Car Tyres

Joe_Horner

AdrianC wrote: »

<raises eyebrow> You reckon?

I know it for a fact (see my last post). In the daf 33, which has £30, 135 profile, Events on each corner and non-servo drum brakes all round I can brake at .92g if I push the pedal hard enough. Or I can stamp on the pedal and find a ditch before any appreciable braking happens.

nobbysn*ts

Joe_Horner wrote: »

Yes, the meter only goes to 1g because that's pretty well the limit that physics will allow.

I have ditchfinders on the front of my Pug, with old "decent" tyres on the rear. I also have a Tapley meter which I picked up for checking the Dafs.

Even with the ditchfinders I can pull 0.95g braking on dry tarmac. Or I can jam the brakes on and lock the wheels, in which case the meter will show that they let go at around 0.7g (it holds the peak reading). The difference is in how they're applied, not how hard they're applied.

Smoothness isn't about not pulling high g - you can drive perfectly smoothly at speed. It's about not loading the forces up too quickly. If you jab a pedal or jerk a steering wheel the tyres will let go long before they will if you apply the control progressively.

And that's the thing, 'braking on dry tarmac', if you lock the wheels then, I agree with you. It's when you get the wet, or slippy roads, or anything that takes the friction down. 'In sunshine, rain, snow, ice, fog, day, night........ ' Check your braking on ice with the daf, I'll bet you can't get close to that. And meters will read anything beyond 1g, it's not physics, it's friction that limits it.

Joe_Horner

nobbysn*ts wrote: »

[...] it's not physics, it's friction that limits it.

Yes, and physics limits the available friction. For dry rubber on dry concrete (a good grippy road surface) the CoF is about 1, giving a maximum theoretical 1g before sliding happens.

Water will reduce the available grip for any tyre (despite the marketing claims), which is why only an idiot would drive the same in the wet as in the dry, regardless of what tyres they have. The same applies to ice. Snow is a little different becaise the best way to stop on loose snow (or sand) is to lock the wheels and build up a wedge in front of the wheels.

But by far the biggest risk is sudden changes in the loading because (a) it unsettles the tyres, causing them to flex in unexpected ways, (b) it unsettles the suspension, causing unpredictable load transfers, and (c) it creates a "shock" load which is far more likely to break grip than a high steady load.

Try pulling a piece of string and you'll find it much harder to break than if you "jerk" it - the same applies to grip from a tyre!

nobbysn*ts

Joe_Horner wrote: »

................Try pulling a piece of string and you'll find it much harder to break than if you "jerk" it QUOTE]

Why?

bod1467

Stretching force versus impact force. Impact concentrates the force over a shorter time duration, and so the effect of the force is much greater even though the amplitude of the force is the same.

Think about pressure, which is P = Force / area ... the larger the area the lower the pressure for any given force. In the case of the string, area equates to time.

nobbysn*ts

bod1467 wrote: »

Stretching force versus impact force. Impact concentrates the force over a shorter time duration, and so the effect of the force is much greater even though the amplitude of the force is the same.

Think about pressure, which is P = Force / area ... the larger the area the lower the pressure for any given force. In the case of the string, area equates to time.

Which is were we are coming from. Even ditchfinders cope with forces generated by normal shear. It's on the sudden impact loads you need the better grip, - long sweeping bends, both hold about as well, heavy braking, better tyres on the front.

bod1467

nobbysn*ts wrote: »

Which is were we are coming from. Even ditchfinders cope with forces generated by normal shear. It's on the sudden impact loads you need the better grip, - long sweeping bends, both hold about as well, heavy braking, better tyres on the front.

I'm with you on that ... I always prefer to have my better tyres on the front. Indeed I got the garage to rotate my tyres front to back at my service last November, as I wanted the tyres with more tread on the front ready for winter - I'd much rather have to cope with oversteer than have no grip on the front.

I was just explaining the physics of what Joe was getting at. (And yes, everything goes back to physics).

Joe_Horner

nobbysn*ts wrote: »

Which is were we are coming from. Even ditchfinders cope with forces generated by normal shear. It's on the sudden impact loads you need the better grip, - long sweeping bends, both hold about as well, heavy braking, better tyres on the front.

Kind of, except it's not heavy braking per se, it's heavy (as in sudden) application of braking that causes most problems.

If you stamp on a brake pedal you'll make the best tyres in the world let go (assuming your brakes are working properly). In that situation, ditchfinders will let go earlier than "quality" rubber. But, if you press the brake pedal hard - even quickly - then the difference is a lot less than you might imagine. The same goes for steering inputs

The difference between "stamping" and "pressing quickly" is difficult to quantify on a computer screen but it does exist. If pressed on the matter pun intentional) I guess it's stamping is more or less instantaneous application of full force, pressing quickly is building the force over 1/2 a second or so. One will "throw" you against your seat belt, the other will press you against it (in fact might not even lock the belt at all).

Find a very quiet straight road and try it sometime - the car will react entirely differently to each technique!

For steering, the difference in feel is an uncomfortable lurch to one side or a firm but controlled settling of the car into the corner.

rrf494g

A while ago I needed to re-equip my "new to me" (but very second-hand) car with tyres (both summer and winter), and spent some time on this (and a couple of others) forum. There seem to be a lot of bigots out there with sweeping, grand, generalised views about price/performance/new/part worn/front axle/back axle etc and it became very difficult to find the reasoned, open-minded views that I was seeking to help make my mind up about where to buy what.

I pass on my summary to you - not to give advice about which tyres to put where on the car - but to give advice about "which advice to listen to". Here are my rules, you may find them helpful.You may, of course, may not. But they worked for me.

1. Don't listen to name-callers. "Ditch-finders" is a common name used, and means nothing. There are obviously some tyres that perform better than others but just calling names doesn't advance any understanding - it perhaps just makes some people feel good and superior when judging products. Stopping distance in the wet is a valid piece of information but names cast about, is not.

2. Don't listen to stock mantras. " Tyres are your only point of contact with the road" or "you always get what you pay for" are examples of typical stock mantras. These statements may be true, but they are then used to support any conclusion the writer is fixated upon. You may always get what you pay for (in a literal sense) BUT this does not mean that the qualities of a product are always reflected in the price. There may be good deals and bad deals.

3. Don't listen to ego-reinforcers. By this I mean those people who views are stated to give opportunity to write about their skills, abilities and exceptional needs. They seem to need to buy products that reinforce their view of themselves as racing drivers on the edge of the car's performance. Advice "from the racing track" has doubtful applicability with most drivers/cars/tyres on modern roads, driving legally (or nearly legally).

4. The official ratings are useful, and sometimes surprising. The ratings for economy, wet braking and noise are at least factual. I found the wet braking rankings to tell an important tale across a wide price range of tyres.

5. There are a wide range of tyres and prices on Ebay. Even if you don't buy from Ebay there are great examples to consider.

6. I found a general concensus (from the open-minded community) that modern tyres (with appropriate approvals) from all sources are much better than, say, 20 years ago. The manufacturing source is not a fundamentally accurate way of judging quality. Little tyre manufacturing companies have been taken over by big, and methods and processes drastically changed.

7. Real comparative tests, still show differences across tyres. They may all be better than 20 years ago, but some are even better than others.

Hope this helps. I ended up buying new or nearly new part-worns from Ebay at around £10 each from private sellers from whom I could collect. I could then throughly examine the tyres before use. But that was just my conclusion. I prefered Continental or Dunlop but I got some high performing budget names as spares.