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Often I find myself driving with my racing bike on the dedicated bicycle lane in my city. The problem with these bicycle lanes is that there is a lot of broken glass (once every few kilometers). Normally I spot it quite late, drive through the glass, and hope for the best in not getting a puncture.

My question is as follows: How can I minimize the chance of getting a puncture while driving through glass?

I am not really looking for material advice, but rather for tips on how to drive through the glass (i.e. does it help to redistribute weight, speed up, or slow down), and does it make sense to check your tires after driving through the glass (as this answer suggests: What is a good way to keep my tires from going flat easily?). This far, I've not gotten a puncture there, but I would like to keep it that way!

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keeping your tyres at a high pressure will help. I'm not sure how practical this is, but if you could bunny-hop, would that help? It depends on the size of the "patch" of glass I guess. –  PeteH Jun 7 at 15:39
    
@PeteH Normally, it is just thrown down beer bottles and things of that size. Typically spans a meter or so. –  Bernhard Jun 7 at 15:49
    
it can be luck of the draw. I was riding in a city once and had to ride through a broken bottle. It was by the side of the road, not in a bike lane, but because of other traffic I had to hold my line and ride through it. Fortunately I too came away unscathed (possibly due to nicely-inflated tyres, more likely just good fortune), but it was when I reflected on it that I thought I might have been able to bunny-hop over the bottle. –  PeteH Jun 7 at 15:56
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It is interesting what you ask about weight distribution though, since a lower pressure pushing the bike onto the road would presumably make the glass less likely to penetrate the tyre. I wonder if anybody has ever quantified how moving around on the bike affects the force on each wheel? Not qualitatively, but actual numbers. +1 for that reason. And welcome to Bicycles SE! –  PeteH Jun 7 at 16:07
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Try to ride closer to the road side edge of the path if you feel safe doing so. There will be less glass there as the cars will pick some up with their own tires, and you can maneuver quicker towards the other side to avoid the glass. It's much safer to swerve away from the cars than towards them. –  Kibbee Jun 7 at 16:09

8 Answers 8

up vote 15 down vote accepted

There are three approaches you can use in combination.

Preparation

Tough tires - people have their favorite puncture-proof tires. Kevlar reinforced tires work pretty well.

Hard tires - pump your tires to close to the maximum pressure written on the side wall. This definitely reduces punctures, and can be combined with puncture-proof tires.

At the time

You limit your question "reducing punctures while [riding] though glass". Obviously, if you could avoid the trash you would.

You cannot always go around the trash. If you are able, you could bunny hop over it. This is also not always possible when commuting, due to lack of time, heavy traffic (including other bikes) limiting your options, or carrying a pack or panniers.

Slowing down is often also not an option when commuting because of following traffic.

After the fact

Brush your tires. It should be acknowledged that there are some who are squeamish about this. But it is very effective at removing foreign objects from your tires. In 35 years since I learned to do it, I've never (seriously) hurt my hand. Of course you should wear gloves. Most gloves have a reinforced patch across the palm to the base of the thumb for this purpose. (Many people mistake this for extra padding :-)

Don't wait: as soon as you hit the trash start brushing your tires. The more revolutions your tires do, the more the trash gets hammered into them. So brush them strait away. It doesn't help to stop to do it; firstly, it interrupts your ride, but mainly because by the time you stop the damage is done.

For the front tire, place your palm (near the base of your thumb) gently on the top of the rotating tire, in front of the brakes. The tire surface must be moving away from anything you could catch your hand in. You only need to keep it there for a few seconds, enough for one or maybe two revolutions of the wheel. Sometimes you feel a slight bump when you have successfully removed something from the tire. Usually, you'll feel nothing.

