"adequate" fender clearance

[ted]

Jan Heine has written that they "... recommend 20 mm between tire and fender for safety."

In the Roadeo write up RBW says you can put fenders over a 33.33mm tire and without fenders can fit a 35mm tire.

That's only a 1.6 mm diff in size. I see lots of photos showing tire clearance that look like there is a lot less than 20 mm there.

Most photos of fendered bikes I see don't look like they have that large a gap either.

Do most folks really leave 20 mm (over 3/4 inch) between tire and fender?

Do most folks measure the clearance from the inner fender surface or from the largest protrusion in there (e.g. mounting bolt for a light)?

[Bill Lindsay]

I think you may have misunderstood what Jan writes.  The only place he goes all the way to 20mm of clearance that I know of is when you are scoping out whether or not you have clearance on your frame.  He writes you shoudl have 15mm between the fork crown and the tire or the rear stay bridges and the tire, but then says 20mm is even better.  Most people do not have 20mm of clearance between the tire and the inside of their fender. 

Most folks measure the clearance from the inner fender surface to the tire when they talk about their fender clearance. 

Some people like about 10mm from tire to fender.  Some consider it good enough if their tire isn't rubbing. 

[ted]

Thanks Bill,

Just for reference, in the 650B group Jan posted:

"If you use these fenders with wider tires, you encroach on the clearance between tire and fender. We recommend 20 mm between tire and fender for safety. If you use a tire that is about 8 mm wider, you reduce that to 12 mm. The bike will be rideable, but it's less than optimal. I wouldn't ride a bike like that on gravel."

Perhaps he feels 10 or 12 mm between tire and fender is acceptable and 20 is optimal, but he clearly is recommending a 20 mm gap there.

[Michael Hechmer]

Metal fenders have a rolled lip about 4 mm wide, so that uses 8 mm of real estate.  If you have 5 mm of clearance on each side that gets you to 18mm.  So a 32 mm tire will want a 50 mm tire or more.  You can cheat on that number a bit but you run more risk of both rubbing and trapping debris.

Plastic fenders don't have the lip and don't curl around the tire as much, so you can get away with a much smaller fender, but 1.5 mm is pretty tight, even if the fender doesn't cover to the widest point of the tire.

I run 45 mm fenders with 29 mm tires on the Ram and  55 mm fenders with 38 mm tires on both our tandem & Saluki.  Jan's 20 mm recommendation makes a lot of sense, if your frame & brakes will support it.

Michael

[ted]

Michael,

Are you saying the 20 mm Jan referred to is the difference between tire and fender measured width, and not the space between tire and fender?

[Bill Lindsay]

Thanks for the clarification, but I still think you misunderstand what Jan is recommending in the post you quote.  In that post, Jan is specifically saying that a 60mm wide fender is intended to use with a 42mm tire.  They recommend that the width of the fender should be about 20mm wider than the width of the tire.  Even though I think Jan's post, separated from context, could be misconstrued, he has consistently recommended over the years that 10mm of clearance between fender and tire is ample and 8mm is acceptable.  There's nothing inherently wrong with 20mm of clearance, except it would look kind of ugly.  You'll never catch Jan saying you need 20mm of space between tire and fender.  That's excessive for road use.   You winter and mud riders may do well to have a lot more clearance, but for the rest of us, ~10mm is plenty. 

[ted]

Bill,
Thanks for straightening me out.
Would it be fair to say that standard practice is minimum clearance of 8 to 10 mm, though some folks go with much less than that?

[Bill Lindsay]

Yes, Ted, I would say that Jan's guidance for best practices is to try to have 8-10mm of clearance between fender and tire, everywhere.  Many do go with much less, particularly for those who try to run the absolute widest tire their bike will take. 

[Jan Heine]

Interesting discussion. Just to clarify, I really do like to see 20 mm of "air" between the top of the tire and the fender. Here is a photo:

https://janheine.files.wordpress.com/2014/02/compass_babyshoe.jpg

Anything less risks trouble when your tires pick up small rocks and drag them through the fender. Even on gravel roads, you shouldn't get the "scrtchh" of a rock rolling between tire and fender more than once every 500 miles... You can ride a bike with less clearance, but it's a little risky for my taste. I know too many people who've fallen on their faces when fenders collapsed and jammed the front wheel.

There really is no reason to have less clearance on a custom bike. The fender lines on the bike shown in the photo above look great, see

https://janheine.files.wordpress.com/2012/06/jhherse_full.jpg

There really is no reason to mount the fenders closer to the tires, except that when you remove the fenders, the clearances may look excessively large. (Just like my car, my bike always carries fenders, so that isn't an issue for me.)

