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Intermittent rear wheel skittishness

dyrlacdyrlac Posts: 735
edited December 2013 in Road beginners
After 4 visits from the PF in 3 days (one self inflicted by a ham fisted attempt to get the tyre back on), I switched from 32mm Sammy Slicks to 28mm Gatorskins. This was not perhaps well thought out given that winter is coming, but I was mad and not responsible for my actions.

Anyway, I have noticed after a particularly hard bump through a pothole or drain cover in the rain, my rear wheel will "feel"-often for the balance of the ride-as if it has gone flat in that it loses traction and skitters all over the road, even on the straights. Oddly, when I get to a reasonable stopping point (usually my destination), I discover that the wheel is totally fine. So what gives? I thought it might psychosomatic: I'm unconsciously putting less weight on the rear of the bike to protect my rims when I think I may have gone flat, but it really doesn't seem to be the case as I try to compensate for any unconscious pulling up.

In any event, I have no confidence in the GS's and could use some alternative suggestions. Requirements are very simple: no p*nctures, grippy on censored London roads but nothing more exotic, and speed, in equal measures :?

Posts

  • bucklesbuckles Posts: 694
    Whenever skittishness is mentioned, tyre pressure (too much of it) springs to mind. However the fact that it happens after a bump is odd.. if it was down to tyre pressure I'd have thought it would happen all the time. How much pressure are you using and how heavy are you? I think about 85psi is recommended for 28mm tyres / 75kg rider.
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  • oxomanoxoman Posts: 8,196
    I currently use gaterskins on my wet weather bike all year around with no problems. I am 85 kg and run at 100psi at all times as below that I managed to get a pinch flat after a pothole. Only times I've had grip issues is when I've gone through diesel spills or wet mud.
    Too many bikes according to Mrs O.
  • dyrlacdyrlac Posts: 735
    I tend aim for about 100 psi (when I remember to top up, mostly when a thumb test reveals any degree of squidge, which seems to be about 80psi), and pack, errrrr, 78 kg of engine. If we're at the margins PSI-wise (and that may be the case; as mentioned, I'm not too happy with grip overall) and my ride profile/weight distribution changes even a little after a bump, that would possibly explain the behavior. Assume dropping down to 85 would improve grippiness generally, but how big is the speed/acceleration penalty?

    Some kind of road fouling would also explain it, and I've definitely had that before, but this "feels" qualitatively different -- exactly like a pinch flat.
  • you're probably over inflating for your weight.. I'm 92kg and run 95psi front and 105psi on the rear (conti gatorskin hardshells).
    dropping pressure is very unlikely to slow you down.
    also - http://www.dorkypantsr.us/bike-tire-pre ... lator.html
    may be of use to you.
    the paragraph below is from the sheldon brown website..

    Rolling Resistance
    "Rolling resistance" is the mechanical friction generated as the tire rolls. As a segment of the tire tread rolls into contact with the road, it deforms from its normal curved shape into a flat shape against the road, then back to the curve as the tire rolls onward. The deformation of the rubber in this process is what causes the friction. A bias-ply tire has some additional friction because of the "Chinese finger puzzle" effect of the bias plies. The edges of the contact patch scrub against the road as a segment of the tread becomes shorter and wider where it flattens out, then longer and narrower as it becomes round again.
    There are four ways to reduce this friction, each subject to trade-offs:

    The thinner and softer the rubber/fabric of the tire are, the more flexible they become.
    The trade-off with this is that the thinner the tire gets, the more fragile it is, and the sooner it will wear out.

    The higher the air pressure, the less the tire will deflect.
    The trade-off with this is that if you pump the tire up too hard, you lose the benefits of pneumatic tires: the ride becomes excessively harsh, and traction will be reduced.
    In addition, extremely high pressures require a stronger (heavier) fabric and stronger (heavier) rim flanges.

    When riding on a smooth surface, rolling resistance does decrease theoretically with any increase in pressure, but with modern, high-quality tires the rolling resistance at correct inflation pressure is already so low that the infinitesimal reductions gained are more than outweighed by the trade-offs.

    In practice, riding surfaces aren't perfectly smooth, and overinflation actually increases rolling resistance, due to vibration.
  • slowbikeslowbike Posts: 8,490
    dyrlac wrote:
    Assume dropping down to 85 would improve grippiness generally, but how big is the speed/acceleration penalty?
    You're definitely faster when the bike is upright ...
  • crankycrankcrankycrank Posts: 1,830
    Did you check for loose spokes after hitting the pothole? This could give a skittish feeling.
  • antflyantfly Posts: 3,448
    Also check for any play in the hub, that would cause skittishness.
    Smarter than the average bear.
  • bucklesbuckles Posts: 694
    dyrlac wrote:
    I tend aim for about 100 psi (when I remember to top up, mostly when a thumb test reveals any degree of squidge, which seems to be about 80psi), and pack, errrrr, 78 kg of engine. If we're at the margins PSI-wise (and that may be the case; as mentioned, I'm not too happy with grip overall) and my ride profile/weight distribution changes even a little after a bump, that would possibly explain the behavior. Assume dropping down to 85 would improve grippiness generally, but how big is the speed/acceleration penalty?

    Some kind of road fouling would also explain it, and I've definitely had that before, but this "feels" qualitatively different -- exactly like a pinch flat.
    There is zero penalty for running the correct pressure. Higher pressure does not equal faster. Higher pressure can even be slower due to reduced traction, especially when accelerating.
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  • Monty DogMonty Dog Posts: 20,614
    Higher pressures = more bounce. When your tyre's not in contact with the ground you're wasting energy.
    Make mine an Italian, with Campagnolo on the side..
  • dyrlacdyrlac Posts: 735
    Dropped pressure to c. 85 psi. Better in every respect. Thanks all.
  • jspashjspash Posts: 107
    Often when I fix a flat (lots of them with GP4000s's) I go through about 30 minutes of wibbly behaviour. I think it has to do with how the tyre or tube is seated. It needs some time to bed in. I also run at 85psi in the front and 90 in the rear since I changed to 23s. So for me, it's not the pressure. High pressure skittishness has a totally different feel.
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