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Thread: Is having different tires on the front and rear less safe ?!

  1. #26
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    Quote Originally Posted by speaksgeek View Post
    I rotate my tyres to maintain as equal as possible tread depth across the car. When they are pushing the end of their life, I replace all 4. If I didn't manage to get that perfect and need to replace 2, but 4 are getting close, I still replace all 4.
    I agree with your approach entirely. It is what we have always done. However, if you strike a tyre dealer that won't rotate the more worn tyres from front to rear (for the reasons given air elsewhere here), then inevitably you wear out the fronts while the rears have plenty of tread left (in FWD car). By the dealer not rotating the tyres with the most wear to the rear, they also see you back much sooner for that new set of tyres!

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  2. #27
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    You've got a jack and tyre iron in the car? Rotate them yourself.
    If you don't know how, it's a good time to practice. It's practically the easiest maintenance you can do.
    206 GTi 180 - Cat Cams, Remapped Group N ECU, AST Camber Tops & Coilovers, -2deg fixed camber hubs by Frogstomp Racing, 24mm Torsion Bars, AP Racing brakes, Yokohama A050, PeugeotSport Baffled Sump, Powerflex Engine Mounts & Bushings, Setrab Oil cooler, Quaife diff, Velo seats.

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  3. #28
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    A few things (not exhaustively):

    First, distinguish various elements in the situation.

    One element is front/rear handling balance. Having different tyre types at each end can affect this. Distinguish dry & wet roads &, within the latter, streaming wet or standing water from merely slick.

    Dry:
    Tyres are somewhere on a spectrum from sloppy response to crisp response. This is a function of two variables: tyre structure & tread element stability. The former is to some extent able to be compensated for by tyre pressure changes but the latter only changes with wear (increasingly stable). So, fit tyres of different types to each end & the handling balance will change (for the technically minded, this is all a matter of relative slip angles). If one knows what one is about then this can be used to advantage. Plough understeer? Try a relatively crisper front. Wayward tail? Try a relatively crisper rear. If one doesn't know the tyres' relative crispness, then fit the same type front & rear & sort out the handling balance by playing with front/rear pressures. (Note that these might profitably change when one shifts from 4 type X tyres to 4 type Y tyres if one wishes to preserve handling balance - roughly, four crisp tyres will be tailier than four sloppy tyres - explanation on demand).

    Wet:
    The above story about slip angles applies in the wet as well but two factors are particularly salient in wet cornering - tread pattern & tread compound. In streaming wet or standing water conditions then the efficiency of the main tread channels in clearing water so that tread elements are in contact with the surface (& not riding above it on a wedge of water - aquaplaning) is of particular importance. But even in the absence of aquaplaning, tyres differ in how well they grip a wet surface. This is a matter of compound & siping. Say one has 4 tyres of the same type but two are more worn & older. Of course tread depth will be less but aquaplaning is a rare problem on Australian roads at Australian speeds & one might think that on merely slick roads, such tyres might be equal. Probably not. Many sipes are not full depth & thus a partly worn tyre has less than when new. Also, the compound has become less effective in the wet for various reasons. So, in our most common wet conditions, it's not tread depth so much as compound change & sipe loss that is the problem with older tyres.

    Of course it's best to change all 4 tyres at once & of course that is facilitated by rotating one's tyres regularly but if just changing a pair, then I'd eliminate slip angle variables by staying with the same tyre type (Unless the current type is total crap - a matter I'll return to.)

    So there will be two older & two newer tyres. Which end to put the new ones? It depends.

    For the above stated reasons, the new tyres will be grippier in the wet but not as crisp in the dry as the old ones. One can, if one knows what one is doing, deploy these differences to serve the cause of how one wants the car to behave.

    The usual industry advice (new at rear) is pitched at average drivers & their behaviour in various cornering scenarios. If a corner is gone around too fast, the average reaction is to lift off. If the incipient slide is a front end one, then that action is (in virtue of weight transfer) correct. If the incipient slide is a rear end one then (unless it's a simple rear wheel drive power slide) lifting off will exacerbate the problem. All of this is worse in the wet. So, when fitting new tyres to such a driver's vehicle, put them at the rear.

