So my wife calls me on her way home from work (40 mile commute) the other night and tells me that the TPMS light came on. She asked me what she should. I asked her if the car handled "funny" to which she responsed no. So I told her to just drive real easy and I'll take a look at it when she came home. When I looked at all the tires I honestly couldn't tell which one was flat, they all "looked" good to me. The tire gauge quickly pointed to the left rear tire. It was at 18lbs. I had her take the other car to work the next day so I could see what was up with the tire. I pumped it up in the morning and drove it to work (8 miles). At lunch time the TPMS light was on and again the tire pressure was at 18 lbs. I drove it for another 6 miles to get air and could not feel any difference in the handling of the car. I was amazed! My "tire guy" plugged the hole today and it's all good again. Now I'm on the fence about replacing these run-flats with regular tires. Had this been a regular tire the story would have probably gone much differently.

 

I had a front RF blowout completely and made it home 40 miles away with no problems at normal speeds.

 

If you're under warranty, that comes into play here. If so, - call the magic number. And, change to non-runflats if you wish & call the dealer when trouble arises. There's value in the roadside assistance we pay.

If the flat had been on a front wheel, it may have proven more serious. She would've felt a difference, anyway. Troubled or scared to drive, even. Runflats may have brought her home safely, but no need to take a chance if still covered.

Right, right-- so there's energy efficiency to consider when spinning heavy runflats. but, do we care? We never re-coup it, really. And in exchange, we get peace of mind that a loved one isn't stuck somewhere unsafe.

 

I run non-runflats & carry "better world" coverage (out of warranty).

But I'm also a guy ready to use a can of fix-a-flat & I'm safer than a wonderful woman is, in dark roadside distress.

 

"If you're under warranty, that comes into play here. If so, - call the magic number. And, change to non-runflats if you wish & call the dealer when trouble arises. There's value in the roadside assistance we pay. "

Interesting. I wasn't aware that the MINI warranty covered modified parts (i.e., normal, non-runflat tires). Do you know this from experience?

 

I got the tire a wheel warranty when i leased my car. I called the company and confirmed that they cover at wheel/tire that is on the car. So they would cover my snow tires if i got a flat. Double check with your own compnay though. I know that mini has used different vendors for this insurance

 

Get AAA for a few bucks a month. Switch to regular sporties.

OMG! Wow! The ride is just as sharp and twice as smooth.

 

as impressive as all of these run flat stories are i'm really excited to get some regular tires after i run flat the run flats ...but i'll miss the boot space as i'll probably carry a snow wheel/tire when i'm anywhere far from home!

 

I have been told by tire guys and SA's at my dealership that run flats could not be plugged like normal tires and they would have to be replaced if they were ever punctured?! Anyone else hear this before?

 

That's a myth perpetuated by folks that want to extract more money from your pocket. The dealer doesn't want the liability either, tires aren't their specialty. Any good tire shop (and I think Discount Tires, etc. are reputable) will plug/patch a runflat if it meets the same criteria as a regular tire i.e. puncture in the tread not too close to the sidewall.

 

My understanding is that if a runflat is driven flat, at some point, the sidewall becomes damaged. This could lead to catastrophic tire failure later. So, some shops wont repair runflats because they can't determine the internal condition of the sidewall, and don't want the liability.

IMO, a runflat doesn't buy you enough to justify the cost and discomfort.

In the OP's case, with non-runflats, she could have limped slowly to a gas station at 18 lbs and inflated the tire to normal pressure. She just needed a tire gauge and a little knowledge. Freeway speeds would not be a good idea, because the tire could get very hot. However, runflats should not be driven over 50 mph, and it would have been a good idea to inflate the runflat to protect it from damage. Also, safety is compromised when any kind of tire is low, so limping back would be best with either kind of tire. So, IMO, not a lot of benefit to the runflat, in this stiuation.

If the OP had been carrying a ContiComfortKit or other similar device with a water-soluble sealer,* she could have added air to the tire and driven wherever she wanted. Or, with a quicker leak, added air and sealant. These devices consist of a 12v air pump that runs off the cigarette lighter socket. There is also a canister of Slime in the device so you can have the pump inject the sealant while pumping up the tire.

With a catastrophic tire failure, there may not be enough tire left on a runflat to protect the wheel, and you would be calling roadside assistance no matter what kind of tire you had.

With a simple air leak on a non-runflat one would have to stop and get out, check the tires with a gauge to find the low tire, connect the pump, and wait five or ten minutes for it to properly inflate the tire (whether or not the sealant is used).

In foul weather, having to get out could be a pain, and you might find runflats worthwhile. Runflat snow tires (if they exist) would be convenient. In sunny weather, the benefit would not be so great. You have to put up with a harsher-riding, much more expensive tire. Running it while flat may mean it needs to be replaced, or could be dangerous.

To me, there seem to be few scenarios where a runflat would really save you much. I run nice light-weight Rota Slipstream wheels and non-runflat tires. The wheels improve the ride, performance, and braking by having less rolling inertia than the heavy factory wheels. They also only cost $170 each. So, my wheel and tire together cost about the same as an OEM runflat. If I needed to limp out of a dangerous neighborhood on a flat tire, my financial risk isn't much greater than if I were running a runflat.

*Slime is a water-soluble sealant that can later be washed out of a tire. It claims to be safe with TPMS sensors in the wheel. What I've heard is that Fix-A-Flat cannot be cleaned out of the tire later, and most shops wont fix a tire with Fix-A-Flat in it.

 

I just had a hole so big in my right rear runflat that any air pumped into the tire whistled out as fast as it was pumped in. It happened 110 miles from home and I didn't experience any adverse handling for the ride home (before I knew it was flat) or the 30 mile drive to the dealer three days later. That tire was pretty hot after I was done, tho, it wouldn't have gone much farther than the distance I drove it. The tire was replaced, and I doubt there was any way to fix it even before the long distance drive. Given the heat it generated, I can see why some shops won't fix these.

