Ka Diary
– 73,017 – 29 July 2005 – Tyres
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ended my last Diary Entry by discussing how age is creeping up on the
kid, how his bonnet and front bumpers are marked, chipped and scratched and how
he doesn’t look perfect any more. But
you know, I think I like this because it shows that Kermit has been used, has
been enjoyed, and isn’t just a show car.
This point underscores Kermit - some people call him a project car, but
he’s not - we use him every day. Over a
week I’ll typically put over three hundred miles on to his odometer and I will
only ever not use him to save mileage if we have a service or oil changed
booked and I don’t want to over-run the agreed mileage. I don’t envisage this materially changing
although Charlie has expressed a desire that we get another car so I can use it
and she gets Kermit.
Of
course, the very next day whilst driving to work along a favourite main road a
vehicle coming the other way drove over a dusty section of road and flung
several stones up at Kermy. It was more
a case that we drove through the stones rather than they stuck the lad.
I’m
going to write about Kermit’s tyres again.
The wear rate has been slightly better than I was expecting. To date he has worn his front tyres at the
rate of approximately 5,500 miles per millimetre of wear. I look to replace the tyres when the tread
depth is down to two millimetres from the original eight millimetres, which
gives me estimated front tyre longevity of thirty three thousand miles. I’m quite happy with this. His rears are wearing out at somewhere close
to 10,000 miles per millimetre, so they should last for around sixty thousand
miles.
Of
course, this tyre wear model is very much dependent on how the Ka is driven and
as regular readers will know, much of his mileage is on dual carriageways and
motorways, which are kind to tyres. The
model also assumes that the tyre wear rate (and behaviour) is constant
throughout their life. It’s possible
that the initial one or two millimetres wear away much quicker than the rest of
the tyre.
I’ve
started my research into having the lad’s XR2i
wheels refinished. We originally
bought the wheels as an experiment to see how well Kermit behaved on 185/60/13H
tyres and we’ve found that they work better than we expected. They’re not perfect: there is some sidewall
deflection under hard cornering and this requires careful attention to the tyre
pressures. Too soft a pressure and the
sidewall flops about. The rear tyres
are particularly prone to this behaviour, which is bad news because not only
does it underline the stiffer
suspension set up but it also causes oversteer. I’m
going to write about oversteer in a little bit. On the other hand, too high a pressure and the ride becomes materially
harsh.
Going
back to the current XR2i wheels, unfortunately, the cost of having the wheels
professionally reconditioned and then painted is currently working out to be
“expensive,” getting on for two thirds the price of brand new 15” alloys (yes,
they would be 15” rims wearing 195/45 tyres, so hardly like for like). At this juncture I’m not sure if I want to
get the XR2i rims reconditioned, we may be as well to find another set of
suitable alloys (but in better condition) and use these instead. Now I appreciate that I’m a bit of a
perfectionist with regard to such things but I believe if I’m going to get the
wheels reconditioned, they should look new afterwards.
If
we’re either going to replace the XR2i wheels or have them refinished so there
are advantages in wearing out all four booties at once. Using my wear rate model, if we swapped the
front and rear tyres around at the seventy five thousand mile interim oil and
filter change, both sets should be approaching two millimetres of remaining
tread by the ninety five thousand mile point.
As I write this it should take us between eight and ten months to cover
this sort of mileage, which puts us right back to Spring 2006. The advantage of this is that I’ll be able
to dump all four tyres at once for getting the wheels refinished without having
to store four part worn tyres about the house.
The disadvantage is that Kermy will be running on worn tyres for the winter.
Worse, he’ll have more
tread on the front wheels compared to the rear wheels for almost every single
mile. This means that there will be
theoretically more grip available from the front wheels relative to the rear
wheels and it’s not how I prefer to have Kermit’s tyre balance. I would rather have the newer, less worn
tyres on the rear wheels.
I
can imagine that some people will be wondering why I would prefer to have more
tread on the rear tyres when Kermit is a front engine, front wheel drive
machine. The front wheels do all of the
steering, all of the acceleration and almost all of the braking effort. Surely in an emergency stop it would be
better to have more available grip at the wheel that can make the best use of
it? And the answer to this is of course
“yes” but there is a caveat. It’s an
absolutely perfect set up for when travelling in a straight line but it is not
ideal when cornering.
The
reason for all of this is oversteer.
Oversteer can be great fun when you want it, but on a wet winter
roundabout and you weren’t paying attention enough to spot some idiot pulling
out in front of you before you need to brake firmly, it can be a nasty
surprise. If the manoeuvre is extreme
enough and you brake going around a corner you will encounter oversteer. If the rear tyres need replacing and you have
brand new ones on the front, you’ll find the Ka will oversteer very
quickly. This kind of snap-out
oversteer can be unpredictable and difficult to control. ABS can help reduce oversteer since it
prevents a wheel from locking up because a locked wheel is very difficult to
control.
If
the Ka oversteers, it will spin through one hundred and eighty degrees (at
most) and you’ll be heading in the same direction as you were when you
started to skid, but backwards. This means
you won’t see the tree, sign, lamp post or truck that you hit. The alternative to hitting something whilst
travelling backwards is that you’ll reach a kerb going sideways. If you’re going quickly enough you will roll
the Ka. Rolled Kas are almost always
write offs.
When
oversteering, if you brake you’ll exaggerate the slide as the front digs in but
you will slow down.
If
you have the new tyres on the back and worn ones on the front, you’ll tend to understeer. Understeer is far more predictable because you tend to pile
straight on. It’s also easier to regain
control. You’ll see what you’re about
to hit, the driver at least will benefit from the airbag. It’s harder to roll the Ka from understeer (but
not of course impossible). You’ll
have two wheels theoretically able to slow you down until they lock up (unless
you have ABS).
Of
course, skidding isn’t what I have in mind but if I can influence if we’ll
oversteer or understeer, for Kermit I’ll take the understeer unless I’m
expecting oversteer.
As
for what we may replace the XR2i wheels with, if we ever replace them, at this
juncture I don’t know what we’ll opt for.
I’ve always liked the look of the “propeller” wheels as used on the Ford Puma and I also quite like the 1x6 Ka
variant’s 16” wheels (the Streetka and Sportka no less). I certainly prefer original fit Ford alloys
to the vast majority of aftermarket alternatives out there!