Of course, you don't need (and won't get) 20 mm all around the tire, because your fender doesn't need (and cannot) to be 40 mm wider than your tire! You won't drag debris along the sides of the tire, so less clearance there is fine. You just need enough so that if your fender's position moves a bit after you lean your bike against a wall or something like that, the tire doesn't rub on the fender. 10 mm probably is fine there.

The chainstays and fork blades will encroach a little further into your clearance. It's fine to indent the fenders there - the fenders are firmly supported, so they won't budge. In those places, you just need enough that frame flex, wheel flex or a slightly out-of-true wheel won't cause the tire to rub on the fender. I'd say 5 mm is OK, 8 is better.

Of course, you can ride bikes with much less clearance. I have done so for many years. The fenders may rub a bit when you rise out of the saddle, and you have to hope for the best when riding over gravel, but most of the time, it's fine. 

Jan Heine

Editor

Bicycle Quarterly

www.bikequarterly.com

[Bill Lindsay]

Well there you have it.  I guess there is a situation where you will find Jan say 20mm.  Again it is useful to remember context.  Jan says right here 20mm of headroom when you are specifying the clearance on a custom frame.  Most people posting on these lists are not specifying a custom frame, though.  They are assessing the bike they already have.  My memory of Jan's recommendation for headroom was based on this quote from Jan's blog:

"You can get away with smaller clearances and still have fenders that work fine. You need at least 12 mm between tire and fender, plus about 1 mm for the fender, and another 2-3 mm for the leather washer that cushions the fender against the frame. This adds up to about 15 mm between tire and bridges/fork crown. (Better would be about 20 mm.)"

Most people "get away with smaller clearances", in part because people tend to always want to try the widest tire that will fit. 

[Jim Bronson]

I roll with 7mm, LoupLoup Passes 650Bx38, Rivendell P45 cream Longboards.

Probably not ideal, but I'm not about to change it.

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[Tim Gavin]

Ditto, I have tight fender clearances on two of my bikes:

Riv Road / 650b conversion: 45 mm VO fenders, 38 mm Pari-Moto tires.  Probably 5-6 mm of vertical clearance at best.

Foundry Auger cyclocross/gravel: 45 mm SKS P45 fenders, 38 mm Bontrager CX0 tires.  Probably 6-7 mm of vertical clearance at best.  (no picture with fenders, currently)

I ride both of these bikes in gravel and dirt B-roads, and it's common to have a small pebble get picked up and  zzzzinged out the front of the fender.  

I can't do it on command yet, like Yehuda Moon can.  :)

https://dutchbikeguy.files.wordpress.com/2013/12/yehuda-moon.gif

Maybe I'm lucky, but the tires don't seem to pick up any rocks big enough to jam betwixt fender and tire.  I do have safety QR clips on all fender stays.

These tight fender clearances are no fun in mud, though.  I ride my fat bike when the dirt roads are sloppy.  I agree with Jan that 10 mm or greater would be ideal, and I would definitely design a custom with more clearance.

[Michael Hechmer]

Yes, that is exactly what I am writing and believe Jan would agree with that.

Michael

[Jon Dukeman in the foothills of Colorado]

I'm riding cream SKS P45s with 38 Little Bens and side pulls on my Sam. The fenders are up tight against the side pulls in the front.

I hear pebbles scraping the fenders ALL THE TIME. Cottonwood leaves are a constant annoyance. A few times during a 30 mile ride  I have to stop and pull them out ..I live with it

I just try to be aware  and avoid the sticks.

I did a ride last week in snow and mud and I was glad I had fenders. My bike and I were dry and clean..my friend without fenders...looked like he just finished a cycle cross event.

 I was wondering if I was the only one with close tolerances between tire and fenders.

Thanks to all for the discussion.

Jon

[Steve Palincsar]

Do you have those black plastic safety release things where the fender
stays attach to the fork eyelets?

[Jon Dukeman in the foothills of Colorado]

Yes I do..Thanks.

Jon

[NickBull]

I try to mount my fenders so that the clearance between the rear of the fender and the tire is the tightest clearance anywhere along the fender.  That way, anything that enters between the back of the fender and the tire should be able to continue along the path under the fender, rather than getting caught further along (e.g. at a brake bridge) and locking up the relevant wheel.

Nick

On Sunday, December 13, 2015 at 6:14:03 PM UTC-5, ted wrote:

[Jim Bronson]

Heh. For me, I tend to pick up acorns quite a lot. Leaves are
another frequent visitor to the innards of my fender wells. I ride
99.999% on pavement, but the nearby Southern Walnut Creek Trail that I
like to ride on goes through a heavily treed area as it's in the flood
plain for Walnut Creek.

https://austintexas.gov/department/southern-walnut-creek-trail