Tyre/front fork clearance - danger? Why?

[james01]

I'm putting some fat 700 x 40c tyres onto an old 10 speed racer, the idea being to have a fair-weather, mudguardless, comfortable local runabout. The clearance under the front fork bridge is 2mm. I'm well aware of the danger of road debris adhering to the tyre and causing a catastrophic front wheel lock-up, and we're advised against small clearances. Surely a large piece of debris would be more likely to wedge itself between tyre and bridge on a bike with good clearance? So I'll be more vulnerable to small stuff, but less so to a larger object which would be more likely to drop off when hitting the bridge because it couldn't fit under it. So, playing the odds game, shouldn't I come out about even?

[CJ]

I think bigger things are less easily carried up there in the first place. Think about the stuff that temporarily adheres to the tyres and plasters the frame etc. of any mudguardless bike. Finer sediments predominate: the bigger the grit the less easily carried.

So when you run over a patch of hot tarmac it's more likely that a piece of 5mm gravel will stick to your tyre than 10mm, which being twice the diameter weighs 8 times as much. In rough and ready terms, I'd say safety is proportional to clearance cubed. At 2mm, even a wind-blown plastic bag might be trouble!

All these things are probably very small probabilities, but nobody really knows how small, so it all boils down to how lucky you feel!

[gilesjuk]

You risk serious injury or death. It could happen down a steep hill when you're doing 25-30MPH. If there's one thing I don't want to try in life is sliding down a big hill at 25MPH with my face as the brake.

Just get some bigger forks?

Also: the front is the danger point, since a lock-up will throw you off the bike. The rear will cause your back wheel to lock and you skid to a halt (you may still fall over).

[CREPELLO]

Or a narrower front tyre. Or perhaps devise some sort of flint catcher positioned behind the crown. 2mm isn't a lot though of clearance though.

[james01]

Many thanks to all for the advice, you've talked me out of it! (I was sort of hoping you would, but I wanted a logical reason and CJ's advice makes a lot of sense).

[Freddie]

So does this make close clearance racing and audax bikes dangerous?.

[james01]

I think the point being made is that they're statistically more likely to get debris jammed in the wheel, even though the overall probability is low.

[reohn2]

................. Or perhaps devise some sort of flint catcher positioned behind the crown........

A flint catcher behind the fork crown would make matters worse where debris is concerned,thats why they were always fitted in front of the fork.

Clearly to be safe the answer, as you say, is a smller section tyre.

[Cyclenut]

So does this make close clearance racing and audax bikes dangerous?.

A bit more dangerous, yes. But lack of mudguards makes some of them safer - albeit filthy to ride when there's an "r" in the month.

The really dangerous ones are the audax bikes with mudguards and not enough clearance in case something goes wrong with them. Even a race-blade could a bit too easily get knocked into the gap under the fork crown.