If you have modern BMW (or Mini Cooper, or Toyota Sienna, etc). you may have heard about Run Flat Tire (RFT). Basic idea is you can run when tire is flat (no air). Specifically, you can run about 50 mile at the speed of 50mph maximum (this differs by tire). That's the main advantage of RFT : when you have experienced puncture or damage to tire, you can still go to some (close) location.
Obviously vehicle manufacturers want that location to be their dealers, where you will spend money on these high priced tires. Yes, they are expensive, way more expensive than non-RFT. Let's look at the pros and cons about RFT in more detail:
- Self Supporting RFT (Run Flat Tire)
Sidewall of conventional tires is very thin, yet it is the main part that supports the weight of the car (typically 3000 lbs or more). It cannot support weight of the car by itself without air. That is why air pressure is important. In RFT, side wall is reinforced (thick) to support the weight of the vehicle for short period of the time, in the event of air loss due to tire damage or puncture. This comes with big cost of price of RFT as well as compromised ride quality, due to stiffened side wall.
- How expensive?
If you compare two identically sized tire prices (one RFT and one non-RFT) online...
http://www.tirerack.com/tires/TireSearchResults.jsp?width=225%2F&ratio=45&diameter=17&performance=ALL
Looking at the very common tire size, 225/45R17...
There are more than 160 tires to choose from (Non-RFT 130 & RFT 30).
Non-RFT ranges from $70 ~ $200 per tire, and RFT ranges from $140 ~ $240.
As seen, RFT is way more expensive and also more expensive to install (mount and dismount).
- Weight of the tire
RFT is typically heavier than non-RFT. Heavier tire means greater rolling resistance, which means more fuel to burn to turn that mass.
- Higher Rolling Resistance
Sidewall of the tire continuously changes its shape as it rotates, to ensure optimal traction against the road it touches. Stiffer sidewall requires greater force for it to shape, which also means higher rolling resistance.
Standard Tire
Run Flat Tire
- Is it flat for not?
If you look at the non-RFT, it's quite easy to tell whether air pressure is too low, as bottom will look flat (hence "flat tire"). On the other hand, it's quite difficult to judge RFT's air pressure only by looking, as even RFT without much air still looks more round than non-RFT with no air. This is why TPMS (Tire Pressure Monitoring Sensor) is almost always required when manufacturer equips the car with RFT. TPMS can be convenient, yet it adds more cost to entire car and job.
- Where is my spare??
Cars with RFT typically do not have spare tire. Instead, there's tire sealant, a bottle of chemical that you can inject through valve stem to temporarily seal the air leak. Even RFT with big leak can fail very quickly, so owner's manual states it is necessary to seal the tire with tire sealant and pump air to vehicle specification. Please note that this is temporary fix, just to get to the closest service station.
So what's good about RFT? You have more trunk space because there's no spare tire. You get that much weight reduction (but with added weight of RFT). Even when tire is completely flat, you can go somewhere. However, when you actually arrive at the tire shop or dealer, you will be likely surprised that RFT is not in stock, special order, and extremely expensive. Perhaps it's a good idea to buy non-RFT next time, save a lot of money and use that money for actual spare tire or AAA? It's up to owner's decision.
To be continued...