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kiwi
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Post by kiwi »

Mulley please explain to us the differance between 175/70 tyres and 175/65?
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Post by MULLEY »

The profile :wink:

In all honesty, there isnt going to be a huge amount of difference between them. One is slightly taller than the other, but whats more interesting is why would you would fit them in the 1st place??

175's are a strange size over here anyway, & both those profiles are plain odd. I'm not saying that some cars weren't fitted as standard with them, but i'm struggling to think of any cars i've owned or know about that had 175 70or65's fitted. One of my bx's had 170/65's fitted & i found that a bit weird, even though the car manual does state you could use that size, bit strange when the standard fit was 185/60.
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Post by kiwi »

I think you better check your manufaturers handbook on the BX range again Mulley :lol:

Regarding the profile! Refering to the manufaturers guidelines 175/65 is the correct profile for the BX19TZS/TRS. Yet I find it curious that both these cars when I purchased them were fitted with 175/70. Reasoning I can think of is that the 70 profiles have more selection of brands and cheaper for some reason or because the NZ roading system with its twists and turns and unformed roads is better suited to deeper profile?

When I swapped the TZS back to the correct size I thought it would correct a speedo error discovered. Well the error is not related to the tyre size (but thats another topic). Bit the TRS must have been calibrated on the large size (70s) profile as it is now showing an error.

Anyways thats now opened another topic so to add to it

What is the correct pressure for tyres? I have always thought it was what is on the door piller! Well appaarently not according to Firestone in NZ and yes it comes up as a topic of debate on forums here like this one.

http://www.trademe.co.nz/Community/Mess ... x?id=62344

So reading through the crap there is a Tyre Tech who claims to work for Firestone saying what his company line is! Same company reckons I should have 35 psi in my tyres on the BX regardless of size.

That affects the grip dont it? An overpressurised tyre is going to hold the road less and be firmer on the suspension. Great when you go around a corner and bounce off a dead possum, cat or bunny or even a pothole.
1991 BX19 TZS 04/01/91 (Deceased)
1990 BX19 TRS 27/10/89 (Reborn)
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Post by Linegeist »

kiwi wrote:Anyways thats now opened another topic so to add to it

What is the correct pressure for tyres? I have always thought it was what is on the door piller! Well appaarently not according to Firestone in NZ and yes it comes up as a topic of debate on forums here like this one.

http://www.trademe.co.nz/Community/Mess ... x?id=62344

So reading through the crap there is a Tyre Tech who claims to work for Firestone saying what his company line is! Same company reckons I should have 35 psi in my tyres on the BX regardless of size.

That affects the grip dont it? An overpressurised tyre is going to hold the road less and be firmer on the suspension. Great when you go around a corner and bounce off a dead possum, cat or bunny or even a pothole.
I think you'll find the tyre pressures are only a recommended pressure that can vary with load, altitude, tyre brand and temperature.

As an illustration of this, I recall a session we had on this at tech college back in the heady days when I was an apprentice technician. The lecturer got a selection of a dozen or so cars from us, got us to inflate our tyres to the 'correct' pressures, then set up a shallow water puddle in front of a level patch of smooth concrete floor. We carefully and slowly drove through the water, leaving a tyre trail on the dry floor following it ....................

He thus demonstrated on 4 of the available cars that, at the recommended pressures, the full tyre width wasn't in contact with the ground - 3 of them were overinflated (so the centre of the tyre ballooned slightly and became convex reducing the contact patch) while one car had tyres which were underinflated and had become convex - with the same (but obverse) effect on the contact patch.

Under normal running conditions, and after extended mileages, the symptoms of under or over inflation are premature wear on the shoulders or centre of the tread area respectively (along with reduced grip).

