Is this dangerous?

[853]

I've been cycling for decades, but would very much like your second opinions on the following issue. I'd be particularly interested to hear from those of you who rode time-trial bikes with really tight clearance back in the day.

Introduction

I have two road bikes, which are both at the racing end of the scale, and a little used fixed-wheel bike. They all have rim brakes. The road bikes are designed to take 23 and 25mm tyres, without mudguards, respectively and I use the one with 25mm tyres in the winter. The fixed wheel bike has 28mm tyres.

Back in late September I had a rear wheel puncture and bad tyre cut so I replaced the tyre. I've been using Vittoria Zaffiro tyres since they came out and so replaced the 25mm Zaffiro Pro (not with Graphene) with another one. Since then I've had a series of small slides and 'exciting' moments on wet roads (only) from the back wheel. The front feels well planted, but I ride with some other ex-racing guys, so speeds in the high 20s are regular on the flat and 30+ on the downhills.

At the moment the local farmers are spreading mud on the road, and the leaves are coming down, so I am not sure as to whether the problem is the tyre, or the fact the roads are particularly slippy at the moment. I have tried dropping the tyre pressure, but it is still very 'twitchy'.

The problem

In order to try and work out if I have a bad tyre I have put a Zaffiro Pro with Graphene 28mm tyre on the rear wheel. I have 5mm or more clearances to the chainstays and seatstays but the clearance to the brake bridge is very tight. It's hard to measure it, because the rear rim brake gets in the way, but I would estimate it to be 1mm clearance for most of the wheel going down to about 0.5mm at one point.

I went out, briefly, this afternoon and as soon as I reached a wet patch the rear wheel made "slurp, slurp" noises as it rotated and the water being carried on the circumference of the tyre got squeezed against the brake bridge. It doesn't make any noise and there is no visible contact when the tyre is dry.

The question is, is it dangerous to have a clearance this tight against the brake bridge? I realise that there is a hypothetical risk of something getting attached to the rear tyre and getting jammed, causing the rear wheel to lock, but is this a great risk in reality? Also, apart from the noise is there any danger in having water getting squeezed between the tyre and brake bridge? The frame is made of aluminium.

The frame has vertical dropouts, but they're only just big enough to hold the wheel. I don't think I could fix the rear wheel any lower in a safe manner.

[531colin]

I have only once seen somebody’s back wheel lock up due to fag-paper clearance between tyre and brake bridge; due to a small stone/ big gravel getting wedged

[Biospace]

Hydraulics only works when the liquid is contained or only has a very small exit hole, so I would say no problem with water. That is, unless it started to freeze.

[pwa]

My own feeling is that a rear wheel locking unexpectedly is less likely to produce a very bad result than a front wheel doing the same.

Also, some tyre treads and compounds grip better than others on wet surfaces. Back when I was doing Audax, for instance, I found Conti Grand Prix 4 Seasons particularly good. But I'm not up to speed on what is on the market now, so others will be able to give more precise advice on what works best.

[Pebble]

not that long ago I had tucked my fleece hat under the seat, it had then dislodged its self (may be 25 downhill) landed on rear wheel and had jammed itself under the brake caliper. The bike just stopped without issue. I was a little puzzled as to what on earth had happened,. surprisingly little damage, if any, to the hat.

If it had been the front wheel, could have been very different, could have been a trip to hospital.

[mattsccm]

Wouldn't worry me unless I was worried about physical damage to the frame or paint.

[531colin]

So, on any particular ride, the wheel might not lock up; and even if it does, you might not come off.

Sounds peachy to me.

[Cugel]

One bike I've been riding for over ten years now has always had fag paper clearances - 28mm tyres (actually measuring 29.5mm) in a rim-brake frame with reasonable clearance to the sides but only a millimetre or two from the top of the tyre to the fork top inner and to the seat stay bridge. I've never had any problem from a jam although a hot tar patch did once deposit enough dusty grunge in the gaps to make a noise. (It was easily cleaned out).

Currently I have 32mm tyres on one set of wheels for this bike, which hardly shows a light line under the forks or seat stay bridge although still has enough clearance to the frame parts to the sides of the tyres. Again, no problem at all in riding a few hundred miles on them since they were installed. (I haven't ridden them in the wet, though).

**************

If you think about it, a jam from road detritus picked up by the tyre and rotated 'round to the fork crown or seat stay bridge is far less likely with close clearances. The frame knocks off any such detritus, even teeny gravel bits. A wider gap actually provides room for a lump of twig or branch or similar chunk to go in to the gap, which is when the wheel might jam if the detritus is irregular so rotates then jams in the gap.

Very close clearance to the top of the tyre from the frame is like having those tyre cleaners we used to mount on the brake bolts that consisted of a wire loop configured to just miss the tyre top, for the purpose of knocking out thorns before they could be driven any deeper into the tyre thus causing a puncture. Close frame clearances to the tyre top, of the fag paper type, are actually safer perhaps! I suspect they might well knock out those thorns.

Of course, larger tyres are also wider. Spoked wheels don't change the wheel circumference under riding loads - to any significant degree, at least - so won't frame-rub on the tyre top on bumpy roads or whatever. But wheels can deflect sideways quite a bit in some circumstances of riding so clearances between the sides of the tyres and the frame parts they run past need to have significant gap to them.

Perhaps there are some kinds of road detritus that a tyre might pick up and not want to release when knocked agin' the frame? That tarry road dust I mentioned is an example. But are any such stickier things going to be of a nature to jam in a fag-paper gap with enough resistance to stop the wheel rotating? I can't imagine such a thing; and, as I mention, I've ridden thousands of miles over the years with fag-paper gaps between tyre tops and frame with no issue at at all.

