struggling to get my head round. The bigger the tyre width the less pressure the rim can take. Apparently my rims can take something like 125psi on 23mm but only 80 on 32mm. Why does the tyre width matter, yes there is more energy stored up in a bigger one but if the pressure is the same ![/QUOTE]
Rim width and pressure matters on clincher tyres but not on tubular tyres. The latter only has a constrictive effect on the rim. Clinchers on the other hand have an opening effect on the rim by attempting to force the two sidewalls apart. Imagine the tyre as a letter C with the open side facing the inside of the rim. As tyre pressure increases, the open end of the C wants to open even more. Since it is hooked into the rim, it forces the rim open.
As for increased size and the required decrease in pressure. Take your letter C again. Imagine it exactly 1 inch wide or, deep, if you want. I other words, as you stare at this C, you know that it goes 1inch into the screen. You can't see this but I can tell you that the length of the ribbon that forms the C is 10 inches. In other words, from the tip of where you started to write this C to the tip where you ended, the line is 10 inches. It is a very wide tyre, lets say for a motorcycle. Now at 100 PSI, we can calculate the total force on that piece of ribbon (actually tyre rubber). We kow the pressure is 100 PSI and that we have ten square inches of rubber. Therefore the force on the ribbon of rubber is 100 x 10 = 1000 pounds. That means that the force that the cords inside the tyre have to withstand equals 1000 pounds of force. Similarly, the rim now wants to spread outwards under the 1000 pound force.
If we look at a narrower tyre where the ribbon of C is only 5 inches, we see that the force is 500 pounds. This tyre requires weaker casing cords and the rim doesn't have to be as strong.
1) I never knew rims were pressure rated (I know everything will have a limit but have always preesumed the limiting factor would always be the tyre)
2) And this is the bit I'm struggling to get my head round. The bigger the tyre width the less pressure the rim can take. Apparently my rims can take something like 125psi on 23mm but only 80 on 32mm. Why does the tyre width matter, yes there is more energy stored up in a bigger one but if the pressure is the same ![/QUOTE]
Rim width and pressure matters on clincher tyres but not on tubular tyres. The latter only has a constrictive effect on the rim. Clinchers on the other hand have an opening effect on the rim by attempting to force the two sidewalls apart. Imagine the tyre as a letter C with the open side facing the inside of the rim. As tyre pressure increases, the open end of the C wants to open even more. Since it is hooked into the rim, it forces the rim open.
As for increased size and the required decrease in pressure. Take your letter C again. Imagine it exactly 1 inch wide or, deep, if you want. I other words, as you stare at this C, you know that it goes 1inch into the screen. You can't see this but I can tell you that the length of the ribbon that forms the C is 10 inches. In other words, from the tip of where you started to write this C to the tip where you ended, the line is 10 inches. It is a very wide tyre, lets say for a motorcycle. Now at 100 PSI, we can calculate the total force on that piece of ribbon (actually tyre rubber). We kow the pressure is 100 PSI and that we have ten square inches of rubber. Therefore the force on the ribbon of rubber is 100 x 10 = 1000 pounds. That means that the force that the cords inside the tyre have to withstand equals 1000 pounds of force. Similarly, the rim now wants to spread outwards under the 1000 pound force.
If we look at a narrower tyre where the ribbon of C is only 5 inches, we see that the force is 500 pounds. This tyre requires weaker casing cords and the rim doesn't have to be as strong.
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Nicely explained, thanks.
Gong to your first paragraph "Rim width and pressure matters on clincher tyres but not on tubular tyres. The latter only has a constrictive effect on the rim." Presumably this constrictive (compressive?) force is present on clincher tyres as well, given that the air within the tube is acting "outwards" in all directions. And if that's so, does it result in a net decrease in spoke tension, compared to a wheel without a tyre on it and is such a decrease noticeable?
Yellow Saddle
Guru
- Location
- Loch side.
There is also a constrictive (yes, it is compressive) force on clincher tyres. In other words, an expansive and constrictive force on clinchers but only a constrictive force on tubulars. The latter don't have side walls to "feel" the expansive force. That there is a constrictive force on a wheel when the tyre is inflated will be obvious to anyone who has worked with cheap factory-built wheels. The first time you inflate the tyres, you can hear the spokes ping as tension on them reduces and they untwist from the nipples. There is a net decrease in spoke tension and you notice it depending on how observant you are. The pinging is one clue, but it can be measured as well. Musically it can easily be demonstrated.
Rims are under enormous compression on a bicycle wheel, Each spoke has say 1000N in it and over 32 spokes that's 3.2 tonnes of compression. The entire rim diameter shrinks by a mm or two once the wheel is built and it is this force that prevents simple pinned rims for ever coming loose in use. In fact, if you can hold it all together, you can build a 32-spoke wheel from 32 loose arches of rim material.
