Tyre pressures

It all depends on the optimum operating pressure and temperature for
any particular tyre, dunnit? Then one has to set the cold pressures
commensurately. Yes, it will vary from day to day, let alone between
bikes and rider combinations, but it is interesting to find the
starting point.

I now want to know what a cold pressure equates to when hot - I know my
Bandit handles better when I'm on it if I set the pressures down a bit
from the recommended 36/42 which everbody sticks in 'em. But that
42psi was for the bloody awful Michelin Macadam that it came out of the
factory wearing. 34/38 gives a better ride and handling compromise and
doesn't seem to affect tyre wear in a bad way.

Champ will tell you that riding with a /flat/ rear just feels a bit
strange - until you stop. Then when you get off to have a look at
what's wrong you find that the bike won't stay on the sidestand any
more and you have to catch it.

I've been amazed by steaming tyres - on a humid day, in light rain
seems to be optimum. Pulling up at a junction I was swathed in steam
to the extent that I thought the radiator had burst.

I had an RF900 which was neither slow nor light. The day I had a new
Avon on the rear, it pissed it down for the first 100 miles. I guess
tyres don't heat up well in an inch of standing water, because the
thing wasn't sticking to well afterwards. I was following a var, too,
and desperate to get around it. Up a narrow lane, I had a moment of
straight, so I went for it. They always put cattle grids on straights
it seems, but I didn't see it because I was concentrating on the
swathes of gravel that had been washed across the tarmac by the
previous day's torrents.

Right alongside the car, up to 50mph in second and accelerating quite
hard, I hit the grid. I didn't check whether it was just wet steel
that was frictionless, or whether the grid poles were the
fully-floating, rotating type, but the revs swung into the red while
the bike slowed down. Before I'd got further than WTF!!? I was off the
grid thanks to momentum, just past the car and back on the tarmac.
Laid a FOAD curving darkie from the rear, that did and I had no more
trouble from rear grippiness after that.

The seat pucker is still visible, however. From space, probably.

 

It wouldn't be difficult and it wouldn't be expensive either. If I were you,
I'd be patenting that idea Champ.

In fact.....

 

A guy comes out of hospital after suffering extreme pain and discomfort and
the first thing said is "My carbs need balancing, is Monday Ok"?

Love it.

 

It's all about priorities.

 

Had a really good blast today round the Stokesly TT & all ovber, they
performed superbly & are very confidnece inspiring, great tyres.

 

I imagine 42psi is a lot higher than the recommended cold pressure in your
part of the world. A manufacturer is going to calculate a recommendation
based on average ambient temperature.

 

They do actually recommend it here though. I don't use it. I usually
run 36R cold.

I'm a pissy scared boy on the front so run 32 in there. Don't mind
lighting the rear up though.

 

I may be being thick here, but that doesn't sound right. I mean, if
the *average* ambient is higher then big deal; the tyre pressures will
be set at that ambient temperature when 'cold'.

I could see it if hotter climates see more sun on the tarmac, so the
working temperature is disproportionately higher than when the presure
was set, or some other indirect effect, but just a higher average
ambient doesn't make sense to me.

 

Tyre flex generates more heat than ambient air temperature. If you are
just tootling down to the shops for a loaf of beard, nah, not going to
make much difference. But if you are gunning it, it makes a hell of a
difference. And 42 cold just has the tyre too solid for my liking.

 

Yes, for hard riding I agree and I drop my tyre pressures accordingly
on the rare track days that I do, but we were talking about the effect
of ambient temperature.

Are you saying that tyres heat up *more* in a hotter climate, i.e.
that racing in 30deg C makes a bigger pressure *change* than racing in
15deg C? If anything I would expect it to be lower: gas pressure
changes are affected by absolute temperature, not gauge temperature
and, say, a 50deg rise in temperature.will be a bigger proportion of
the absolute temperature at 15deg C (i.e. 288K) than 30deg C (i.e.
303K). I'm not taking account of moisture though; I have no idea of
the effect of vapour pressure.

 

bad form yada but I worded that really badly; I should have said
"...gas pressures are roughly *proportional* to absolute..."

Something else I thought of: as you're starting from a higher initial
temperature, on the face of it, you don't need so much sidewall flex
to achieve an optimum temperature anyway.

I'm sure someone will correct me, though.

 

No, just saying that the tyres are going to heat up regardless and
when you are punting the effect of flex is going to be greater than
ambient temperature effect. Obviously both have to be taken into
account, but I've always been told to look more at flex than ambient.
I haven't raced bikes, but have raced Sports Sedans and done tarmac
rallies and that was the way we'd operate. I'd drop the slicks down to
24 psi because flex would put them up closer to 40 psi once I'd got
some heat into them. And that was in cold little Tasmania.

 

I think the point being made was that Kevin's probable ambient is much
higher than ours, him being in Oz - so his tyres, when 'cold' are
already halfway up the warming curve compared to ours.

If 42psi is recommended here, then to set his tyres at 42 would make
them run hotter than is desirable when he's on one. I'm not a gas
pressure expert, but I'd think going to 40psi would give Kevin the same
starting point as we'd get from 42psi.

 

Yeah precisely. Even in Tasmania. But I still think 42 is too solid
for a cold rear.

Ride it to the servo and check it at 42, yeah that makes more sense.
But fucked if I am going to put 42 in a cold tyre. By the way it was
5C when I got up this morning, so we do actually have cold here
And that was near Mascot airport where I live, on Botany Bay. The
western suburbs of Sydney had sub zero.

 

Sure, I'm not arguing that point at all, merely the suggestion that
higher ambiernt = lower cold start pressure.

Perhaps the quoted road tyre pressures are, like many things, a
compromise. Road bikes may be ridden very slowly in traffic and lose
all their tyre heat, a road bike may carry baggage loads, and though
pressures for carrying pillions are usually quoted in the handbook,
not many people will alter their tyre pressures just to give someone a
lift. Also (and this is a *really* wild guess), by making the tyre run
cool, the tyre grip might be more consistent over a wider range of
riding conditions, so less likely to catch someone out if conditions
change suddenly.

 

Oh for sure, but in general if you aren't generating heat by fanging
it then the lower pressure (not too low) isn't going to make much
difference to you if the tyre doesn't heat up. But if you do start
pushing it then I'd rather a slightly more flexible back tyre that
doesn't feel like a brick. And the ambient temperature is not going to
play the major part of that. Your riding style is.

 

Indeed. I didn't make the ambient call, someone else did. I just
mentioned that they make the same call in Australia as they do from
the UK. That doesn't allow the difference between ambient temp in
Hobart to Darwin which is wildly different. Which is why I ignore it
and run what feels right. Use it as a guide, not as a golden rule.