The back tire is a bit harder to brush, because you have to reach down to an awkward place and if you have to look it means you're not looking where you're going. So you'll want to practice this skill before you use it in traffic. Again, brush your tire in front of the brakes, in front of the back stay, where the tire is moving away from the brakes. Because of the low clearance between the back tire and the down tube on most race bikes, you need to be careful not to let you hand get pulled down into the gap. That hurts. Also avoid poking your finger into the spokes. That hurts too.

After reading this you can see why some people avoid doing it. Just be careful while you are learning, and practice before doing it in traffic. It's a valuable skill to have.

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So you would really advice against stopping and brushing the tires from there? Would be a lot safer, and the stopping distance would not exceed the distance that it takes to brush the on the move, I suppose? –  Bernhard Jun 8 at 7:10
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In my experience stopping doesn't help at all. By the time I've stopped the damage is done. I've never seen anything to brush off, nor felt it. But on the move I do notice success. In the group I ride in, there are those who brush and those who don't. That divide matches the number of punctures we get: close to zero for the brushing group, and one every week or two for the non-brushing group. YMMV –  andy256 Jun 8 at 7:54
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Ok, then it would be good for me to practice the brushing a bit. I am not sure if bunny hopping will work though :) You can easily end up landing on some splinter a bit further away, which will then be more forceful –  Bernhard Jun 8 at 9:14
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While you are stopped you could brush the glass off the path to save other riders. –  JamesRyan Jun 9 at 13:30
    
@andy256 Tried to brush the front wheel this morning, worked pretty well. For the rear wheel I couldn't find the guts yet, but I am halfway there :) –  Bernhard Jun 9 at 14:16

As for during
"How can I minimize the chance of getting a puncture while driving through glass?"
Coast and distribute your weight evenly.
Don't brake - it will grind the glass in.
If you think you can clear it then hop it - be sure you can clear it with both tires.
And a hop - not a bunny hop - a bunny hop is for height not distance.

There is lot you can do before and after.

Nature of a glass flat

To avoid a glass flat it is important to understand the nature of a glass flat.

Rarely get a glass flat in one pass.

Glass is very different than a nail, tack, or screw. Glass is brittle.

Two type of glass:

  1. Big chunks
  2. Little shards that you can't easily see

Big chunks:
Not often you will get sliced open by a big chunk as the sharp edge has to be up and since glass is brittle you often break it. But like the base of coke bottle with some 1/4 points sticking up. Tires loses pretty much every time. At least those are easier see - steer clear.

Smaller shard:

A long narrow sliver of glass does not have the structural integrity to puncture a tire in one pass - glass will fracture. A sliver of glass is rarely standing upright.

The are 4 stages to a glass shard flat:

  1. The glass penetrates the rubber surface.
    There is minimum or threshold force per area for the glass (or any sharp object) to penetrate the surface.
  2. Work into the rubber. This takes many revolutions.
  3. Work through the belt.
    First the glass has to make it to the belt. If the tire is thicker than the belt then the glass will not make it to the belt.
    Kevlar and other fibers resists cuts.
    But these fibers cannot stop abrasion. If you leave the glass on the belt long enough it will work through.
  4. Work though the tube.
    Tube is designed to hold air. Tube does not resist puncture, cut, or abrasion.

I have pulled glass out tires at least 20 times. Only twice out of a flat tire. From inspection that glass had been in the flat tires a while. A glass flat rarely happens in two seconds or two minutes. A glass flat can take days.

As for before - Lots you can do

  1. Puncture resistant tire
  2. Fatter lower pressure tire
  3. Slime

Puncture resistant tires

  1. Harder rubber to resist penetration.
    I personally don't like harder rubber.
    It is a harsher ride and not as much grip.
  2. Thicker rubber.
    Some puncture resistant tires tires even have a spacing foam.
  3. Puncture / cut resistant belt.
    This is where you get the best bang per weight

Before you go out and buy THE most puncture resistant tire consider it comes at a cost.
They are hard and heavy.

Fatter lower pressure tire

I know people say higher pressure is better for puncture resistance but I disagree.