Image

I suspect that, with the more pliable tyre compounds in use today, the effect might be even more pronounced. The 'rolling wet check' is a test I still carry out today whenever I get a fresh car - often with surprising results.
Last edited by Linegeist on Tue Nov 17, 2009 8:51 pm, edited 1 time in total.
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Post by DavidRutherford »

Always better IMO to have a tyre over-inflated rather than under-inflated. Yes, Ideally it should be "spot on", but it's generally safer to have them slightly over. Not only does it mean that you are not going to overheat the sidewalls, but you'll get better economy, the handling is less affected and the tyre wear will be more appropriate for clearing water. Plus, if the tyre loses any pressure, you've a "margin of safety" in that you'll catch it before the tyre becomes dangerously under-inflated.

Under-inflation only has one benefit, and that's increased traction when off-road due to the greater contact area, although you have to reduce tyre pressures a lot to achieve that.

Case in point, the van handles like a wayward shopping trolley with 40psi in the tyres, despite the tread being in good contact with the road. Increase that to 60psi and it's much much safer. That, and I get 2-3 more MPG, which will massively offset any slight decrease in tyre life.
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Post by kiwi »

I understand the logic of tyre pressures affecting wear and do agree with David on having them slightly over pressure. Except what seems to be recommended by Firestone is 35psi as a standard for car tyres which in most cases (including the BX)is aound 5 psi over, then throw in an altitude factor and some inert gases they like to use.

So changing tyre designs and obvious corporate interests you got to wonder what the correct pressure really is for your tyres.

So next question on tyres is this thing about the profile? whats the benefit of a deeper profile over and shallow profile.
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1990 BX19 TRS 27/10/89 (Reborn)
1992 BX19 TXD (Ex UK - K 744 SDF) 15/06/92
1990 BX19 TZS Auto 06/11/1989
1992 BX TZD Turbo Estate (Ex UK) 1/07/91
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Post by Dollywobbler »

Deeper profiles are generally softer, so less jittery. Plus, a deep profile will happily cope with you walloping the car up a kerb (my wife had to do this once in her poor Mini to avoid a head-on smash...) whereas a low profile won't offer much protection to the rim.
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Post by DavidRutherford »

Conversley, a lower profile tyre will not deform and flex as much under cornering, and so the car's handling will be more presice. This is typical of many modern cars. Very low profile tyres giving very sharp handling and an awful ride.

As for how over-pressure is over-pressure... I used to run my BX DTR turbo with 165's on it at somewhere around 36-38psi all round. I could generally feel if any tyre got below 30 as the handling would deteriorate. Yes, they did wear very fractionally more in the centre band compared to the outer edge, but it was very marginal. 32-34psi would probably have been ideal, hence my running slightly higher. I also used to do a fair bit of laden high-speed motorway work, so having pressures higher was no bad thing.
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Post by docchevron »

Even more interesting stuff!

Ok, I know I'm veering back to buses, but I know about them see!

With each bus comes a chart of tyre pressures to be used. A large list of pressures, all of which relate to individual tyre manufacturers.
Whilst the company has long since given up and now put 100PSI in everything I have the list for a Dennis Lance, and the recommended pressures (which camr about from consultation between tyre manufacturers and Dennis themselves) vary A LOT.
From 85 PSI (rear) to 125PSI (rear) dependant on tyre manufacturer.

Ok, bus tyres are like concrete compared to car tyres but the difference's can be staggering, hence I do often wonder about whether to listen to the tyre maker, or Citroen when it comes to pressures..

I find running the valver at 32PSI int he front and 30 in the rear (more than cit recommend) works well on the Uniroyals, running them at the stated 30 front 27 rear gives excessive flex in the side wall and some odd wear characteristics..
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Post by BX Bandit »

Are we talking tyre manufacturers recommendations as per what is stamped on the tyre? As that wouldn't accommodate the axle weights of each individual car.
On the BX I've always run the tyres at what is stamped on the drivers door pillar and always had good even wear. The Xantia on the other hand used to eat the tyre edges for breakfast!

In the event of skidding, due to lock up, wouldn't a slightly under inflated tyre be preferable to a slightly over inflated tyre? Admittedly though at a cost to cornering/handling?