    So far, this is just about cornering behaviour. What of braking & traction under acceleration? Traction obviously depends on which end is driven. As for braking, it is true that most braking is done by the front & that grip lost there means no steering. With ABS vehicles, grip will be but transiently lost & steering will be maintained but, with increased cycling of the ABS, stopping distances will still be longer. So, a case from braking in the wet for new at the front.

    So far, my inclination for the average driver is to recommend new at rear on event frequency grounds (cornering problems more than braking ones) especially for ABS vehicles. This judgement is reinforced by one other argument.

    Most cars wear their front tyres out before the rears. Put the old ones on the front to chop them out in the cause of getting a more closely matched set ASAP. This argument has extra force if the old tyres are a crap type & a process of improvement is in train. Better, though, is to junk the crap tyres.
    Last edited by 4cvg; 21st September 2017 at 12:01 AM.

  4. #29
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    I was wondering if you were going to contribute (and looking forward to it).

    What is the sipe?

    In RC we change Ackerman when tyres become a bit tired (ha! - how about that?). Our tyres are slicks or foam and we don't drive in wet conditions (though there are tyres for wet track - it just doesn't happen in Oz). Plus, there are cars that use different size tyres front to rear so no front to back rotation option (we do regularly swap right to left as a matter of course - tracks being broadly speaking circular, the outside tyres would go to bits prematurely if we didn't). This compensates for the change in slip angles as tyres wear out (to some extent).

    Not that I would suggest one would be trusted with Ackerman adjustments on a real car even if it were possible.

    There is also easy access to changing camber, toe and roll center, spring rates and damping if need be (keep in mind tyres are regulated so you have to work with what you signed up for).

    I think this is where driver training comes in strongly, and though I don't want to open again the debate, I deplore the lack of driver training in Oz. Early learning to control any car misbehaviour would probably reduce accident rates a great deal and give scope to raising speed limits.

    I would like to see your more in depth comments about the impact of crispier tyres and the difference to sloppier ones (over-under steer shift), but I think you should qualify the behaviour by considering on and off power situations (as mentioned above). In my experience, these conditions have categorically different outcomes.

    Again, referring to RC where the effects are obvious (and I am free to push beyond the limit with no consequence to my or others' safety), a set of good tyres will promote oversteer off power coming into a corner especially if the corner is tightening radius, but not so much as a set of worn tyres. Coming out of the corner off power simply doesn't happen (unless you broke down).

    On power (it is rare that anyone would enter a corner on power, but it can happen depending on track layout), my experience shows that good tyres will promote mild oversteer, whilst poor tyres would see you overshoot (understeer), whereas on corner exit poor tyres will go into oversteer whilst good tyres will give mild understeer or stay neutral.

    These examples need to be considered in the context of completely unrealistic power to weight ratios as far as real cars are concerned, but I think they provide some indication of the behaviour induced, which I think (based on my attempts to replicate such situations in real life) is valid in full scale.
    Last edited by schlitzaugen; 14th September 2017 at 02:54 PM.
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    yep; all noted.

    Sipes are the narrow slits within the tread blocks (as opposed to the channels separating those blocks).

    cheers! Peter

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    Quote Originally Posted by speaksgeek View Post
    Rotate them yourself.
    If you don't know how, it's a good time to practice. It's practically the easiest maintenance you can do.
    Used to do that as a youngster after school & on weekends in parents service station. The thought of moving around the four 19" rims with 11" wide tyres seems like hard work at this age!
    Last edited by turnbull151; 19th September 2017 at 08:01 PM.

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    Quote Originally Posted by turnbull151 View Post
    Used to do that as a youngster after school & on weekends in parents service station. The thought of moving around the four 19" rims with 11" wide tyres seems like hard work at this age!
    Among many virtues, my 4 toys use but 165/70-14, 185/60-14 & 185/60-13 tyres. They are rotated regularly (X pattern alternating with H pattern) at 5,000 km services but are also swapped around by me on occasion for various purposes. In my case, despite personal decrepitude, not much of a chore :-) YMMV

    cheers! Peter

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    Quote Originally Posted by turnbull151 View Post
    Used to do that as a youngster after school & on weekends in parents service station. The thought of moving around the four 19" rims with 11" wide tyres seems like hard work at this age!
    What are you driving with 11" rims?
    206 GTi 180 - Cat Cams, Remapped Group N ECU, AST Camber Tops & Coilovers, -2deg fixed camber hubs by Frogstomp Racing, 24mm Torsion Bars, AP Racing brakes, Yokohama A050, PeugeotSport Baffled Sump, Powerflex Engine Mounts & Bushings, Setrab Oil cooler, Quaife diff, Velo seats.

