Tandem Cycling - Another tandem tire/wheel question -
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08-01-03, 08:45 AM
I'm really looking for a solution to my newfound fear of fast descending on our tandem.
After briefly seeing 60mph on the descent of Slumgullion pass, and getting over 50mph many other times during our weeks tour in the southern mountains, the front tire blew off the rim on our first casual ride when we returned home. It happened at about 30, after a hard, but fairly short application of the front brake. After sanding most of the dings out of the Fir rim, I put on another set of the same Conti GP3000's. Removed them after reading stories of other Conti blowoffs, and put on a set of Ruffy Tuffys.
Still being uncomfortable with the dinged up rim, I had a set of Velocity Deep V's built up on Phil hubs, but now the 27mm Ruffy Tuffys look like a stretch for the narrower rims. Does anyone know of a chart showing tire/rim width compatability? How about a tried and tested tire for a 300 lb. team on these rims?
I'd like to regain the confidence to be as relaxed descending with the tandem as I am on my single, but with my beautiful wife in the stoker seat, I need to trust the bike completely.
Thank you for any comments -
08-01-03, 08:59 AM
Here's a link to Sheldon Brown's rim/tire width compability chart:
Your Deep-V rims have an Outside Rim Width of 19mm and an Inside Rim Bead Width of 14mm; it's the inside bead width that you're interested in. A 700x27c tire is at the upper end of the size range for the Deep V's.
At 300lbs, you're far from being a large team; in fact, 300lbs is on the low end for many tandem teams. As for candidate tires, Avocet's Carbon Fasgrip K series are used extensively, we use 700x23c and 700x25c Vredestein Fortezza's (280lb team weight), Continental's Gatorskin 700x25c and 700x28c are becoming a common sight as well.
Front tire blow-offs would indicate you're using your front brake more than your rear so you might want to focus on your braking technique with an eye towards making sure you split up the work by alternating (modulating) your use of front on/back off, then front off/back on braking. On the bright side, the deeper section Deep V rims will be more effective as heat sinks than the FIR rims since they have more mass and more wind-swept surface area.
Of course, there's always drag brakes....
08-01-03, 09:48 AM
Thanks, Mark. That chart is exactly what I was looking for. I've used the Fortezzas on my singles, usually at pretty high pressures. Maybe some 25's at 120 pounds or so would be just the ticket.
Also appreciate your braking tips. We use an Ariai drum, but probably not enough. Our usual rides involve climbing up one of the three roads out of town, then screaming back down. When we started doing these rides on the tandem, I was a little overwhelmed by the instant acceleration, and the amount of braking needed to keep things in control. I've always been a little afraid of toasting the drum or melting my hub bearings.
08-01-03, 01:00 PM
Originally posted by Kingofbeers
Maybe some 25's at 120 pounds or so would be just the ticket.
The 25's are usually a special order; almost no one keeps them on the shelf. Also, you'll want to run them at the suggested 135psi as it will actually give you a stronger bead seal -- noting that a bad/weak bead seal is the root cause of most "blow-offs". The tighter the bead seal the less likely an overheated tire is to soften up enough to be lifted away from the rim by the tube. Most blow-offs seem to occur on bikes that, in additional to being subjected to some really nasty heat on very hot sun-baked asphalt roads also had older tires, tires that didn't fit the rim as tightly as they should, a bad bead strip or a defective / worn-out / failed rim braking surface. It's usually the combination of two or more of these things that leads to the blow-off, not just heavy brake use. When folks recommend running larger size tires what they often assume is that the greater volume of air is what helps control the heat when, in fact, it's just the larger surface area of the tire casing that provides the majority of the heat management.
I've always been a little afraid of toasting the drum or melting my hub bearings.
You'll know when either of those things are about to occur (and it's rare).... first by the sound of your brakes glazing over as they start to melt, next by the brake fade that accompanies the glazing and then by the smell of the brakes and/or drum cooking.
It's very instructive to make a point of doing some of your own benchmarking with regard to how hot your rims, tires and drag brake get under heavy braking conditions. In other words, make a point to pull off after subjecting your brakes to what you feel were pretty strenuous conditions and check the tire's temperature with your hand. If it's not too hot, check the rims noting that they can get hot enough to give you a 1st degree burn so you might want to lick your finger first before touching it as you would to check an iron. For the drum, drip some water on it from a water bottle and see if it boils on contact.
A note of caution: It is not at all uncommon for folks who stop to let their brakes cool to have a blow-off right after stopping. The reason is actually pretty straight forward - so long as you continue to keep your tires and wheels rolling the heat loads will continue to be dissipated by the ambient, lower temperature air flowing over them. However, if you come to a rapid stop while you already have overheated rims and tires, not only do you increase the heat load with the physical application of the brakes, you also remove the flow of air that was cooling your wheels. Therefore, the heat loads in the tires and rims quickly goes up expotentially and kablam; blow-off.
OK, so what now? The key is staying ahead of the heat build up by using a drag brake when you plan to take on extremely steep and twisty descents that you're unfamiliar with OR to force yourself to stop to let your wheels and brakes cool when you know you're pushing them to the upper end of the heat range. Therefore, that's why it's a good idea to do the benchmarking so you learn to recognize what your brakes sound and feel like as they begin to reach those upper limits under familiar conditions and when there's no real time pressure.
Regarding the benchmarking, we're not talking about a major science project. All it takes is making a point of bringing your tandem to a stop on relatively steep and fast descents that you ride on a regular basis for a momentary check (touch) to see how hot they are. To push the brakes up the heat index just experiment by over using the rear brake.
Just some food for thought on how to learn more about how your equipment performs under harsh conditions.
08-01-03, 04:30 PM
That is really some helpful information, Mark. Thank you.
I've got some Fortezza 25's coming from Excel, and I appreciate your comments on the right pressure.
Our next trip down from the Bells will include a bunch of stops to give myself the hot-finger and get a better feel for what the brakes can take. Our drum has started to squeal on a few of our longer descents - possibly the start of the glazing process.
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