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Tyre pressures - hot and cold

Discussion in 'Technical and Troubleshooting Torque' started by sjestory, Mar 20, 2006.

  1. Part of the scrutineering process at SBS is checking and adjusting tyre pressures. I asked the official what pressures he was putting in my tyres and his reply was 30lb front and back - same for all bikes! Given that this is considerably lower than my normal I asked him the rationale. His reply was that manufacturer's recommended pressures were influenced by concerns about tyre life whereas the only concern at SBS was maximum traction.

    I also figured that maybe the sort of work tyres do on the track would heat them up and increase the pressure more than everyday road riding. So I checked my pressures yesterday before and after a 2-hour, reasonably spirited ride. The result: front increased by 2lb, rear by 3lb. I had expected a lot more difference.

    Any comments about tyre pressures? Cathar said something recently about Pilot Powers liking more pressure? Does that help steering, traction, comfort, longevity?
  2. heat kills tyres as far as overheating and general lifespan.

    Getting enough heat into them for good traction is the other side of the coin. One of the major problems with pneumatic tyres is that correlation between sidewall stiffness and internal pressure. You need enough pressure to achieve a suitable carcass stiffness, but not so much that the profile distorts or they don't get enough flex to warm up.

    Horses for courses, and for 600/1000cc supersports rolling on 120/180 tyres, that pressure is probably fine across all types of bikes.

    I'd be disappointed if they tried to put 30psi in the tyres on my bird for a braking and cornering course though! Much heavier for the same tyre size = huge sidewall flex.
  3. I like 2psi extra front and rear in the Pilot Powers due to their sidewall flex when at the edge. Sometimes when tipping it in the sidewall will flex quite a lot if the tyre is cool, and give a brief moment of "oh crap!", before they readjust and stick.

    It's odd - some people report this happening frequently for them, and others not. I guess it depends on how quickly you throw the bike onto its side. The quicker you throw it in, the more it seems to do it if you run "typical" tyre pressures, but that's just my opinion. Others may differ.

    I personally run 2PSI more when running Pilot Powers than when running most other tyres, and that includes at the track where I'll run 32PSI, instead of 30PSI for other tyre types. Again, this is just my preference.

    The main reason for lowering pressures is to increase tyre temperature more quickly and have the tyre running at a higher temperature. With a lower pressure (30-33psi) the tyre carcass flexes more as you can imagine, the tyre generates more heat and actually the pressures will rise further than the 2-3PSI that you observed when street riding on, let me guess, 36-38PSI cold pressures?

    Ideally you want pressures to rise between 3-6PSI between cold and hot. For typical street riding, for a tradeoff between maximum tyre longevity and grip, you'd be targetting the 3PSI rise mark, which tells me that whatever pressures that you're running right now for the road, they are essentially correct. Maybe drop 1PSI from the front, but I wouldn't be too concerned.

    At the track, you typically target 35-36PSI hot pressures from a 30PSI starting cold pressure. If you're riding hard enough on street tyres though they will get substantially hotter, the pressures will rise further, and they'll start to slip and slide about as the rubber starts to delaminate. This typically occurs at 70-75C rubber temperatures for many street rubber compound tyres, and it is an issue when riding on very hot days at the track (>35C). Generally though, when starting out from 30PSI, most street tyres will warm up to the 65-70C range, rise to 35-36PSI pressure, and offer about the highest level of grip that they are capable of, but at a substantial reduction in tyre life.

    If you're riding at the track hard enough to get road tyres to start slipping about regularly, then it's time to start looking at fitting proper track rubber. In my opinion, this is only really necessary if you're achieving much less than 1m50s times around Phillip Island - again - that's just my opinion. If you're not getting lower times than that, and your tyres are slipping (and so long as the tyres aren't hard touring rubber) then generally the problem is the rider and/or suspension, and not the tyres at all.