Like many bike commuters, I have a tendency to obsess over flat tires. Like many bicyclists, I'm also a nerd. As a nerd who obsesses over flat tires, one of the things that intrigues me is the problem of understanding flat tire rates, particularly as it applies to comparing various tires.

It's well known among bike commuters that flat tires are essentially random events. You'll go eight months without getting a flat tire, then you'll get three in two weeks. It's just totally random, right? Well, I'm not giving up that easily.

One of the main problems with flats being random events is that it calls into question the possibility of comparing two different models of tires without using both for a long, long time. Nevertheless, as humans we all form opinions based on small sample sizes and can't be convinced otherwise. If I try tire A and get a bunch of flats then switch to tire B and don't get a bunch of flats you won't be able to convince me that tire A wasn't significantly more flat prone than tire B.

But is that really true?

That's one of the questions to which I wanted the answer. So, being a pseudo-scientific type, I set out to collect data. For the last three years I've been compulsively recording all information that seemed relevant about my flat tires -- the date, where I was riding, what the weather was like, how many miles were on the tire, front or rear, cause of the flat, etc. Now with three years worth of data, I'm starting some analysis.



So, I've got two tires, which I will call tire A and tire B. I used tire A for about 1900 miles and got 6 flats. I used tire B for 2000 miles and didn't get a single flat. Obviously tire B is more flat resistant, right? But how to quantify that?

What I decided is that I'd imagine a simplified probability model. I'd choose a somewhat arbitrary probability that I'd get a flat in any 10 miles of riding and then apply that probability to these two tires to see how well it would explain the data.

Let me say that I am aware of the crudity of this model. For one thing, the probability of getting a flat isn't actually consistent over time but seems to increase with tire wear. It also varies with weather and riding location. I'm ignoring these factors.

So, returning to my model, I made the guess that for any 10 miles of riding there was a 3% chance that I'd get a flat tire. Applying that (by means of the binomial formula), I find that in any given set of 200 10-mile trips, there is about a 60% chance that I'd get 6 or fewer flats, so that seems like a reasonable fit for tire A. However, with that probability, there is only a 0.2% chance that I would get zero flats in 200 10-mile trips. If both tires actually had this same probability of getting a flat, there would be about a 1 in 1000 chance that I would get more than 5 flats with tire A while getting no flats with tire B.

Conversly, in order to get as much as a 1 in 4 chance that I could have used tire B for 2000 miles without getting a flat, I have to assign a probability of 0.7% for a flat in any given 10 mile trip. Applying that value to tire A, there would be a 99.7% chance that I'd get fewer than 6 flats. This yields less than a 1 in 1000 chance that I would get more than 5 flats with tire A while getting no flats with tire B.

So, my conclusion is that given two tires both used for 2000 miles in similar conditions if one tires gets 6 flats while the other gets 0 flats then I can, in fact, trust my belief that the tire that got no flats has better flat protection.

The next thing I'd like to know is how many flat tires you need to get before you can conclusively say that a tire is not as flat-resistant as another tire that got no flats.

Yes, I have too much time on my hands.

Yup. You do Einstein Go for a ride.

6 flats in 1900 miles is too much for me.

You obviously don't realize that the geeks will inherit the earth.

Yeah, me too. But let me attempt to pre-empt the inevitable Marathon Plus recommendations by saying that I'd rather have 6 flats in 1900 miles than ride 1900 miles on hard, heavy tires. Hence the urgency of this research. BTW, tire B was not an SMP.

"Flats are random?"
Yes, unless someone meticulously lays out thumbtacks in a meticulous grid pattern..

Yeah, yeah, yeah, I'm surrounded by geeks all day long.

Yeah, go ahead and give Andy more reasons to do his cyclescience

My flats are... um, not happening much at all. How about that?

Nah, I'm the opposite because the flats have a tendency to happen in the worst possible conditions, like early on a cold, rainy morning or that one day you're late for work. You could probably figure out, I suppose, with enough math and statistics, when and where the flats will happen and avoid them though

Exactly! So far I've figured out that they mostly happen between April and June on tires with more than 2000 miles on them. Tire B above just hit 2000 miles, so I'm leaving it parked until July while I ride on something else.

Way too many flats. I average one a year, which is about one every 3500 miles. Half of those are pinch flats because I don't always check my tire pressure as often as I should, and sometimes the county throws down very rough gravel on the roads including some nearly fist-sized rocks, and I might hit one of those at night. I really can't remember the last time I had an actual puncture. I think it was about 3 years ago, a sliver of wire IIRC. It was a slow enough leak that I made it the last 2 miles home after noticing it.