[rareposter]

+1 for everything Cugel says ^^

My SS road bike has very slight clearances; that was my commuter for many years (on filthy city streets, in all weathers) and I never once had an issue with anything getting trapped in there or the tyre catching on the frame.

Where it does become an issue (on any frame) is either if the frame is flexy enough that the wheel starts rubbing on the chainstays under hard pedalling (like out the saddle on a climb for example) or when riding in really thick heavy mud where it builds up enough to start rubbing through paintwork and potentially frame material.

The famous "Unbound" gravel race in the States this year was horrifically muddy and the consistency of it was enough to keep the wheel rotating but clog everything solid and there were quite a few cases where the mud actually wore through carbon or steel chainstays:

https://www.cyclingweekly.com/news/ardu ... d-worth-it

https://www.reddit.com/r/gravelcycling/ ... bound_mud/

[853]

Thank you for all your replies - most helpful

It would seem that it is safe to try the 28mm tyre to see if my lack of rear wheel grip is down to the 25mm tyre, or the current condition of the local roads.

[Brucey]

......If it had been the front wheel, could have been very different, could have been a trip to hospital.

IMHO this is correct. Also the gap between the fork crown and the tyre is usually wedge-shaped so it is only a matter of time/dumb luck until something sticks to the front tyre and causes a rather nasty jam. i have often wondered if a tyre saver might mitigate the risk from running close-clearances.
Needless to say I think cugel is living in a dream world; I just hope he doesn't wake up in hospital.

[Cugel]

......If it had been the front wheel, could have been very different, could have been a trip to hospital.

IMHO this is correct. Also the gap between the fork crown and the tyre is usually wedge-shaped so it is only a matter of time/dumb luck until something sticks to the front tyre and causes a rather nasty jam. i have often wondered if a tyre saver might mitigate the risk from running close-clearances.
Needless to say I think cugel is living in a dream world.

Ha ha - Cugel lives in quite a real world, with slippery leaf roads and also the detritus happy to cause wheel jams should the bicycle be configured to allow them.

But in fact, the only wheel jam I've ever had was due to a crumpled up front mudguard (a cheap and nasty thing) that jammed in the rather large gap between the tyre and the frame on an unsuspended MTB with cantilevers, set up for road riding. It's big chunks of gnarl that will jam between a wheel and the frame, not teeny bits of gravel in a fag paper gap. The big gnarls need a big gap, not a fag paper gap, to get in and jam. And it takes a big gnarl to resist the forces of a turning wheel - to not snap and to stop the wheel turning instead. Big gnarls can't get into fag paper gaps.

Of course, you may be able to point at examples of very close clearance wheels & frames that were jammed when a 10 micron piece of dust got in there ..... ? Despite riding thousands of miles with others in various events and club runs, many riders having fag paper clearances to their wheel tops, no such incidents have happened. But you may know of, oh, hundreds.

[Pebble]

......If it had been the front wheel, could have been very different, could have been a trip to hospital.

IMHO this is correct. Also the gap between the fork crown and the tyre is usually wedge-shaped so it is only a matter of time/dumb luck until something sticks to the front tyre and causes a rather nasty jam. i have often wondered if a tyre saver might mitigate the risk from running close-clearances.
Needless to say I think cugel is living in a dream world.

Ha ha - Cugel lives in quite a real world, with slippery leaf roads and also the detritus happy to cause wheel jams should the bicycle be configured to allow them.

But in fact, the only wheel jam I've ever had was due to a crumpled up front mudguard (a cheap and nasty thing) that jammed in the rather large gap between the tyre and the frame on an unsuspended MTB with cantilevers, set up for road riding. It's big chunks of gnarl that will jam between a wheel and the frame, not teeny bits of gravel in a fag paper gap. The big gnarls need a big gap, not a fag paper gap, to get in and jam. And it takes a big gnarl to resist the forces of a turning wheel - to not snap and to stop the wheel turning instead. Big gnarls can't get into fag paper gaps.

Of course, you may be able to point at examples of very close clearance wheels & frames that were jammed when a 10 micron piece of dust got in there ..... ? Despite riding thousands of miles with others in various events and club runs, many riders having fag paper clearances to their wheel tops, no such incidents have happened. But you may know of, oh, hundreds.

aye, I doubt my hat would have made it into a very small clearance. In fact I'm not sure how it managed to get under my rear caliper, its not a big space, I doubt I could re-create it.

[Brucey]

years ago,I got bored of being seen as a bit of a know-it-all in my own club, so I just bit my lip when I was out with a bunch of strangers in my GF's club..I kind of wished I hadn't when, just two weeks later I was told that the very same guy [who was very experienced, but riding close clearances much as cugel describes] I had failed to warn was currently in hospital because he'd broken his neck when his front wheel had jammed.
The shape of the gap between the fork crown and the tyre is what is important here not the size. What cugel is doing is unwittingly taking a risk quite unnecessarily, IMV.

[Jezrant]

I'm probably missing something here, but seems pretty obvious that the risk of coming off due to skinny HP tyres and slippery roads is far greater than the risk of coming off because the rear wheel locks up from something getting wedged under the brake bridge. I don't know the tyres mentioned by the OP, but I'd be trying another tyre with a different compound and/or lower pressure (tubeless maybe?). This time of year I'm happier on my MTB with 2.2" tyres.