My first forum question (I'm a beginner).

What are the size specs for road bike fenders?
Is it the fender's width and its wheel diameter specs?
If so, how do I measure a fender's wheel diameter spec?

Thanks?

The diameter is spec'd by the wheel size, ie. 700c, 26", etc. But the width is the actual width, so a 35mm fender will be 35mm wide.

You want a fender wider than the tire, but you don't need it much wider because the vast bulk of water thrown comes off the center zone of the tire. For mud, you might want a more generous overhang.

One thing they don't specify is how much radial clearance there is. But fenders are flexible, and that will be mostly determined by the height of the fork crown or seat stay bridge, with you adjusting the braces to maintain that all the way around.

On a couple of sets of fenders I bought it specified on the packaging that your for 25mm tires you should have 35mm fenders, and 35mm tires 45mm fenders. This is a bit overkill, and it looks ridiculous with the 25mm tires. Like FBinNY said, you don't need much. In fact, My 35mm fenders work and look great on the 35mm tires. It's a pretty tight fit, but that helps keep the water spray down.

One other thing about fenders is that the front ones are much more important than you think. Make sure it comes down low enough so that the spray doesn't get at the BB, and hence your feet. Long mudflaps are good for this.

in rare cases you have to cut the front fender in half & mount the halves independently on both sides (front/rear) of the fork crown. example cases include: some frames w tight clearances or when squeezing in larger than original tire sizes

Thanks for the helpful replies!

If I'm clear, the wheel size (like 26") is the same number people use to describe their "bike size" -- a 26-inch bike.

So, sorry, now I wonder how I can learn the size of my bike. I bought it used and don't know much about it.

From where, to where, should I measure it in inches?

Thanks again. Very helpful forum!

Forget about bike size which is often confused by folks who don't know bikes. You only care about wheel or tire size, since that's what the fender wraps around. That's real easy because it's molded into the tire's sidewall.

So, check your tire and is should say something like either 26x1.75, or anything from 26x1.25 to 26x2.25, all of which are 26" tires.

Or it might say something like 700c x 28, or 700x28c, or anything 700x some width. Tat would be a 700c and take the corresponding fender. There are other sizes and fenders made for them, ie. 20" on folding bikes.

BTW you might also notice the ISO size designations which would be something like 40-559 or 25-622, which would 26" and 700c respectively.

Def take a look at CRUD Road Racer series fenders, especially if you don't have proper mounting points for other systems.

I can't say enough good things about them and I think there are some others around here that would agree.

Excellent durable product.

center axis of BB, to top of seat tube.


Only children's bikes just use wheel size, because they only make one frame size.

I really appreciate the helpful replies! It's a great forum.

Everything was clear except one point:
BTW you might also notice the ISO size designations which would be something like 40-559 or 25-622, which would 26" and 700c respectively.

Does 40-559 mean a 26" wheel/tire with a 40mm tire width?
Sorry, it's probably because I'm a beginner.

I'll keep reading the forum. Hope one day I can offer advice!
Thanks!

Yes, in theory. So 559 is the bead seat diameter, 40 is the tire diameter (the section of the tire, that is). So the overall diameter of the wheel, with a 559-40 tire on it, is 559+40+40=638. That will usually be pretty close to the actual diameter of the wheel.

Note there are two different diameters: the wheel (rim), and the tire thickness. If you bend a fender out to make it fit a bigger wheel diameter, it will become narrower. If you bend it in, to make it fit a smaller wheel, it will get wider.

Actually that's not true ...

I believe that the OP is referring to the ETRTO (ISO-5775), which lists width of a tyre and the diameter of the wheel (see Figure 1).

https://en.wikipedia.org/wiki/ISO_5775

I'm sorry, but I don't follow. What part of my post is erroneous?

When using the ETRTO, the overall height is not 2 x FN + SN (first number and second number). In the case of ETRTO the first number is the width of the tire rather than the height.

Sorry for the pedantry.

Abbreviations like ISO and ETRTO are all well and good, if one uses them carefully, which is why I was careful to NOT use them. And I have now edited the quote to clarify which part of the post I was addressing. The numbers OP used, and which I quoted, were just numbers. Of those, 559 refers to an actual measurement of a nominal "26 inch" rim, and 40 refers to an actual measurement, the width (which is approximately the same as the height) of the tire. So to turn those numbers into an overall diameter of the wheel (wheel being defined as a rim with a tire on it) you really do need to count the tire twice.

The "26 inch" rim that measures 559 mm, for what it's worth, assumes a 2.0" (50 mm) tire to add up to 26 inches; so 559+50+50=659 which is, of course, just a little shy of 26 inches, but you get the idea.

no worries, let's just get this right and not confuse anyone!

I recommend u chick out topeak fenders, they are universal and very light and flexible, they have models according the tyre sizes. have one installed on my bike and they saved my clothes from many splashes.