tyre rating dilemma

Ultrasonic

Joe_Horner wrote: »

It did start to because there's no such thing as a really "straight line" on the road and...

Indeed, at which point the reduced rear grip due to braking shifting the weight forward makes the tyres more likely to lose lateral grip, all without the brakes locking.

In case it has got lost, my point is that the reduced rear load caused by braking means that I don't think you can really conclude anything about the relative performance of the front and rear tyres from your test. I know you aren't making any claims for this being a rigorous test, but this was your original conclusion that I was responding to.

I'm sad enough to find thinking this through interesting .

Joe_Horner

Nothing sad about it, far better to think the possibilities through now and then than have them sneak up on you out on the road

As for your suggestion, I can see where you're headed with it, but weight transfer won't remove enough load from the back tyres for them to lose lateral grip in a (basically) straight line as long as the wheels are rolling. You'd have to practically be lifting the rears off the ground to manage that and a Pug's front disks aren't that good

You're absolutely right that it was a pretty unscientific test, including lots of possible factors - the rear tyre age / possibly happening to have mud under one of the rears just as I braked / etc.

But those are all real-world conditions which, potentially, make it a fairer test than some carefully controlled test-track experiment and suggest that the real-world differences may not be as big as the tests imply.

Ultrasonic

Joe_Horner wrote: »

As for your suggestion, I can see where you're headed with it, but weight transfer won't remove enough load from the back tyres for them to lose lateral grip in a (basically) straight line as long as the wheels are rolling. You'd have to practically be lifting the rears off the ground to manage that and a Pug's front disks aren't that good

I don't really see how you can argue that the rear of the vehicle moved sideways without losing lateral grip.

Also, grip isn't a binary thing, laterally or in the direction of travel. You don't have full grip or no grip with nothing in between. I've not thought about this before but presumably there is always some extent to which a tyre slips on the surface, but 'grippier' tyres slip less, resulting in better acceleration, cornering and braking. Weight distribution affects grip, resulting in for example lift-off oversteer.

As for the comparison of your tyres from your braking test, I'm struggling to come up with a way it could have panned out that would make you conclude that the new front tyres were worse than your part worn premium rear tyres?

facade

The mistake people make with new tyres is putting them on the front. I'm surprised that whoever fitted them didn't at least mention they are best fitted to the back.

The best tyres should go on the back, as unless you are a Driving God, you want more grip at the back, to stop the back end overtaking the front, which usually results in a sideways impact, where there is less crash protection.

Having the fronts let go before the back will cause you to crash head on giving your airbags, crumple zones, belt tensioners and head restraints the best chance to save you.

A Driving God can avoid the crash by snatching the handbrake, or opening the door into a lamppost or something.
A true Driving God likely wouldn't be pushing it so hard that he was going to crash

Mr Average like me, who might make a mistake and overcook it, is better off going straight on.

Joe_Horner

Ultrasonic wrote: »

As for the comparison of your tyres from your braking test, I'm struggling to come up with a way it could have panned out that would make you conclude that the new front tyres were worse than your part worn premium rear tyres?

Because car braking systems are designed so that, if the grip is (roughly) equal at all 4 corners, the front wheels WILL lock up first under braking in a straight line. The fact that they didn't means that there was considerably more grip at the front than at the back.

The back end can step out when the wheels lock from braking because, when they do lock, their grip suddenly reduces to almost zero. It's literally as if they're on ice. At that point any tiny lateral force (well within their normal ability to cope) will start to make the back move out of line. This can be something as small as the camber of the road.

Then, as soon as the backs are no longer straight behind the (still gripping) fronts, simple momentum takes over because you have the weight of the back wantng to carry on at the same speed, while the front is slowing down (rapidly). So the car effectively tries to pivot around the front wheels.

As for being a Driving God (in reply to facade) I think I already admitted to that in the first post?

facade

Joe_Horner wrote: »

As for being a Driving God (in reply to facade) I think I already admitted to that in the first post?

You just gave a technical description of what I meant: you want the grippier tyres at the back (if they are the same tyres, the ones with more tread have more grip in the wet) to avoid locking them up.

It is contrary to what you would intuitively think: the locked wheels will move to the front not drag along at the back, it is why an artic will jacknife if the trailer wheels lock up.

But which sort of Driving God are you?

The sort I see drive rings round the police in Police Stop! Shock Horror! who can do 120 on stingered tyres and make a 90 degree turn in an alley by opening the door into a lamp post. :eek:

Or the sort who read the road and never need to demonstrate extreme skills

:D

Joe_Horner

facade wrote: »

But which sort of Driving God are you?

The sort I see drive rings round the police in Police Stop! Shock Horror! who can do 120 on stingered tyres and make a 90 degree turn in an alley by opening the door into a lamp post. :eek:

Or the sort who read the road and never need to demonstrate extreme skills

:D

Type 3: I could do all that handbrakey police-losing stuff if I wanted to, but prefer to keep all 4 wheels going where nature intended, so haven't needed to do anything close to an emergency stop in something like 28 years - the last one was on my test cos some plonker hit the dashboard with his folder.

As "proof of anticipation", a coupe of years ago I drove from Birmingham to Colchester, only touching the brake pedal 11 times (when actually stopping for roundabouts, junctions and coffee). The fuel consumption on that trip went from my normal 50mpg in the Daf I had at the time to just shy of 65mpg. The concentration needed wasn't worth the saving, but it was a good illustration of what's possible!

facade

Seriously,
I wasn't tongue in cheek about the better grip tyres on the back, last time I had a pair of tyres at ATS (on the front obviously, as they wear faster) the fitter recommended they go on the back, but did what I asked them to do.

(If I put the 3/4 worn back tyres on the front they would be illegal in no time I'd already swapped them before, thats why they are 3/4 worn)

molerat

Both my local National Tyres and Ford dealer asked why I wanted the new ones put on the back as surely I need the better grip on the front !

Ultrasonic

Joe_Horner wrote: »

The back end can step out when the wheels lock from braking because, when they do lock, their grip suddenly reduces to almost zero. It's literally as if they're on ice. At that point any tiny lateral force (well within their normal ability to cope) will start to make the back move out of line. This can be something as small as the camber of the road.

But the rear wheels don't have to lock for the lateral grip of the rear tyres to be exceeded, just think of a car going around a corner with no brakes involved. For the current example, braking and thus throwing the weight distribution forwards makes a loss of control of the rear more likely without the wheels locking.