Twitchy rear end - Alfa Romeo Forum
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(Post Link) post #1 of 14 Old 4 Weeks Ago Thread Starter
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Twitchy rear end

Ive decided that, as the title suggests, the rear end is a little bit too happy to step out in the JTS.
Particularly so in the wet. There dont seem to be any concerns in the dry.

It happened once by accident and ive since then took 2or3 opportunities on known roundabouts to test the handling and it surely gives way much earlier than i expect it too. The electronics kick in fairly efficiently it has to be said.

My first thought is tyres. They look new and have loads of tread but its the ones that came with the car and im not sure they're of a great quality(im being kind). Ive replaced the fronts with some nice Pilot sport4s and im wondering if this has had a paradoxical effect on the rear end

The fronts had to be replaced due to wear but i left the rears as the seemed new and it didnt make economic sense.
Other that this, is there anything else anyone can pitch in or would tyres be my first port of call in any case?
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(Post Link) post #2 of 14 Old 3 Weeks Ago
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Perhaps the new front tyres are showing the rears, have you had suspension aligned since you got it?
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(Post Link) post #3 of 14 Old 3 Weeks Ago
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Been waiting for someone to demonstrate why I always fit new tyres to the rear.

Yes, grippy low slip angle tyres like PS4 (good tyre choice, BTW) do go where they are pointed and it is something a bit lacklustre on the back, the back end will twitch...er..

Anyway, fitting the rubbish ones to the front does limit cornering speed but at least the tyres can be felt they are at their limits rather than the rear pop out of line unexpectedly.

Much safer with the good ones on the rear. Also, new tyres are subjectively better in the wet, worn tyres are subjectively better in the dry. I find the handling to be more consistent the way I do it and giving new tyres an easy time on the rear to begin with helps them last longer.

Years ago I mixed Michelin Pilot things and Avons, I think. I had to put the Michelins on the rear due to rear end wander above 65mph when Michelins were on front- even when going straight!
Putting good ones on the front is a bit like radials on front and crossply on rear.
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(Post Link) post #4 of 14 Old 3 Weeks Ago Thread Starter
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Well....thats what's going to happen soon then Fruity. When i come back from France there'll be two new decent tyres going on the rear and then I'll have a better balance hopefully.

To my knowledge btw, theres been no messing with the geometry of the car
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(Post Link) post #5 of 14 Old 3 Weeks Ago Thread Starter
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BTW, if what you say/have experienced is correct 4 new tyres all at once is prob not a grwat idea?

Ideally we should fit new tyres to the back, give them 1000mls and then buy two more and put THEM on the back and move the rears to the front??
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(Post Link) post #6 of 14 Old 3 Weeks Ago
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My understanding:

A tyre generates greater slip angles when it is new and its' tread blocks are deepest, and smaller slip angles as the tread blocks wear down. The tread blocks distort, they move sideways and twist with the lateral and axial steering forces imparted into the blocks. They do this more the deeper the blocks are, the smaller area of the blocks, and also the softer the tread rubber is. The more the tread blocks 'squirm' under lateral load the greater the difference between the direction in which the wheel is pointing and the orientation of the tread rubber at the road / tyre interface. As the tyre wears down this difference decreases, so the tread face has an increasingly greater fidelity with the direction in which the wheel is pointing.

This is at least one reason why higher 'performance' tyres tend to have larger tread blocks (greater distance between tread grooves and fewer grooves), and often shallower grooves. The larger shallower blocks tend to 'squirm' less than smaller deeper blocks, especially with the softer rubber used in 'high performance' tyres. And, at least one reason why dry weather racing tyres have no tread grooves (or only vestigial ones).

So all else being equal, newer tyres (that generate larger slip angles) fitted on the rear of the car (with somewhat worn tyres fitted on the front) will encourage oversteer, more so the greater the lateral forces acting on the treads. Fitted to the front (with somewhat worn tyres fitted on the rear), they will generate understeer.

Personally, over the years a number of times I have fitted new tyres to the rear of a number of different cars (with more worn tyres on the front), and each time I did not like the result much. The back end would feel 'vague' and 'wooly' compared to the front end, especially as cornering forces increase. Understeer decreased, but not in a nice way. The rear end isn't all that stable with full depth tread on the rear tyres (and less depth on the front tyres, maybe 1/2 depth, or so, give or take).

