Is it really a 29er?

[steve_sordy]

I was taking a look at the Isla Bikes website, particularly the BEINN 29, which will take up to a 2" wide tyre. The spec says that the wheels are 700c, which puzzingly is the same as on their adult road bikes .

I have a Trek 7100 in the garage with a 700x35c tyre, and the wheel and tyre combination measures 27.5" diameter. How can the same wheel be used for a 29" mtb bike? Is this Isla Bikes stretching things a bit, or are all 29ers based on the same sized wheel?

I'm assuming that as my 26" mtb wheels actually measure 26", that the 29ers will actually measure 29", so how do they do that? Where does the extra 1.5" diameter come from?

[fredmundo]

29" wheel = 700c wheel in size dude, i think it's just smoke and mirrors to hide the fact....

One of the reasons I've decided my commuter is going to be a 29er is that if I wanted to I can whack on commuter tyres to ride longer jaunts on road...

[gorehound]

Could be time to get the tape measure out Steve.

[bongo_bill]

I thought 700cc was a 28er? You live and learn!

[EnglishLion]

As I understand it the wheel size (rim) for a 29er and a 700c is the same. However mountain bike have been classified by total size (including tyre) which is where the 26" and 29" measurements come from. This may explain where 700c = 28" comes from if (and I don't have a bike to measure here ATM) a 700c + tyre = 28" due to the smaller tyre.

Of course any measurement including tyre is silly because that all depends on the tyre that's fitted.

[DISGreece]

I mentioned this on the Olympics post... but thought I'd bring it up here too...

When racing BMX in the 80's larger wheeled bikes 'Cruisers' were not allowed to race the 20'' BMXs
Why are 29ers allowed to race with 26''???

Shouldn't they be classified as a hybrid as mentioned by Kev?

[steve_sordy]

I thought 700cc was a 28er? You live and learn!

I've since had a look on the Schwalbe website and it says that a 700x35c tyre (French system) is the same as 37-622 (ETRTO - European Tyre & Rim Technical Organisation). Both are equivalent to the 28" x 1.40" (Imperial) wheel and tyre combo. The French system is the approx outside diameter of the wheel & tyre combo in mm (27.56") and 28" is the approx outside diameter under the Imperial system. Only the 37-622 ETRTO gives the actual tyre width of 37mm (1.46") and inside dia of 622mm (24.4"). ETRTO are predictably sniffy about the French and Imperial system for not being precise.

Schwalbe say that the imprecision of the French and imperial systems is historical and was based upon the size of the rim brake, which in turn was influenced by tyre size.

I still couldn't understand how a rim designed for a 622 inside dia tyre could support a 29" outside dia tyre (737mm), so I had a go at some sums.
If we allow for 5mm of tyre to fit inside the wheel rim, a 622 tyre inside dia implies a rim outside dia of 632mm. The outside dia of a 29" tyre is 737mm, so the tyre has to bridge a gap of (737-632)/2 = 52.5mm. in other words 52.5mm of tyre projecting beyond the rim. As a sense check I measured how far my 2.35" Igniters project beyond the rim and they measure 50mm. 2.5mm is less than 1/10th of an inch, so we are close!

It looks as though an appropriately designed tyre fitted to a rim otherwise suited to a 700mm (French), a 622mm (ETRTO) or a 28" (Imperial) tyre will make up a 29" wheel & tyre combo! But it still seems like cheating to me!

The good news is that it seems that all I have to do to make my Trek 7100 into a 29er is to add some bigger tyres! Must take a closer look at the frame clearances (and those rim brakes).


Many thanks to the Schwalbe website. http://www.schwalbe.com/gbl/en/techn...nPoint=Technik

[Gunwitch]

I've just measured my 29er. The wheel is 24.5 inches rim to rim. Where the hell do they get their measurements from?

[steve_sordy]

I've just measured my 29er. The wheel is 24.5 inches rim to rim. Where the hell do they get their measurements from?

24.5" is 622mm, which is the ETRTO size (see my post above) equivalent to the 700 and 28" "conventional" wheel sizes. What is the outside diameter of your wheel including tyre? Is that 29"?

[Gunwitch]

Aye Steve. Outside tyre to outside tyre is 29".
kinda weird when you think about it that the tyre is what makes it a "29er", well it is to me