    Sandown - 1:31.5
    Winton - 1:45.6
    Phillip Island - 1:58.4
    Nürburgring - 10:23.ish (Fiesta ST)

    Previously, 2x 504 Wagon, 505 Wagon, 505 STi, 405.

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    Wheel and tyre on a C5 is about 10 inches wide. Heavy b's with no studs to hold it.

  10. #35
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    Oops, meant to say 10" wide. 245 x 40 x 19
    Last edited by turnbull151; 21st September 2017 at 10:40 AM.

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    Quote Originally Posted by seasink View Post
    Wheel and tyre on a C5 is about 10 inches wide. Heavy b's with no studs to hold it.
    <pedant>My randomly chosen C5 is showing a 215/55-16 tyre on a 6.5" wide rim. A 215 tyre is 8.46" wide across the tread, which to me is noticeably smaller than 10-11"</pedant>

    There is a bit of a trick to getting rims on and off easily, especially with the wheel wheel studs used on PSA cars. I sit cross legged, quite close to the car. It avoids me having a leg under the car, but I can lift the wheel onto the locating lip, and hold it in place with my toe, while lining up the holes.
    206 GTi 180 - Cat Cams, Remapped Group N ECU, AST Camber Tops & Coilovers, -2deg fixed camber hubs by Frogstomp Racing, 24mm Torsion Bars, AP Racing brakes, Yokohama A050, PeugeotSport Baffled Sump, Powerflex Engine Mounts & Bushings, Setrab Oil cooler, Quaife diff, Velo seats.

    Sandown - 1:31.5
    Winton - 1:45.6
    Phillip Island - 1:58.4
    Nürburgring - 10:23.ish (Fiesta ST)

    Previously, 2x 504 Wagon, 505 Wagon, 505 STi, 405.

  12. #37
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    turnbull151 fixed the tyre size. I wish I could sit cross-legged, though a leg under a C5 is a bad idea. The small locating hub projection is far too small for these big wheels. I''ve been meaning to get a stud to keep in the boot, because they are [email protected]#$s to fit on the road side.

    Tyre changing is a pain on my DS3 too. The supplied scissor jack fits onto a projecting fin on the main rail (there's a plastic skirt at the sides), and lifts both wheels off the ground. It is decidedly unstable, and I've had it rotate on the fin and drop the car.
    Last edited by seasink; 21st September 2017 at 11:03 AM.

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    I wonder how long a lever would you need to allow you to use one of those F1 style jacks on our cars. Granted a 205GTI is not that much work to lift, but you'd struggle to find a point where you could place the jack coming from the front F1 style. The first strong chassis point you have access to is the back of the under frame, nearly under the bulkhead, so way past the engine.
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    During my 50+ years of driving, I always placed my new tyres on the back!
    They are the tyres that the cops following you can easily see!



    Really though, new tyres on the front every time. They do the braking and the steering.

    Anyone that thinks they can drive in the wet on a public road like "Brocky" could on the track, is a dick head!

    Good Luck.
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    An extreme example of mismatched tyres front and rear was experienced by me in the late 60s. My Dad's EH had, for some reason Michelin X on the front and Dunlop crossplies on the rear. A lethal combination, as I found out in a hurry. The old man drove everywhere at 50mph, so he didn't have a problem, but when Harry Hotwheels (me) gave the mighty 179 it's head it swapped ends at about 75mph.
    The 3 great pretenders, all in the front seat bounced off one another as we rolled it twice into the shrubbery. The old boy was not impressed! Needless to say I have been meticulous in having matching tyres front and rear ever since, and hang the expense. Killing your two best mates is not an option.
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    Michelin X had a very low slip angle, even less than modern low profile tyres, due to the 3 layers of steel under the tread,so would have be diabolical mixing with cross plies. My 203 on Xs would not slide, it would just get up on 2 wheels if you cornered hard enough. Trouble is Xs would sometimes let go for no reason, so could be dangerous, they were toned down in the Zx and then further in the XzX, these tyres didn't have the response but were safer, particularly in the wet.