As far as runflats costing you efficiency, I don't see that as a significant factor in every-day driving. Yes, it takes more energy to accelerate a heavy wheel than a light one, but once you're up to speed, that ceases to be a factor. Since you're spending a lot more time at a constant speed than accelerating during normal driving, the efficiency lost to heavy wheels is unlikely to be noticed in the mass of other variables.

Now, if you're racing the Mini, that's a whole other story. Then the unsprung weight becomes important for handling and you want to minimize drivetrain inertia for better acceleration. Since a race course requires constantly changing speeds, a lighter wheel will have a greater impact on the car's overall performance under these conditions.

 

I wonder if Slime would have plugged it? I also carry a Dynaplug kit. If you could find the leak, that probably would have fixed it.

Don't forget braking. To slow a heavier wheel it takes more friction in the brakes. That means more heat, more brake dust, and a shorter life to the pads and rotors. Yes, the difference in energy used might be subtle for freeway driving. It would have more effect in city driving with the frequent starts and stops, or mountain driving where brake fading can be an issue.

Where it might be most noticeable in everyday driving is in ride comfort. A heavy wheel that is rapidly forced upward by a bump will impart more energy to the suspension, and eventually, to the occupants. This isn't going to make a huge difference, but combined with the harsher ride of a runflat, the difference is noticeable. I was quite pleased with the improvement in ride when I switched from the factory 22.5 lb. Crown Spoke 17" wheels with Dunlop SP Sport 01 DSST 205/45-17 runflats to 16 lb. wheels with Michelin Pilot Exalto PE2 205/45-17 non-runflats.

The advantage to heavier wheels are that they can be less expensive to make because they can be cast. They can be stronger that lighter cast wheels. Good quality lightweight wheels can be made strong through forging, and good engineering.

 

Great points, Robin, but let me respectfully disagree with you on one of them.

I don't think a heavier wheel makes for a worse ride in the way you say it does. Yes, the greater inertia of a heavy wheel means more force at a given velocity, but a heavy wheel needs more energy to get it to the same velocity as a light wheel.

If we took both of these wheels over the same bump at the same speed, the amount of force applied to each wheel will be equal. However, the lighter wheel will respond with a greater change in direction of motion than the heavy one, based on Newton's 2nd Law, Acceleration=Force/mass. However, the light wheel will also respond faster to the opposing force of the weight of the car through the suspension, for the same reason, because it has less inertia. The heavier wheel will respond to both of these forces with smaller accelerations.

Ride smoothness is more a function of the car's overall inertia and how much that is effected by the forces fed into it by the wheels. Part of that is a function of how much of that movement is absorbed by the tires, which is why the stiff-walled run-flats ride rougher than regular tires; RF's have much less give than non-RF's, so more wheel motion is transferred to the car, instead of absorbed by the tire.

Inertia is also why light wheels have more traction. The time needed for them to complete the cycle of being pushed up by the bump and pushed back down onto the pavement is less than for heavier wheels, which means a light weight tire/wheel is in firm contact with the road for a greater amount of time, resulting in greater traction.

You might be interested in an book called RACE CAR ENGINEERING & MECHANICS by Paul Van Valkenburg. It's out of print, but you still should be able to find it in used bookstores. It's a treasure trove of information regarding car performance engineering.

 

I'm not so sure you can assume that the two wheels will receive the same force. The wheel has to get over the bump. The bump is not going to absorb much of the force.

The heavier wheel may force the tire to deform more, but I would think that more compression in the tire would impart more force to the wheel.

Some of the froce from the bump may be converted to heat in the tire (not a good thing). I would think that a more compressed tire would impart more force to the wheel.

Another part is how much force can be absorbed by the suspension. If, as I suspect, the heavier wheel will receive more force from the tire, it will impart more force into the suspension.

If the light wheel receives the same force from the tire, wouldn't that would mean the tire has to absorb more force? If so, the tire wall becomes more deformed, and more heat is generated in the tire.

 

>>I'm not so sure you can assume that the two wheels will receive the same force. The wheel has to get over the bump. The bump is not going to absorb much of the force.<<

Actually, I'm assuming the bump does no energy absorption at all. I think I see what you're saying, however. Given a bump of fixed size, the magnatude of the wheel's movement will be the same, but the time needed to move that distance depends on the speed of the car. Faster the speed, the less time the wheel has to cover the same distance, which requires greater vertical velocity, which requires more acceleration, which equals more energy.

A heavier wheel, due to it's greater inertia, will require more energy to move a fixed distance in a fixed time than a lighter wheel, ergo, more force is generated by the interaction between the bump and wheel. That means you're right, the force transmitted to the car by a heavier wheel is greater than a light one, which would infer that lighter wheels ride better than heavy ones.

My error was to pay too much attention to the magnatude, and ignore the acceleration and inertia aspects of the problem.

Great discussion, Robin, thank you.

 

I had sensor light come on yesterday and did not notice difference in ride.
Checked and air pressure was down to 28PSI on left rear tire.
Pushed car and their was a nail..YUK
The other day in wifes SX4 I was nailed with a stone and cracked windshield,
the second in 4 months.YUK again.

Just put truck tire plug into Mini tire and will watch and see..
These plugs are really gummy and sticky and hope it holds.

I would say most of my riding is local riding and if I were doing Interstate driving
I might consider putting new tire on to replace nailed tire..

One thing for sure if it goes low in pressure I have the sensors to tell me.
Also I would not put anything in these tires to seal them in any way for I am
sure this could do damage to sensors inside that read and send information
for low pressure readings...
Joe