Has anyone fitted deeper than recommended profile tyres? I'm considering it. It comes down to Kitch's classic quanta of the comfort:handling ratio! I'm sure with the right instrumentation or even mathematics the subjectivity could be removed and we could get car manufacturers to put actual suspensiony springy stuff back into cars instead of differing grades of concrete block!
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Post by Linegeist »

BX Bandit wrote:In the event of skidding, due to lock up, wouldn't a slightly under inflated tyre be preferable to a slightly over inflated tyre? Admittedly though at a cost to cornering/handling?
That raises the interesting point of what d'you say to the nice plod-person if you have an RTA, and he checks the tyre pressures (yes, they certainly do) and they're all 5psi high/low?

Given our porcine patroller's penchant for promptly prosecuting people that present half a chance of procuring patsy points (sorry, couldn't help myself there :wink: ) you could be looking at a prosecution for having a dangerous vehicle.

If they'll do you for 33mpg in a 30 zone, they'll be on you like a rat up a drainpipe for over/under inflated tyres methinks. :evil:
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Post by MULLEY »

I use 32 for fronts & rears on my bx's, wear seems fine to my untrained eye.

I think the problem with just using the tyre pressure chart stamped in the doorwell is that it relates to tyres of that era. Which is quite a few years ago now, & tyre technology has marched on significantely.

The other problem is that tyre manufacturers can only recommend a pressure rating based on an average axle weight of a car (i presume?), which in the case of the bx which is quite a bit lighter than modern cars, what should you use? I think the only way is to try various pressures to see which is the best, as every tyre these days is different, so what one tyre will need, may not suit another.

Interesting point about being outside of a recommended tyre pressure rating, regarding what the filth would do if you were involved in an rta. No idea, i'd just say i used a supermarket tyre inflator (these are notorious for being inacurrate), might get you out of that one.
2002 C5 2.0 HDI Estate - Jasmine - Now SORN
2011 Mini Cooper D Clubman - SOLD
2016 Mercedes A180D Sport - Auto refinement
1992 TZD Turbo - Bluebell - My daily
1991 Gti 16V - Blaze - crash damaged, will get repaired.
1990 Gti 8Valve SOLD - looks like it's been scrapped
2002 Mini Cooper S - SOLD - i miss this car
1992 TXD - Scrapped in March 2014
1988 CX 25 GTI Turbo2 - SORN
1996 - AX Memphis 1.5D - Dream - SORN

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Post by Matt H »

I think, to be fair, the Police would only do you if your tyre pressures were dangerously under/overinflated. like having 12psi in one of the front tyres and 58psi in one of the rears.
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Post by Linegeist »

I'm afraid there's evidence to the contrary, especially since constabularies started having to meet targets.

When an RTA is attended, under new guidelines Police are now under pressure to establish a culprit and, if the evidence is less than clear-cut, they'll start checking over the vehicles involved for defects. If they get more than one prosecution out of this procedure, then so much the better because, statistically, it looks good on their results sheet. Wayward tyre pressures will do just fine thank you very much and can even be used to establish the probability of liability. Certainly insurance companies will take a dim view of the matter and may even try and wriggle out of paying out if they find out you're driving around a bit flaccid (Oooer! :shock: )

A case in point on the silly charging issue, my mother-in-law hit a breeze block late at night on the M4 in Reading last year at around 65mph. The impact snapped off her nearside track rod end and her car skidded up the embankment. No other vehicle was involved. The Police/CPS prosecuted her for dangerous driving and won. Her only defence would have been that brick, but the Police 'tidied up' at the scene of the accident, and there were no other witnesses. I examined her car and found the snapped track rod end but, without the offending breeze block, it amounted to nothing as evidence. She was fined heavily and got 6 points.

Disgraceful? Certainly. Unusual? Not if you believe even a fraction of the stories coming off the internet from reliable sources.

I suggest the nice piggy-wiggy with the slashed-peak cap, the beltful of assault weapons and the punchy attitude is actually no longer our friend and should be treated in the same way as prosecuting lawyers and stoats - with great care, and avoiding where possible.
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