Velonews: Technical Faq: Road Discs And Tire Pressure On Wide Road Rims


Jan 3, 2005
Lennard Zinn considers whether higher pressure might actually lead to a higher incidence of flat tires. Photo: Caley Fretz |
Dear Lennard,
My wife has just ordered a 2015 Cannondale Synapse with Shimano’s ST-R785 Di2 hydraulic levers, and I’m wondering if your 2009 advice about hanging bikes with hydraulic disc brakes also applies to road bikes?
At the end of your article you concluded, “Hanging the bike by the front wheel will most certainly not compromise your hydraulic braking, but hanging it by the rear wheel could, although I suspect that it is unlikely.”
— Greg
Dear Greg,
I have hung my Di2 hydraulic disc bike equipped with those R785 levers both ways on many occasions and have never noticed a braking issue afterward. I also have a SRAM road hydraulic-brake bike, and, that, too, I’ve hung both by the front wheel and by the rear wheel with no noticeable change in performance when I hopped on the bike.
Here is an answer about hydraulic road brakes from SRAM’s brake product manager, Paul Kantor:
“If there is enough air in the system, hanging by the rear wheel or the front wheel could change the contact point. Most reasonably-bled systems on road should handle it just fine. We spent a fair amount of design time trying to accommodate this scenario. It is possible to force air into the system if you pull the brake while the bike is hanging (understanding that all systems have a little air in the reservoir). You can tell by a soft contact point or longer lever travel. If this does happen to occur, pulling the lever three-four times should agitate it back to the reservoir.”
So I wouldn’t worry about it.
― Lennard
Dear Lennard,
You’ve illustrated a nice example of the road disc brake problem in your recent article. Doesn’t the KE=1/2mvsquared also factor in? Given that road bikes typically also run higher speeds (and the velocity squared would have more effect than the mass)?
― Eric
Dear Eric,
Yes indeed. With the velocity term being squared in the expression for the kinetic energy of the bike means that the energy dissipated when slowing the bike goes up geometrically with higher speeds.
The kinetic energy of the bike and rider is four times bigger for the same guy when he goes twice as fast on his road bike as on his MTB. It does beg the question of why road rotors are smaller than MTB rotors and how big a concern boiling fluid might be on a road bike.
― Lennard
Dear Lennard,
I have a PowerTap [hub] laced up to a Hed Belgium+ rim. I didn’t think it was going to be that big of a difference compared to other rims. I’ve had bad luck with tires on the rear. I’ve sliced four tires since the year started ([Continental] GP4000s, Conti 4 season, Specialized Turbo Pro, all 25mm tires). I run them at 105psi rear; am I doing something wrong? These are clincher tires.
― Manny
Dear Manny,
You “sliced” four tires? If you sliced them due to sharp objects on the road, then I guess it’s a coincidence that you hit those objects when riding the wide clincher rims. But if the tires exploded, leaving a long slice in the tube, that’s a different story, and the story involves the increased force on the inner walls of a bigger air chamber if it is at the same pressure as a smaller one.
I ride wide clincher rims (generally Zipps) most of my time on the road, and I rarely get flats, despite my 174 pounds, and never a slice. But I never pump them up that high. When I ride 23mm tires on wide rims, I never go above 85psi, and on 25mm tires, I stay below 75psi.
I got two answers from staff members at Hed Cycling Products. First, Chris (“Dino”) Edin, North American sales director, writes, “We have not personally experienced more flats or slicing issues nor do we field many calls of this sort if any. There are of course a few variables involved here: those are all pretty good tires for performance and not necessarily puncture resistance. Winter is also not the cleanest of roads in most places. It is worth noting however that most of us here ride the 4000’s and flat them on rare occasion and personally it is always when they should be thrown out from wear. I am not sure if it will make a difference in slicing but no one around here rides the 25mm tires on a Plus rim at more than 90psi. 105psi on that rim is hard as a rock.”
And Andy Tetmeyer, Hed Cycling Products’ product manager says, “Like Dino, I think the problem is tire pressure. It is much easier to slice a piece of fabric when it is stretched taut. Two years ago I tried running 90psi in some 23mm tires on Plus rims (up from the 75-80psi that I had been riding). I got three flats in two days. I dropped the psi back down and went back to my normal rate of one or two flats per year. Additionally, one of our guys here suddenly started complaining about the durability of the latest Conti model at the same time he started riding the plus rim. He put new tires on when he switched to plus rims, and he was convinced that the tire construction had changed. He was running 95 psi in 23mm tires. I was never able to convince him that his tires were too hard but I believe that was the case.
“I weigh 170 pounds and run 22mm tires on Plus rims at 82psi max, 23mm at 78psi max, and 25s at 75psi max. I have let 22mm tires fade down to as low as 55psi and 25s down to 50psi without pinch flats. They’re supremely comfortable at those pressures, but it makes my 13-mile commute one to two minutes slower.”
I recommend that you back off on the pressure. You don’t need so much to avoid pinch flats if you’re using those wide rims, and the softer tires will give you more comfort, and, probably, to a point, lower rolling resistance as well.
― Lennard
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