Replacing My Kumho's (some technical discoveries too)

[davis449]

Well I've finally had enough with the constant trips to Discount to get these things re-balanced. The last straw came this morning (actually Sat. when I took them to get rotated and balanced and they left out the balance part) when I went in to have them rebalanced. They just cannot seem to get these things balanced. Well, I got online with national customer service and talked to one of their reps, he said that the Kumho's should've never been reccommneded for this car anyway. He told me to call national customer service, so I did. He suggested I call the store and that if I had any problems getting them replaced, call him back. So I call my local shop and they said they'd pro-rate the value on them (which is going to be over 90% cause I've had them all of 6K miles) and get me a new set. I ended up choosing Pirelli P Zero Nero M+S in 225/50/16 size.

The interesting part came when I called my S.A. at the dealer and told him what I was going to do. He said to go back to stock size 205/55/16, but I plugged in the numbers into Discount's tire calc., and also into one from a Miata site that I got a link to from here by using the good old search button, and found some interesting results that has pushed me to keep on with the 225/50/16's. The difference between the stock size and the siz I am going with is SO miniscule. The only big difference is the tread width is significantly wider than the stock size tire. Everything else is minute. Check it out:

The differences are as such
Sidewall Height: .1
Section Width: .79
Overall Diameter: -.2
Circumference: -.7
Revs per mile: .68


The miata calc. found no difference at all. Now the dealer has said that it's the mass of the tire (too heavy) that's causeing the problem (215/50/16 vs stock 205.55.16) and that my suspension can't handle it, but with this little difference between the stock tire size and the 225/50/16's that I've ordered, I don't think I should have the same problem with these as I am currently having with the Kumho's. What do y'all think?

[c55m8o]

Originally posted by davis449
... The miata calc. found no difference at all. Now the dealer has said that it's the mass of the tire (too heavy) that's causeing the problem (215/50/16 vs stock 205.55.16) and that my suspension can't handle it .... Don't buy that BS!!!

[davis449]

I figured they were full of shit. Mass problems my ass, I think it's the damn tires just suck or have a manufacturer defect. So the bottom line is that I'm gong to a better tire. The guy at the online customer service for Discount Tire said that I should stick to Michelin, Pirelli, or Continental for European cars.

[davis449]

Well, I got my new ones put on, 225/50/16 Pirelli P-Zero Nero M+S, and now I have the same issue again. These things just won't stay balanced. I mean they were smooth as a baby's ass right after they were put on and after normal dirving for four days, they're screwed. Vibration like hell. I don't get it. Maybe you can't plus anything on stock rims and keep the tires in balance. I'm in quite the pickle now, cause if I take it to the dealer, they'll point immediately at the tires and won't even try to check the suspension (which they keep telling me is fine) and if I take it back to Discount, well, there's just the embarassement. I think I'm gonna go back to stock. I have an appt. at 2:30 this afternoon. God this sucks!:( :angry:

[c280nz]

are you running enough tire pressure?
take it to ANOTHER tire shop and ask them to balance them, and then see what happens, those tires are good quality tires and shouldnt come out of balance

[davis449]

Yeah I am running proper pressure, I've already been down that road a million times. Put up another thread that explains what has happened with this problem.

[JAY4]

I have been running crappy Kumho 711's on my '98 C280 sport for a little over a year now--stock 16" wheels. 225/50/16 up front and 245/45/16 in the rear. no unusual vibration shakes or anything from the plus sizeing even on the same width wheels with staggered tires. Just my 2 cents for free. BTW Porsche used this same tire size setup on early 90's 928's.

[davis449]

Yeah, I wish I had been able to get them to admit this BEFORE I put new tires on it, then I probably would've gone with something a little lower profile like my Kumho's were, but I am pretty happy with the setup I have now and will be happy when it just works right again. BTW, if you didn't check the other thread, it was control arm bushings causing the vibration problem.

[tpliquid]

maybe ur rims are bent,
shold try a diff set of wheels from some one


edit
225/50/16 = 24.858 inch
205/55/16 = 24.877 inch

shuoldnt matter at all. i say 225 is good to go

[davis449]

Well here's where we are right now. The tech happens to drive a 94 C 220 so he swapped his own rims and tires on to mine (195/65/15) and got the vibration to subside. I drove it so I felt it too. Put mine back on there with the Pirelli's and it still vibrates like hell, I even went +4 psi to 40 psi cold pressure and it still hasn't helped. So we're back at tires AGAIN (he said the rims were fine). I am going in on Fri., all Discounts in the area have Michelin Pilot Exalto A/S tires in stock in a 205/55/16 91H (exact stock recommendation for the C 280 Sport) so I'm going with those. If that doesn't solve it, it's going back in for some more tearing down. I'm inclined to beleive the tech on this one though cause that rim/tire switch seemed to work. He says the steering is tight and so is the rest of the suspension. Plus, the work up on it includes not only bushings, but two alignments and a road force balance. It's got to be those tires, there's just some reason that car doesn't like anything but stock. I'll keep you guys updated.

[davis449]

UPDATE: I went in this morning to Discount Tire and put Michelin Pilot Exalto A/S 205/55/16 91H tires on the car, immediately noticed a difference. There is no more vibration, no more shaking in the steering wheel, everything seems to be kosher (I hope I don't eat my words here). Things I've noticed from the three tire changes I've been through: the turn in is definitely quicker with stock sized tires (I knew I would sacrifice some of this on a wider tire), the cornering is much more (to my surprise) stable, lane change I think has suffered a little here, the ride seems smoother on the stock sized tires, and the acceleration grip seems to be about the same (although I'll need more time to really evaulate this). These are a $600+ set of tires so, dammit, it better be good! They look alright on the car, not better than the Pirelli's or even the Kumho's for that matter, but at this point, if it drives right, I'm ecstatic. Michelin's are good tires (overpriced IMHO), but the wear rating on them is the same as the Pirelli's so I expect to get good life out of them (better than the Yoko YK420's I burned through in 27K on my last car) and I think overall I will be happier with them; plus they'll be alright for the occasional high speed run. Thus, hopefully, this thread is concluded. Thank you all for your input and if you have any more input, please continue to share. Thanks for listening.

[davis449]

OMG! I ate my words so fast on this one I'm choking on them! Yep, that's right, it's going back to the dealer this afternoon about 4:30 to get test driven AGAIN by the tech. Same problem, but this time a little less severe. My hunch is that the plus sized tires on it aggrivated the problem and that's what made it REALLY noticeable before. I think's it's the steering rack. We'll see. I have now dumped over $500 in tires at this point, out of my own pocket, to no avail. This is really pissing me off.

[davis449]

I'm getting my suspect rim re-done today and I'm picking it up here in a little while. After driving on the spare for a day, I have to say that I firmly beleive this will solve the vibration problem. I still have some with the spare on, but I'm attributing that to the fact that the spare isn't balanced with the rest of the tires. I haven't had the usual symptoms that I get every day going to work. We'll see.