Funny pressures again

[cdavis]

Greetings, All. First-post newbie here, so please be gentle.

On Friday I picked up a magnificent 500SE - a near-clone of our existing 420SE - for an embarrassingly low price, and I'm still in a haze of delight. I'm hoping the two will be a breeding pair and we'll soon have lots of little MB sports cars around. But enough of that - here's the issue:

Everything else on this fabulous ship is immaculate, but strange things are going on at road-level: instead of the spec 205/65R15 - 94V tyres on 15" rims, this is fitted with 17" rims and Pirelli 235/45ZR17 - 94Ys all round. Nice enough tyres, to be sure, but the pressure is a mystery.

I note from other postings in this thread that the official position is that the filler cap values are supreme - obey them and all will be well. This just doesn't seem to be the case here.

When I first took the car onto the highway all was well until I exceeded 60mph, at which point a mighty shudder (at wheel frequency, I believe) started. I was so alarmed I pulled over fearing a wheel was about to come off. It wasn't - the tyres were just hugely underinflated at about 20psi.

So I pumped them up to the spec F30 R33, as per the filler cap and manual, and the problem improved dramatically, but didn't entirely disappear.

Looking at the car at this pressure, the tyres all seem pretty squished - very compressed and low at the contact point. So I pushed the pressure up to F33 R36. Again the residual shudder decreased.

Seems to me that the road contact 'patch' on these tyres is very different from the standard: wider and shorter that the stock ones. I'm assuming that the theory is to have sufficient tyre pressure on each patch to balance the relevant portion of the car's weight.

Am I barking up my fundament here, and/or is there anyone who can comment on this? The filler-cap pressures just don't look or feel right, though of course the remaining judder could be down to wear, tracking and balance problems. I'm a bit lost.

Sorry for the long post: I'm garrulous and excited here. Bad combination!

CD

 

[Brian WH]

Just get the fronts balanced and ask them to ensure that the tyre is "sitting" on the wheel correctly. As they have been under-inflated they might have moved. I am sure that you must use the tyre pressures as stated, any overinflation will affect the wear rates.

 

[kwakdonut]

Welcome!
I'm in agreement with Brian WH - get them balanced and checked over at a reputable tyre place. Maybe worth checking that this wheel/tyre combination is ok for the car too??

 

[cdavis]

Grateful thanks to you both. I shall indeed have this whole balance thing checked over in detail - though not by the local Merc dealer, alas, which has shown itself to be a combination of incompetence and greed. Happily we've found an MB genius nearby.

I've also talked to a super-engineer friend, who suggests that the pressures might benefit from a little calculation if the tyres are sufficiently different from the spec. We'll discuss this when he gets back from zooming around the world.

Thanks again for the help.

CD

 

[BTB 500]

If the car has been standing it's possible that one or more tyres has developed a flat spot. Over inflating would make this less noticeable. Or it could be a mechanical problem with a tyre (delamination etc.), causing it to go out of round.

So I agree that a thorough checkover by a good tyre shop would be the first priority. Good luck!

 

[DITTRICH]

There are two points here, I think.
Firstly, the pressure reading on the bit of kit you were using to pump the tires up. If it was a garage forecourt 'dalek' then I'm not sure I would trust the readouts on these things. Secondly, I did have flatspots on some tires after parking up one of my cars for 6 months but strangely enough over time they disappeared. The vibration wasn't very much - just a little between 50-55mph - but it eventually disappeared. These were continentals on 17" amg staggered alloys fitted to a w202 c230k.
Les