Non-runflat shennanigans ....

Q 2. Can you fit normal tyres to a car set up for Run Flat tyres?

No, you should not put non-Run Flat tyres on a car set up for Run Flat tyres! There are two main reasons for this.

Because Run Flat tyres are very hard due to the reinforced sidewall, the suspension on the car is set much softer so the ride is normal for occupants. If a normal tyre is fitted then the ride will feel very soft, the handling will be adversely affected and could present serious safety concerns.
As the car will not have a spare or anyway to deal with a flat tyre, if non-Run Flat tyres are fitted, the car will be stranded if a puncture does occur.

*Copied from site*

:tumbleweed:
 
It would seem Q2 is what BMW forgot to do with the suspension :-) :rofl: I wonder if the newer BMW's have suspension compensated for in this respect?
 
It's all arse.

Overall ride rate:

1/tyre rate + 1/wheel rate = 1/ride rate

Realistic figures for an RFT are in the region of 300kN/m

For a non-RFT think in the region of 200kN/m

Wheel rates of about 25kN/m are about right for a Z4


Thus

RFT
1/300 + 1/25 = 1/ride rate = 23kN/m

Non-RFT
1/200 + 1/25 = 1/ride rate = 22.2kN/m

A whopping 4% loss in ride rate.

To compensate for this, you would add 4% to the spring rate to get back to the 'ride rate' after going to non-rft. So BMW must have done some VERY fine tuning softening the springs for RFT use so the ride for occupants was nice haha!

Consider also that adding a tank of fuel or a passenger, or just a fat or thin driver, would impact the ride rate more dramatically.


Damping is another issue, but this is why RFT's are so horrible. You can't use the damping of the suspension to compensate for the stiff tyre. Non-RFT's classically have very good damping vs spring properties. RE050 RFT are so rigid that they essentially don't damp forces going through them.

I didn't work with BMW, nor did I work with Bridgestone, so the above is a critical analysis purely on the points Halfrauds used.


Lastly, RFT wheels are a concession FOR RFT tyre fitments, not an exclusion on non-RFT. An RFT wheel is simply one that allows RFT to be used as well as non-RFT.


Tyre places need a kick up the arse on this one. There is so much mis-information it is sickening. I personally think RFT's are so detrimental to the ride/handling on UK roads that you are safer without them full stop! Horrible horrible things.

M owners don't seem to have problems either :D

Dave
 
My tyre fitter (acting on behalf of Black Circles iirc) asked me to sign the paperwork acknowledging that I had agreed to non runflats being fitted when runflats were the norm. Covering their arses I guess but I was happy to do so.....
 
WLH said:
Even the BMW dealers on this side of the pond here will swap out run-flats for non-run flats. I notice over the year that this seems to be a problem in the UK and I wonder why. It is they want to sell you the more expensive run flat or is it a legal concern that if you have an accident and they would be held liable for installing the wrong tire?
Click to expand...

Its the nearest thing to the G himself in UK: HEALTH & SAFETY EXECUTIVE! :rofl:

no such thing over here though :o
 
Mr Whippy said:
It's all arse.

M owners don't seem to have problems either :D

Dave
Click to expand...


NO, we just get a bad back as standard :rofl:

Tried a mate's 2.2 convertible. Verdict- more comfortable seats (as in squishier) and softer ride.
 
i think the tyre fitters think of the wrong reason as to why you shouldnt fit a non RFT to a RFT car, they think the non RFT on a RFT rim in some way will slip off mid corner, the real reason is in my mind if you had a blowout where the tyre deforms fast and causes you to loose control this would cause an accident. The accident investigation (if a large accident involving multiple cars) would look at every detail and may see you had non RFT tyres fitted, in that case you could say the accident may have been avoided by fitting the recommended tyre to the car. On the flip side though you could have that dim person who doesn't care or know what the red symbol is lit up on the dash and drives round with it to the shops all week thinking everything is fine then gets on the motorway gets up to speed the tyre then has lost its RFT characteristics and that causes an accident..

Overall the bottom line is you are safer with them ...... its all chance i guess..... :driving:
 
I'd say they are more dangerous from my experiences :D

My car came with front right tyre at 9psi, rear right about 12psi, front left about 14psi and rear left about 10psi... all really quite flat. TPMS must have been reset by some idiot, or god knows what else, so it didn't trigger.

I bought this car from a trade seller (useless idiots), and had been going 80-90mph at times thinking the tyres were fine till the TPMS (or equivalent in UK cars) went off. Oh dear I thought, slow down. Check the tyres. Eeek, puncture, 9psi on front right noooo! Best check the others. Oh, no, not a puncture at all, just ALL my tyres were flat.
Dodgy dodgy dealer!

Imo, RFT mask problems with tyres, and it's only a rare case where they can be saved. It's just a very expensive convenience to be able to "run on flat" imo. I'd rather stop, foam the tyre, and then be able to save it later at a tyre fitter/repairers.
Or, if the place to stop is so dangerous, amble along on the tyre/rim with hazards on till you can stop safely, then call the AA and get recovered. Pay for a new rim/tyre if needed (long-term this failure likelihood is probably cheaper to run on rim and replace rim, than run on flat ALL the time at higher cost of doing so :) )


My 2p anyway :D

Dave
 
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