DarkStar66

I've been having a series of handling problems recently which have mostly been fixed by some geometry and tyre pressure adjustment.

But now when I'm driving down certain stretches of straight dual carriageway my car pulls to the left or right where there's no obvious rut or camber. Also, around some roundabouts it feels like the back is going out slightly like a RWD car. My near side rear tyre is different (P6000) to the others (Potenza) but I can't see that causing this problem.

So I was wondering if my rear or centre diff is sticking when it shouldn't? There's no strange noises though.

Jolly Green Monster

iTrader: (4)

I would point the finger at the tyre

try swapping the tyres front to back and see if it does the same or now understeers.

DarkStar66

I was going to get a new pair to replace the odd one at the back and put the front ones at the back but they are still in good shape so I wanted to check out if there's another reason before spending a lot of money. But I'll keep it in mind, thanks.

Don't forget, oversteering isn't the only problem.

Jolly Green Monster

iTrader: (4)

I wasn't talking about spending any money..

jack up one side and swap wheels, fornt to back and back to front and see how it goes then..

They are very sensitive to tyre pressures but seem usually okay around the 30psi mark.. mine has felt very light on the steering all week and like I was on ice all the time, took 2psi out of each front tyre this mroning and it is back to gripping the road like on rails.. just 2psi difference!

DarkStar66

Yeh, I knew what you meant but was a little nervous of putting the mismatched tyres on the front. Also not sure if the standard jack can be used to lift both wheels on one side, I'd probably need a stand (don't have one) at one end whilst jacking up the other eh?

As for tyre pressures, the place that did my geometry also checked the tyre pressures with a very accurate gauge. Apparently they were quite far out even though I'd checked them myself at a petrol station. The standard settings are a bit more than 30psi, maybe I'll reduce them a little if I can find an accurate gauge.

Jolly Green Monster

iTrader: (4)

if you only have the standard jack then get the space saver out, jack the front, put the space saver on, jack down, jack up the rear and take of wheel and fit the one from the front and then jack the front again.. pain in the bum but cheaper than a trolley jack.

I bought a little silver digital gauge from halfords which seems pretty accurate, garage pump gauges are not very accurate.
Or buy a foot pump, the double barrel one with a gauge is fairly accurate as I have one.. try altering them etc..

DarkStar66

OK, thanks for the advice. It looks like Halfords will be making some money out of me this weekend!

DarkStar66

Well I bought a foot pump. I thought I'd go for the Michelin one instead of the Halfords own but regretted it because of the flimsy construction, it does the job though.

I attached it to the first tyre, it read about 24psi!!! I thought the gauge wasn't working properly so I tried another gauge and it confirmed the first reading. Now I remember the guy who checked my geometry and tyre pressures saying that my pressures were way too high. He had a brand new pressure gauge which I suspect he didn't know how to operate!

Once my tyres were up to 30psi I felt the improvement. Although it still exhibits a strange instability in a straight line over certain bits of road, especially in the wet. I think its down to the miss-matched tyre at the back.

I'll give it a week to see if things settle down. If not I think I'll put the matching front tyres at the back and some new Eagle F1s at the front. I've decided on F1s because they came top in a Evo test a couple of years ago and I've always heard good things about them. Any opinions to the contrary?

DarkStar66

This is for the benefit of Jolly Green Monster in case he was wondering about the outcome.

I had the mismatched rear tyres replaced with Eagle F1s. The back feels nice and secure but the front seems to follow ruts and dips more than I remember it used to.

Then last night I was leaving work. This involves a slight dip onto the road as I turn left. There was fairly loud 'ping' noise from the front suspension so I thought 'Oh b*gger! What was that!'. Next thing I know the car wasn't tramlining nearly as much, it was back to it's good old self!

All can think it was that one of the springs had got dis-located when the car had been lifted up at some point and had just popped into place.

Engineer@Uni

DarkStar66, tyre gauges are notoriously unreliable. I remember getting a 6 psi difference between the gauge my tyre-specialist and my local petrol station has. Ultimately, what you need to do is always use the same gauge. Best way to go about that is buy your own hand-held electronic gauge (as you have) and when you go to inflate them, always overdo it. Then when you're somewhere private you can bleed them out till you're happy with what your gauge reads. Always compare pressures from the same measuring equipment.
Also take into account that the gauge you're using might be out too. (although obviously less than your garages) so on a saturday morning, inflate them a tiny bit more and have a spin. Repeat until you fell that they're deffinitely too much, and then take 1-2 psi out. That will probably be closer to the recomended 30 psi than any gauge says!
Lastly, different brand/wear tyres are very bad karma on 4wd cars. You'll find that the contact patch is different for each tyre, making one wheel move more than the other. This should make accelerating and breaking non-linear (you'll be moving off at a tangent, sort of like power-oversteer) and will also potentially over-heat your rear LSD oil if it's very pronounced. Get those tyres sorted! It's not as crucial to have the same brand/ware tyre front back as it is side to side.

DarkStar66

Well that spring pinging back into place theory was incorrect. I think I just happened to avoid any dodgy patches of road on the way home because it was still tramlining after that.

This week I finally decided to get new Eagle F1s for the front as well as the ones I got for the back earlier. Oh, and as an after thought I asked Kwik Fit to check the tracking at the front. They did this and had to adjust the near side wheel. For about 250 miles now I've not had any tramlining or feeling of the back end going.

I'm not sure if it's all down to the adjusted tracking or if the new tyres made some difference, I suspect the former. Which means that not only did PMW of Chelmsford get my tyre pressures wrong but also the alignment!!!

Engineer@Uni

thought so. It was highly unlikely it was the diffs anyhow.