Rick Onanian writes:
>>> This may be all true in a perfect world where roads are
>>> fresh, clean pavement. I, however, experienced a
>>> controllable side-slip on my rear tire, followed by whole-
>>> bike drift, while doing some 35 mph downhill, around a
>>> curve, on an older road whose pavement is probably
>>> imperfect (probably not as flat a surface as a taut
>>> string, for example), and it was probably not perfectly
>>> clean.
>> If it was pavement with no sand or other solid
>> (lubricant) I am reasonably sure that you did not drift
>> around a curve on pavement.
> It probably was not perfect, fresh, clean, 100% black,
> unmolested pavement. I'm sure that it includes some dust
> and dirt. Many driveways on that street are dirt. No dirt
> was visible on the pavement, but it wasn't the 100% dark
> jet black of brand new pavement either.
I'm not quibbling about common dust on roads. The dry
pavement you describe fits the definition of clean dry
pavement and on such a surface, no "drifting" or sliding is
possible without crashing. This has been measured often
enough and we have not seen anyone who can demonstrate a
slide in a curve since the lean angle for that is greater
than 45 degrees to the pavement. At that angle, no one I or
any of the fastest descenders that I know have seen anyone
do that and not crash.
> In short, it's a real world condition.
It doesn't take that much definition.
>> I am sure you experienced loss of traction but it was not
>> on clean dry pavement, that condition where cornering
>> ability of a tire can only be assessed by measurement at
>> the moment of break out and crashing.
> Agreed. It was a real road, the kind where bicyclists
> depend on tire traction.
>> That is not a reasonable thing to attempt and that is why
>> I asked how handling of one tire over another under
>> similar conditions is assessed.
> Your paved-drum/leaned-bike experiment sounds like a
> reasonable, if imperfect, test method for fictional roads
> made of perfectly clean and perfectly flat pavement. I
> rarely find roads like that, and when I do, the new-
> pavement fumes make riding somewhat unpleasant.
Maybe you can explain what is "imperfect about this test. We
have a few riders in this area who corner at the limit of
near 45 degrees and the testing machine predicts break out
at 47 or so with slick tread repeatably.
>> I have experienced such slips often and even done so
>> crossing smooth paint stripes in the rain, but I don't
>> attribute those incidents to the tire but rather to sand
>> on the road or a slick wet spot. We ARE talking about
>> handling ability of one tire over another.
> Yes, but what good is it to know the handling ability on
> perfect pavement when we don't ride on such surfaces? We
> ride on roads with a bit of sand or a slick wet spot.
> Knowing the handling ability of a tire for such conditions
> is immensely more useful.
Let's not get into philosophy. You claim to have slid tires
on clean dry pavement and I said that is not a recoverable
condition so it cannot be the criterion for handling among
different tires. We generally don't ride beyond the limit
of traction so the criterion must be something else. I'm
trying to get to the bottom of how you can give comparative
ratings to tires of similar size, inflation and essentially
smooth tread.
>>>> You test it on a machine. I originally performed such
>>>> tests on a 16ft long paved plywood tiltbed increasing
>>>> the angle until the bicycle slid.
>>> I suspect that the different compounds, as well as tire
>>> casing constructions, react differently to the changing
>>> conditions of a road while moving than to the static
>>> conditions of a stopped bicycle on a tilted platform.
>>> Per my example above, where I theorize that the pavement
>>> is probably slightly wavy (although I've never noticed
>>> it, a tire on a 35 mph bike would), the tire's casing
>>> would have quite an effect on how the tire's tread stays
>>> in contact with the road.
>> Who's talking about static condition? The test bed was
>> ridden across at about 15mph. That is dynamic enough to
>> give a reading and to
> I think I understand; the bed was tilted sideways while
> you rode across it, leaning to the high side of the bed
> to keep yourself plumb. That makes more sense; I
> visualized you on the non-moving bike holding the brakes
> as the front of the bed raised (in a dumping motion),
> until the bike slid.
I didn't lean, I rode across it upright. The test bed was
tilted. When the tilt was too steep, the bicycle slid out
unrecoverably as it does on a road.
>>> The casing also must compensate for imbalanced wheels.
>>> We don't balance our wheels because it doesn't bother
>>> us; but if you lift your bike, hold it in your hand, and
>>> pedal the rear wheel up to 25 mph, you may feel the bike
>>> jumping up and down. Mine does; I taped weights to the
>>> wheel until it was balanced, just to see if I could. I
>>> suppose it could be something odd with the bearings...
>> I guess you missed the balance experiments that were done
>> by placing lead weights at one spoke location and other
>> such balance and imbalance conditions that are used as
>> excuses for all sorts of rider errors. This has all been
>> done. None of the best descenders I have talked to has
>> ever considered balanced tires as a benefit.
> Well, then we're not talking about a lot of precision
> here. Wheel imbalance can bounce a bike up and down in my
> hand at >20mph; that lifting/weighting force must affect
> the tire's load (and therefore, contact patch) each
> revolution.
I doubt that. Having descended at more than 50mph often
without having balanced wheels, I have not felt so much as a
hint of imbalance from my conventional wheels that are not
balanced. Besides that, as I said, I have done balance and
imbalance tests with no perceptible effect and reported the
results here. These tests were done to prove that shimmy is
not related to wheel im/balance.
>>> Additionally, that test is certainly irrelevant for
>>> lateral traction; it only tests fore-aft traction.
>>> Sideways, the tire bends (again, involving the casing),
>>> and also the rider leans, exposing a different part of
>>> the tire to the road.
>> I think you are not visualizing what occurs when a
>> bicycle leans in a curve. The tire testing machine leaned
>> the tire that was loaded in-plane onto a large paved drum
>> while a set of sensors triggered the recording of the
>> angle at which the wheel slipped out. This was repeatable
>> and was done at about 20mph. There are no side loads any
>> more than there are side loads on a bicycle when
>> cornering.
> A different portion of the tread, supported differently by
> the sidewalls, is in contact with the road. However, the
> tire testing machine of which you speak, and of which I
> was unaware in my previous message, would test that.
I don't understand what you propose here. The tires tested
were typical of available major brand tires.
>>> I disagree that a rider can't tell; and more
>>> importantly, if I'm wrong and the only way to know how a
>>> tire handles is to wipe out, then it would seem _very_
>>> important to know how the tire handles before buying it;
>>> at that point, one's life could depend on a tire's
>>> handling ability.
>> Could you explain what this paragraph means with respect
>> to a rider being able to feel that one tire handles
>> better than another?
> A rider can tell if he got through his favorite curve
> (which has real-world pavement) at a higher speed without
> any traction reduction.
Yes? How do you determine "traction reduction". This is what
is at the root of this subject and I propose that you cannot
sense this without exceeding the limit and crashing.
Therefore, claiming that one tire handles better than
another is an undefined subjective claim.
> More importantly, however, you've failed to address the
> big issue -- if tires really do break out without any
> warning as you say, then it would seem extremely important
> to be able to choose a tire based on it's actual tested
> and reviewed handling. A high speed traction failure
> accident sure sounds dangerous! I doubt I would have
> ridden home if my slip incident turned into a complete
> loss of control; at 35 mph or so, I would have wrapped my
> body around a tree or a stone wall (the two types of
> object found on the side of that road). --
I repeat, you didn't slip on clean dry pavement. I don't
claim that you didn't slip but it was for some reason other
than traction limitation of the tire. It was more likely
some foreign object on the road or a spot of some lubricant.
Tire testing machinery is important in this business and as
far as I can tell no one has one other than Avocet, a
company that is not currently performing such tests.
Jobst Brandt
[email protected]