I recall one occasion when the then current car had newish tyres fitted on the rear only (enough kms to be scrubbed in, but still effectively full depth treads). I was attacking one of my favourite corners no harder than I had easily done many many times before, and the rear end just lost grip, badly. The car travelled a long distance at least 45 to the direction of travel, a really big opposite lock slide, requiring huge skill to save (or maybe it was huge luck...).

Swapping the new tyres from the rear to the front certainly feels better to me. The steering and handling is more stable. I've tried this experiment a few times over the years, and the result has always been the same, the car was better with the new tyres on the front. These days I just put the new tyres on the front straight away, and the more worn pair on the rear.

However, I don't like new tyres much. I can feel the blocks moving, the steering becomes vaguer and handling diminishes compared to part worn tyres. I like my tyres best just as they are in need of replacement due to lack of tread depth. At minimal tread depth I find that tyres are more precise, steer better, handle more 'solidly' and are more consistent.

Where new tyres are probably better is in rubber hardness, because rubber get harder with age and heat cycles. So for me, all else being equal the best tyre is a new one that has had most of its' tread shaved off, though not a practical nor economic option...

Regards,
John.
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(Post Link) post #7 of 14 Old 3 Weeks Ago Thread Starter
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Maybe a semi racing tyre then, for track days but, as you say, we're stuck with buying new so have little option really.

The ps4s on mine are pretty new, theyve maybe covered 3k and i dont push them that hard that often.
Will probably make little difference (to me) as I'll be getting new ones for the back pretty soon and after another 2or3 hundred miles, There wont be a huge difference between them. It'll be interesting to see if the oversteer lessens or not.
By all accounts different cars react and are quite different in their sensitivity to tyre choice too.
Thanks
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(Post Link) post #8 of 14 Old 3 Weeks Ago
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I've had PS4s and liked them a lot, even when they were new...

Have you experimented with fitting the PS4s on the rear and your lesser quality tyres on the front?

Regards,
John.
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Quote:
Originally Posted by johnlear View Post
I've had PS4s and liked them a lot, even when they were new...

Have you experimented with fitting the PS4s on the rear and your lesser quality tyres on the front?

Regards,
John.
No....but there's no reason why i shouldn't, before i go and spend 160.

I'll report back!
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(Post Link) post #10 of 14 Old 3 Weeks Ago
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Yes, specific applications etc all have their effect and that's before throwing driver preference in the mix.

Following on from John's comments, road cars are engineered to understeer with increasing body roll. Handling is most balanced in low grip conditions and understeer the prevalent trait on dry asphalt. That in itself is normally the best argument for fitting the newest (best hopefully- unless you've taken a chance on a pair of "Lucky Land- Happy Galop"- yes, these actually exist- in that spelling) tyres on the rear.

Another benefit of the rear first method is the wear bias which develops is lessened and there is less rounding of the front tyre shoulders by the time they are worn.

Tyres also tend to wear at a slower wear rate when worn. Again, John discussed the reasons why (more rigid tread blocks= lower heat build up= lower wear rate (normally). This is the opposite of brake pads which wear faster when worn (heat reflected by pad backing plate back to disc= higher temperatures= faster wear rate).

Last edited by Fruity; 3 Weeks Ago at 11:03.
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What are the tyres on the back exactly, Brand and model, you haven't said ? some of the cheapo brands are frankly dangerous in the wet, very poor grip compared to the dry.
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Quote:
Originally Posted by Alexie View Post
What are the tyres on the back exactly, Brand and model, you haven't said ? some of the cheapo brands are frankly dangerous in the wet, very poor grip crompared to the dry.
Cant remember..its in the garage.
Its something ive never heard of so no doubt put on there to pass its MOT last Nov! I'll post pics later
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(Post Link) post #13 of 14 Old 3 Weeks Ago
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I had a pair of Wanlis on the rear of my JTS when I got it... Ok in the dry but tail-happy on roundabouts in the wet. I was always advised to put the grippier/new tyres on the back as it's more consistent with most people's expectations of driving - for front and lesser extent 4-wheel drive cars. It's "safer" to go straight on at a corner than spin round and round and round... ok, it's "easier" to work with as speed can be moderated into the corner more readily to combat the understeer.
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Tyres are "Vanti Touring"
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