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    Quote Originally Posted by BIGRR View Post
    During my 50+ years of driving, I always placed my new tyres on the back!
    They are the tyres that the cops following you can easily see!

    Really though, new tyres on the front every time. They do the braking and the steering.

    Anyone that thinks they can drive in the wet on a public road like "Brocky" could on the track, is a dick head!

    Good Luck.
    well said! which reminds me of Bathurst in 1987 - which Brocky won - and which provided
    some terrific examples of wet-weather driving when a deluge descended in the afternoon.
    the best was 22 year old Glenn Seton in the Skyline, on slicks, incredible car control, unforgettable.

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    Quote Originally Posted by BIGRR View Post
    During my 50+ years of driving, I always placed my new tyres on the back!
    They are the tyres that the cops following you can easily see!



    Really though, new tyres on the front every time. They do the braking and the steering.

    Anyone that thinks they can drive in the wet on a public road like "Brocky" could on the track, is a dick head!

    Good Luck.
    I suggest that one doesn't have to be driving in a manner which you would decry for a vehicle's limit behaviour to be experienced. Two scenario examples are the emergency swerve & the overfast corner entry owing to inattention. Presumably neither scenario would ever apply to you but for lesser mortals . . .

    Of course the front tyres do the major part of a vehicle's steering & braking but to note such point is trite & doesn't attend to the detail of various dynamic scenarios.

    cheers! Peter

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    Just on my bookshelf are a couple of texts dealing with vehicle dynamics/mechanics. Over a hundred pages in one full of charts and equations, and calculation of steering effects, some sudden - all as described in this thread. Not really bedside reading.

    I don't understand now why so many people think they know this stuff. I learned the lesson once, the scary way, on a public street. I only looked it up afterwards and felt stupid. I changed the rear tyres.

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    Quote Originally Posted by 4cvg View Post
    I suggest that one doesn't have to be driving in a manner which you would decry for a vehicle's limit behaviour to be experienced. Two scenario examples are the emergency swerve & the overfast corner entry owing to inattention. Presumably neither scenario would ever apply to you but for lesser mortals . . .

    Of course the front tyres do the major part of a vehicle's steering & braking but to note such point is trite & doesn't attend to the detail of various dynamic scenarios.

    cheers! Peter

    Peter,

    you are probably correct on all counts, though I am too long in the tooth to change my practice.

    I wonder at what ratio the following events might occur per annum to an average road driver: Emergency stop, enter a corner too fast (for any reason) and an emergency swerve. 17:3:3 ?
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  21. #46
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    If one's driving is almost entirely in suburban circumstances then braking "moments" are more common. As soon as one is outside such circumstances, then people having "moments" on corners in the wet is more frequent.

    cheers! Peter

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    I've always put new pairs at the front, for a FWD.
    Front does most braking, all steering and perhaps more lateral forces when the front weight is higher (like most Peugeots, I'd imagine).
    However, also have decent rear tyres, under all circumstances. Cheap tyres are not worth the saving, when the unexpected happens...
    I don't have an issue with different tyres front/rear but suggest overall diameters be similar i.e. width x profile product should be similar.
    My 2 cents worth.

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    Believe me, if you are able to do the maths involved in steering, you'll change your mind. Your car has four wheels.
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    Quote Originally Posted by seasink View Post
    Believe me, if you are able to do the maths involved in steering, you'll change your mind. Your car has four wheels.
    My old fuego would 'cock its leg' so to speak when pushed hard, so its rear tyre (singular) had a rather important job to do on tight fast corners.


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    I drove 7000 miles on a set of Bridgestone M+S front and a set of Dunlop SP3 at the back to Darwin and back in 1970 in a 16TS. The M+S were needed between Cunnamulla and Winton or we would have gone nowhere in the mud of the blacksoil plains. The rear tyres dutifully followed the fronts.... all the way.
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