Actual wheel size

I need one of you experts to help me get the wheel sizes straight.

I’ll put down what I think and you can correct me.

What we refer to as 20", 24", and 26" wheels and tires are generally measured at the outside diameter of the tire. The rims are significantly smaller in diameter. The 700c rim, however is a measurement of the rim without a tire.

The 700c rim can be fitted with a tire that will make the outside diameter either 28" or 29" depending on the depth of the tire mounted.

I have also heard that there is a British tire and wheel size that is a 28" measured in the customary way of 20", 24", and 26"…but it is not on a 700c rim.

I am looking at a 28"/29" unicycle for my next ride and want to be sure that I end up with a 700c rim which is a more universal size, but the rumor of the off-size 28" has me spooked when I look at the sales pages. Who uses those, if they in fact exist, because I don’t want to get one.

Thanks–this would all be nicer if we spoke of rim size versus tire size, like we do on cars, and most other rolling things.


Re: Actual wheel size

Those I know are the diameter from one side of the rim only to the other. The tire is not involved in the with the sizing. Like a 24 inch muni is really with a down hill 3 inch wide tire is more like 26 inch+. Tires are very different in sizes how tall they are it wouldnt make sence to size rims with the tires on them.

I dont really know 700c rims. 28 inch would be 28 inch in diameter not as comon as 700c but more then 29 inch I believe but I’m not to sure.

so a 19" trials uni has a 19" rim, right? and the tire is an extra like 1-2 inches on both sides…
so a 19" trials uni actually has like a 22-23 inch wheel, right?

Wrong. The trials is said to have a 19" rim because with the 2.5" tire, it is 20". With a 3" DH tire, bikers usually run a 26" rim and tire on the front, and a 24" rim and 3" tire on the back so they are the same. I think most rims are made so that with the standard size tire, 1.95", 2.125" or whatever, makes it the listed size. My torker LX actually has a 16" or so rim. A trials rim is I think around 15".

All tyre sizes are nominal, because the tyre section and pressure vary so much. Rim sizes are standard. So, you can buy a standard rim then fit any one of a number of different tyres to it.

700c with a standard 32mm tyre on it gives a rolling diameter of almost exactly 28. With a fatter tyre, the rolling diameter is nearer to 29 inches.

700c = a nominal size of 70.0 centimetres.

Old UK sizes included the 27 inch wheel (standard before 700c became popular) and the 28 inch wheel (already a bit of a curiosity even 20 years ago).

Tyres are not intercahngeable between different rim sizes.

Therefore, in this size bracket, 700c is to be regarded as the standard. That’s the size you’ll get from a reputable supplier.

Worry less.

I had one Dawes bicycle with two 27" wheels.
Try explaining everyone that 27" wheels are larger than 28" wheels.
…or try to find tires outside the UK for it!
…and so I had to lace new wheels when my tires needed to be replaced.

Tire sizes are based on the outside measurement of the tire for lots of information see sheldon browns article

But all the 28" and 29" unicycles I have heard of (Kris Holm, Nimbus, Sun) have 700c rims which actually measure 622mm outside diameter

my advice is to use the metric tire sizes e.g the 60-622 on the big apple 28x2.35" tire, the 622 is the diameter of the bead/rim in mm 60 is the width of the tires in mm

I have a tire labelled 21x1 3/4" tire that has a 24"outside diameter.

Old Schwinn were notorious for obscure tire sizes


Back in my youth, I had bicycle with 26 x 1 3/8 tyres, which were noticeable large wheels than a 26 inch mountain bike/MUni wheel. Then I went onto 5 speed “racers” and 10 speed “racers” which as standard had 27 x 1 1/4 tyres. Real hardnuts had
27 x 1 1/8 tyres.

I occasionaly saw bicycles with “28 inch” wheels. These were massive wheeled granny bikes - super dreadnoughts in the Dutch style.

Around 1986 - 1990, I was heavily into bicycle touring and tandem touring, and I remember having to choose between 27 inch wheels or 700c wheels. As a bit of a conservative (small “c”) I went for the imperial size.

We unicyclists only refer to 700c wheels as 28s or 29s because wheel diameter is critical to our sport, and easy to compare. Bicyclists would I suspect refer to 700c or “700c x 32mm” etc.

The fact that a 700c wheel with a standard road tyre is almost exactly 28 inches diameter is purely coincidental. There is no connection between what we refer to as 28s (700c x 32) and the old imperial 27s or 28s.

I’ve only seen a unicycle with an “old-school” 28" wheel once. That was in 1983. I’m pretty sure nobody makes them now, unless you order it from DM (in which case just ask to make sure).

Foot feel the warmth of the mouth. I think I understand now. So road tires are set up so that they end up having a 24" diameter but then Big DH tires are set up for that rim size also but they are likely more then 24".