Let me be clear.
I am not suggesting you run A tire at a lower pressure.
Run a tire that has a lower design pressure.
A fatter tire has a lower design pressure.
If you weigh 180 lbs or more then run your 25mm at the max (e.g. 110 psi).
I am suggesting if you run 35mm that has a maximum pressure of 80 psi then it will have better puncture resistance.
For rolling resistance, grip, pinch, and other factors run the tire at the design pressure.

There is a threshold force for the glass to enter the tire.
A larger tire has less PSI and a bigger footprint.
For the same weight 50 PSI will have twice the footprint size as 100 psi.
Larger tire makes contact with more pavement around glass such.
So a larger tire avoids penetration from a range of glass that would have penetrated the smaller harder tire.

Even if the large tire makes contact with more glass it is more points of contact.
The force is spread out over more points of contact.
A person can lie on a bed of nails but not stand on it.
If you had to walk through a bed of glass would you spread your foot to spread the load or would you walk on your heels to minimize the pieces of glass?

If a fatter tire more puncture resistant

Slime

Slime is not much good for glass. Glass tends to have larger cuts.

After

The best thing you can do to prevent a glass flat is to remove glass.
First two seconds, two minutes, or two days.

If you leave glass in a any tire longer enough it will puncture the tube.

On the road can use key or a knife to gently pry it out.
At home I use hemostats.

Recommendation

Don't go out and buy the most puncture resistant tires without evaluating the trade offs.
I have a pair and I don't like the ride, grip, or roll.
And they are going to last a long time.

You have not had a flat yet.
Consider a tire that is a mix of ride, grip, roll, and protection.
A Marathon Plus weighs almost twice as much as a Gator Hardshell.
And inspect your tires on a regular basis.

If you can go 28mm or 32mm then do so.
Not just for the reduced pressure but you also have a lot more options in flat resistant tires.

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+1 for this: "a person can lie on a bed of nails but not stand on it. So a larger tire avoids penetration from a range of glass that would have penetrated the smaller harder tire." –  Carey Gregory Jun 8 at 6:18
    
Love the analogy with the need of nails, but I think it's flawed :-) We are riding over only one or a few sharp objects, not a whole bed. The bed of nails analogy works when there are many points to spread the load over. –  andy256 Jun 9 at 23:15
    
@andy256 But if you are not sharing the load with glass you are sharing the load with the road (even better than nails). If I asked you to step on a single piece of glass would you point your heel and put the smallest amount of skin on it? –  Blam Jun 10 at 1:06
    
There seems to be some debate about whether tires at their max pressure or at a lower pressure is optimal for avoiding flats. You seem to say that high pressure is better. @Andy256 (above) says that higher pressure is better. Do you have a source to quote or are you just citing your own personal experience? –  sixtyfootersdude Jun 10 at 16:12
    
@sixtyfootersdude No, I say lower is better. I am citing my own experience, physics, and material science. I have a degree in engineering. A degree in engineering does not make me right - but that is my engineering analysis. bicycles.stackexchange.com/questions/21874/… –  Blam Jun 10 at 16:28

One solution not mentioned so far: tyre savers. Rather than trying to stop and wipe glass off the tyre and embed it in your gloves or hands, install a small piece of wire that does this continuously. They might even work. I've seen them used by a couple of people, and those people thought they were great.

The theory is that they brush off any protruding glass before it has a chance to get pushed into the tyre. If you think that brushing the tyre off is a good idea, these obviously work much better since they act immediately rather than trying to brush stuff off after it's been round the wheel a few times and is well embedded.

It's easy to make a set up using old spokes bent into shape, the tubing is fairly optional as long as your wheels are round. If there's a bump or irregularity some sort of flxible coupling is needed.

tyre savers

tyre saver fitted

IMPORTANT: the tyre save must be attached so that they trail the movement of the tyre, never leading. If they catch on the tyre you want them to be pulled off the bike rather than jamming between the wheel and frame. At best they'll get pushed into the tyre and you'll get the puncture you were trying to avoid.