I've concluded that the black tires get more flats than blue tires and I didn't have to work nearly has hard at it as you.

This is of course no help if your bike is red because the blue tires would clash. Imagine the horror if you did get a flat in a blue tire that was mounted on a red bike. I bet no one would stop to help you.

I believe you would want to ride several individual Tire As and several individual Tire Bs for 2k miles each to find a distribution of flats/2k miles for each model and at the end of that you would know how typical your 6/2k and 0/2k results are likely to be.

For me flats are not random, generally when a tire starts throwing too many flats in a short span I chuck it regardless of apparent wear. That has only happened to me past the 1k mark, if it was the 1st week then I might evaluate differently...

Clashing tire/frame is probably the equivalent of not wearing clean underwear and then winding up in the emergency room for some reason.

The trouble here is that riding location is a huge factor in flats. I'm convinced that how many flats you get on your route has essentially no numerical correlation to how many flats I would get with the same tires on my route. Only by comparing two different tires on the same route do I trust the data.

It's true. I could have just gotten a particularly good or bad sample of either set of tires, but I think it's a decent start given the extent of the disparity.


This is absolutely true. Four of the six flats for tire A were in the last 600 miles of use. But how do you decide how many flats grouped together are significant? One flat is obviously not meaningful. Two in a week could still be coincidence....

You'd have come back a broken man from my last trip to the Picketwire dinosaur trackway...63 punctures in 16 miles. I had to carry the bike the last (and worst) quarter of a mile. That's what I get for crowing about not getting a single flat on the previous trip and teasing the woman who got 20 of them.

Holy crap! Goatheads, I assume?

Thanks for the detailed study. I see a bit of a hole that needs filling, though... you did not correct for weather, riding conditions, etc. I don't think your results are truly valid unless you had two identical bikes and riders riding through exactly the same debris on exactly the same road every day.

Although I agree with your conclusion. I am pretty lucky when it comes to flat prevention. I have been commuting to work on and off for the past six years on bad pavement and gravel, 14km each way, and never had a flat during a commute. I may have had two or three flats I discovered hours or days after I parked my bike, but I have actually never had to fix a flat on the rear of my IGH equipped bike (I would remember that).

Acres and acres of them. 2011 was a bumper crop.

Make an iPhone app!

I think your sample size is too small to predict with any accuracy. You need to test about 1000 tires before you can get any assurance that your results have any validity. As I recall, a sample size of 1,000 would mean your results would be accurate to within about 3% 19 times out of 20. We need a better way of doing flat prediction. We know that if you ride on any tire long enough, there is a 100% probability that you will puncture it. I would propose n km before puncturing would be a more meaningful starting point. The idea would be that you'd have a 95% probability that the tire would go n km before going flat. That last 5% accounts for completely random behavior. But the closer you get to km n, themore likely you are to puncture! So your Tire A would have a rating of 333 miles (533 km) before puncturing. Tire B would be 2000 miles (3200 km) before puncturing. Sort of a mean time before failure rating.

Luis

MOst of my flats have been random. I've never had a planned flat.

Flats aren't random. They always happen at the most inopportune time. Such as 5F outside in the middle of a snow storm. That was my last flat, and it was this winter.

Wouldn't you also have to randomize your ride times to determine if flats are random?

This is why I've always liked physics better than engineering. I prefer to make sweeping assumptions that simplify the calculations and then sort out the details afterward.

As it happens, tire A got all of it's flats in dry conditions while most of the use of tire B has been on wet roads, so if I include that level of analysis things get even worse for tire A. (It's intended as a racing tire, so all of this is to be expected.)

For the curious, in the three years that I've been collecting this data I've gotten 16 flats in about 12000 miles. Of those, 12 occured between April 1 and June 30. Of the 10 flats in 10000 miles that were not on tire A in the example above, 7 happened in the rain. Four of the 10 non-tire-A flats occurred on tires with more than 2000 miles on them. I believe that every one of the 16 occurred in a designated bike lane (though to be fair 75% of my commute is on designated bike lanes). Only three of the 16 were on the front tire.

I could really complicate things by introducing tire C, which got five flats in 2400 miles but 4 of which occurred at over 1880 miles.

I'm pretty sure over 35 years of riding that I have proven that new tires are much less likely to get a flat than tires with a lot of miles on them. Of course, I don't really test that theory much any more, because if I get a flat the tire is gone soon thereafter. Life is too short to nurse an older tire along to get a few more miles on it.