DS tires: rounded or flat tread?
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  1. #1
    GGR
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    Default DS tires: rounded or flat tread?

    Hi All,

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    I read somewhere that the Michelin XAS was best for the DS because the tread section is "rounded". The design of the suspension is such that the wheels move vertically relative to the car. As a result, they end up at an angle when cornering due to body roll. The rounded tread section does a better job is such conditions compared to flat ones.

    OK. But then, what about the SM? They have the same suspension system as the DS (inverted in the front) and were equipped with Michelin XWX tires, with a flat tread section...

    I'm asking because my DS is currently equipped with 180 x 15 XAS tires and I am considering replacing them with 205/70-15 tires on wider SM rims in the front. What are specialists' views on this?

    Thanks!

  2. #2
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    Quote Originally Posted by GGR View Post
    Hi All,

    I read somewhere that the Michelin XAS was best for the DS because the tread section is "rounded". The design of the suspension is such that the wheels move vertically relative to the car. As a result, they end up at an angle when cornering due to body roll. The rounded tread section does a better job is such conditions compared to flat ones.

    OK. But then, what about the SM? They have the same suspension system as the DS (inverted in the front) and were equipped with Michelin XWX tires, with a flat tread section...

    I'm asking because my DS is currently equipped with 180 x 15 XAS tires and I am considering replacing them with 205/70-15 tires on wider SM rims in the front. What are specialists' views on this?

    Thanks!
    First: I would avoid both the Xas (prone to aquaplaning & poorly compounded for the wet) & the XWX (better longitudinal & lateral drainage but also poorly compounded for the wet) unless you have fierce tribal loyalties to classic Michelins or an insistence on originality (implausible given your remarks about rims).

    So, something modern then but how profiled & what size & on original wheels or not. In turn:

    Profile.

    A complex story & I will but sketch it (more detail on request). Note first that the ID/DS design also used very square-edged Michelin X "Arrete" pattern tyres early on. This could work laterally because its very soft sidewalls allowed accommodation of distorting forces by sidewall distortion & migration of a still fairly flat tread towards the inside rim. The merit of the Xas was not better accommodation of positive camber but that, because of the split design of the steel belts (two half width belts at different cord angles), the belts generated a castor-like stabilising force that assisted in stopping any tendency to high speed wander. The rounder shoulder profile did indeed help a more stiffly sidewall-structured tyre from climbing on to its outer belt edge under (positive cambering) cornering & thus lifting the inner edge but my understanding is that edge profile was not Citroen's motivation & that belt structure was. I very much doubt that the current Michelin "Classic" range Xas has that split belt structure (perhaps some kind soul could cut a worn-out "new"one up - jig saws are good - & look; the easiest sign will be the little narrow cap ply just above the split in the middle). An extra merit of the split steel belt arrangement would be some tendency to "hinge" at that point & thus allow some further accomodation of camber. As far as I know, it was unique to the Xas & was even dropped for the XVS. (Some modern types have other ways of allowing some belt distortion to maintain footprint.)

    But many (most) modern types also have rounded shoulder profiles & have superior compounding to the modern "Classic" Xas & XWX. - See an earlier thread on a German test of classic car tyres including the horrible (Nankang) "Retro" faux-Xas & including the XWX.

    Size:

    But there is no modern type available in 185/80-15.
    205/70 preserves the original gearing but is also an obsolescent size & doesn't give access to good modern types.
    So, perhaps entertain some other size with the motivation being access to better tyre types whilst retaining a tolerance for the move to positive camber under lateral loads. My other recommendation with any classic is to prioritise wet grip over longevity & to prioritise benign limit behaviour (in case one has an emergency in the wet).

    In my judgement (derived mostly from published magazine road tests here &, via the excellent Tyre Reviews site in the UK, European tests), the tyre size which fits standard DS rims & has the best (by reference to the above criteria) range of available tyres is the very popular 195/65-15.

    There are two immediate apparent objections, one real but trifling & one which is merely apparent.
    The former is that the fitment will under-gear the car by a little under 6.5% (compared to 185/80). I don't consider this to be a worry on a long-legged DS & a bit more top gear flexibility is a trade-off.
    The latter is that, even with a bit of extra width, the lower profile 195/65 will be shorter in the sidewall & thus less able to allow the distortion & lateral tread migration spoken of above. This is so but I suggest that it won't prove a real issue. I have long used 185/60 tyres on swing-axled rear-engined Renaults. This suspension also moves to positive camber under lateral load & the sidewalls are shorter than 195/65. They accommodate the change well enough anyway. Certainly the available tyres are so much better than those available in the original sizes that the plusses outweigh the minuses.

    Within 195/65 lie some splendid tyre types. Four stand out as a short-list. Michelin's new Primacy 4 seems like a good thing &, on the basis of the one test I've seen of it, it seems to be new class leader (a welcome return to form by Michelin whose earlier efforts in the Primacy & EnergySaver ranges have been underwhelming). Possibly now deposed as class-leader but splendid is Continental's PremiumContact 5. Also excellent are Goodyear's EfficientGrip Performance & Dunlop's FM 800. Each of these is smoothly contoured at the shoulder & softly enough structured in the sidewall area away from the bead to accommodate some positive camber. Each is also well patterned & compounded for the wet & benign at the limit.

    I would personally get the PC5 as a very well known proposition but any of the four would suit nicely. Each also has the benefit of being tubeless (fine on your rims & do not fit tubes to tubeless tyres - generally a bad idea &, anyway, not an available option with 65 profile - rationale for this on request).

    Rims:

    So, you can do very well indeed without changing rims & I see no rationale for doing so with a 195/65 fitment.



    I hope that that is of some assistance to your deliberations.

    cheers! Peter
    Last edited by 4cvg; Yesterday at 08:41 PM.

  3. #3
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    I ran 205 70 15 on DS wheels for years, when there were several options in that size. Better traction and less squeal. The only downside is they’ll rub/polish the inner guards on full lock and make a loud noise, so winding in the end caps on the rack is a good idea.

    Now that there isn’t much choice re 205 70 15, maybe some 215 65 15 on the front would be worth a try as they’re only 3% different. Also 215 65 15 will fit an SM all round, which gives some much cheaper options than buying four ridiculously priced XWX.
    Cheers, Marc.

    1987 CX GTi T2 Maikonics
    1972 SM 2.7 carb
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    GGR
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    Thanks Peter and Marc. Here in the US we still find some H rated 205/70-15 tires so I may go with that size.

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    Quote Originally Posted by GGR View Post
    Thanks Peter and Marc. Here in the US we still find some H rated 205/70-15 tires so I may go with that size.
    My apologies; I simply did not notice where you were writing from. The US is an appalling place to find good mid-range tyres & high-mileage A-S tyres of dubious wet grip & soggy dynamic qualities dominate. A quick browse through the Tire Rack site suggests that available tire types might be better in 195/65 than in 205/65 which is better than in 205/70. I suggest a protracted browse through the Tire Rack tests for your short list.

    Perhaps, given the fouling issues, 205/65 is a better compromise than 205/70. Four seemingly OK tires: Michelin's Premier A-S, General's Altimax RT43, Yokohama's Avid Ascend GT & Continental's ProContact are available in 205/65 but not 205/70.

    cheers! Peter

    Edit: sorry, ProContact, not PureContact
    Last edited by 4cvg; Yesterday at 09:18 PM.

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