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I didn't mention them because I haven't seen any for years (and I couldn't remember what they were called). They do work; I used them on my touring bike (it has mud guards). –  andy256 Jun 12 at 8:43

The only solutions I can think of other than trying to dodge, using "thornproof" tires (not a complete solution but they do help), getting good at changing tubes (and inspecting the tire to make sure there isn't retained glass that will promptly puncture it again), or campaigning/volunteering to get the glass cleaned up... is to assert your rights as a vehicle and take the travel lane rather than the bike lane. Car drivers will get grumpy about it, but if the bike lane isn't safe, it isn't.

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The municipality is quite good in cleaning up the glass, but not always within a day or two. The travel lane would not be safer in this case, see, e.g. maps.google.nl/… –  Bernhard Jun 7 at 15:48
    
Sometimes there is no good answer and you have to pick the least-worst... –  keshlam Jun 7 at 15:50
    
Just a note: In some countries (such as the one in question here) it is illegal to cycle on the travel lane if there is a 'compulsory cycle lane/path' (which is the de facto case nearly always in the Netherlands). –  David Mulder Jun 8 at 12:16
    
Good to know; thanks @DavidMulder –  keshlam Jun 8 at 16:04
    
@DavidMulder: True, but usually (at least in Germany) there are exceptions if the cycle path is "unusable" (due to ice, snow, debris, construction...). "full of glass" might count as an excuse (I'm not a lawyer, though). –  sleske Jun 9 at 9:36

Pretty much, aside from spotting it and veering around it, all you can do is run tougher tires. Belted tires are a good start, like the Schwalbe Marathon Plus for general use.

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Kevlar tyres or puncture proof inner tubes would also help. The inner tubes can be heavy though. –  JFA Jun 7 at 18:26
    
The Marathon Plus tires have a Kevlar layer, and you almost need to fire a gun at one to puncture it. They laugh off road debris. But they're quite heavy and rather annoying to mount. The Marathon Supreme tires may make a good alternative. –  Michael Hampton Jun 8 at 12:34

Some riders will reach down with their gloved hand and let it brush the wheel as it turns, to remove any glass or small pebbles that have adhered.

Long-distance riders have been known to use a formed piece of wire or molded plastic attached to the front "brake bolt" (center bolt hole in the fork above the wheel) that continuously brushes the tire. There used to be a product sold for this, but I haven't seen it in maybe 15 years.

I don't quite have the nerve to reach down and brush the wheel with my hand, but as luck would have it my front fender is quite flexible and I find I can reach down and press the leading edge of the fender into the wheel to brush it off. I do this when I hear/feel something on the tire.

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In addition to brushing your tires as soon as it happens (as soon as it's safe to stop and do so of course), checking your tires before every ride and picking out small peices of glass is also important (there's often glass on the road that gets picked up by your tire that you don't visually see of course). I follow this simple procedure and I haven't gotten a flat on my road bike in 2 years, and only get flats on my commuter once every year or so (it goes through much more glass).

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I work at a bicycle rental shop and we use Schwalbe Marathon Plus Tour tires on all our rental bikes, and we never get punctures. They are better than the solid rubber tires as they weigh less and have more spring to them. Also, if you get a broken spoke, getting the solid rubber tires off and back on is just about impossible! So check out schwalbe tires, and than get the highest level protection available from them with tires that fit on your bike. The marathon plus tour are the highest level protection (level 6). But level 5 might be all you need.

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Welcome to Bicycles SE. We tend to frown on self-promotion on this site. Please do so only if it is necessary to answer the question at hand. Also, the OP specifically stated "I am not really looking for material advice, but rather for tips on how to drive through glass" so your answer does not really address what he is asking for. –  jimirings Jun 20 at 15:59

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