What's the deal with 135mm O.L.D. road bike wheels?

Question

I've been shopping around for replacement wheels for my 10 year old Specialized Tricross, which came with an AlexRims Ace 19 rear wheel with a 130mm O.L.D. hub. I measured the rear dropout at 131.3mm with a set of digital calipers.

Now I'm getting confused about hub width. Sheldon Brown's article on hub width lists 130mm for road 8-10 speed wheels (my bike came with a 9 speed cassette), and 135mm for MTB 7-9 speed.

However, I'm seeing a lot of wheels that I think are meant to be road wheels but which come with 135mm hubs. I understand there's some bleed over between standard road and MTB hub widths for particular disciplines like Cyclocross and touring and such, but I'm confused about why I'm seeing a lot of wheels I'd consider firmly in the road discipline, but built up with 135mm hubs.

Here are a few examples:

Bontrager - Connection

The Wheel Shop - Mavic A119/ Acera

This AlexRims DA22 wheel

The latter is notable because it's specifically a narrow road rim aimed at "Road Racing" according to AlexRims.

Is there some industry trend that explains why so many wheels seemingly intended for road bikes are using MTB hubs/ hub spacing?

Is it just a fluke that I've stumbled on a bunch like this? Are these wheels, admittedly at the cheaper end of the spectrum, actually aimed at hybrids, not road bikes? (follow up: do hybrids use 135mm generally?)

Or, as discussed here, does this have something to do with a proliferation of disc brakes on road bikes? If so, why does that affect these rim-brake wheels? Have frame builders moved towards a generic frame spacing that can then be finished with either disc or rim brake mounts based on trim level?

Thanks.

Answer 1

The DA22 you link to is a normal 130 road wheel.

The other two are 135 touring/hybrid wheels.

"Road" has a few different definitions. Some use it to mean all drop handlebar bikes, some use it to mean all bikes ridden on the road, some would use it to mean only a proper full-on racing/club type road bike, etc. The notion exists that drop-bar loaded touring or gravel bikes are road subcategories that are still types of road bikes, but others find that farcical.

Few non-disc 135 wheels have proper road racing rims. Usually non-disc 135 is most associated with hybrids, but within that genre some are more fitness-y than others, and some non-disc 135 fitness hybrid bikes do have rims that more or less would fit in on a pure road bike of the same era. Aftermarket replacement wheels of that sort exist but are not a major category.

One historical piece of the above is there have been a few different takes on how to execute bikes in the flatbar road / fitness hybrid milieu. One interpretation has been make it a pure road bike where literally everything is the same except the handlebars and controls. Keep it 130 for non-disc, etc. Bikes like that were pushed for a while and do still exist, but in reality basically everyone drawn to them really wanted at least some more hybrid-y type features, especially gearing lower than pure road bikes. To me it feels like there was a point or period where manufacturers just stopped caring about doing purist flatbar road bikes, and afterward the fitness hybrid took over, most of which are 135, probably because the OEM hybrid component groups are based on 135. But, some bikes of that sort veer pretty hard to being pure flatbar road bikes, and that's where a 135 wheel with a pure road rim can make sense.

Some companies did 135 rim brake cyclocross bikes. It was always a terrible mistake because they couldn't take road wheels without compressing the rear triangle and they couldn't take cross cranks without screwing up the chainline. The main reason to make or buy a bike like that was not knowing about bikes. But they exist and do want a 135 rim brake wheel with a road rim.

Within disc bikes, there is a chronology to be understood about the OLD wars in the early days of road disc. 130 disc road bikes exist, but never became very populous and are now weird. Most road disc QR bikes are 135.

Answer 2

First, the article seems to be outdated: MTB are using now 141mm O.L.D. (or even larger, to fit larger tires) and have to 12 speeds. Road/Gravel bikes are have up to 13 speeds, and the switch to 135mm O.L.D. That doesn't appear in the article.

Road bikes also have longer freehub bodies than MTB: because MTB big sprockets are huge, they can overhang over the freehub body. But road cassettes are smaller and it's difficult to do that, so to increase the number of speeds longer freehub bodies are necessary. Pure speculation on my side, the switch to 135mm could also have been made necessary to accomodate 11/12-speed cassettes (on top of the disc brakes).

For your follow-up question: hybrids/touring are a mess in term of components. 135mm is indeed very common, sometimes 135mm road (with the longer freehub bodies, for city "fitness bikes"), sometimes 135mm MTB. Hybrid e-bikes are now increasingly using 141mm, to be able to use larger tires for comfort (55mm or more).

(Note to add to the confusion: the numbers I give relate to the spacing on the frame, that shouldn't be confused with the axle length: 135mm O.L.D. = 142mm axle length in case of through axles, that are also common on road bikes)

Answer 3

Is there some industry trend that explains why so many wheels seemingly intended for road bikes are using MTB hubs/ hub spacing?

Road bikes are using far higher pressure than MTBs, and MTBs often have suspension. MTBs are usually ridden at slow speeds as is evidenced from their ridiculous gearing having a chainring as small as 20 teeth and still a cassette with as many as 50 teeth. Also MTB riders ride on soft terrain. About the only obstacle an MTB rider might encounter that isn't completely soft is a tree branch, and even that obstacle is softened by suspension and low tire pressure. Thus, because of not encountering concrete or rock curbs like road bikes do, and because of having low pressure and suspension, spoke tension is usually not an issue on MTBs.

The problem with spokes is that once a spoke loses tension due to riding over an obstacle at high speed with heavily inflated tires and no suspension, the vibration can cause a nipple to unscrew. A rim can usually take only about 1200 Newtons of spoke tension per spoke. With a road bike, the high speed, lack of suspension, high tire pressure and curb make of hard material, the impact forces will be greater than they are with MTBs. Also sometimes road bikes are used in long trips where you need to carry heavy luggage (thus increasing load on the wheel), whereas MTBs typically carry no luggage. Thus, all things being equal, a road bike needs higher minimum spoke tension.

A factor in spoke tension is the asymmetry of the rear hub. The right side needs the cassette and to make space for it, the right side spokes are more vertical. Because of this, the right side spokes pull very little to the right. The left side spokes that are at a shallower angle then will naturally have lower spoke tension to have an equal pull to the left. For example 2:1 asymmetry in spoke flange distance causes 2:1 asymmetry in spoke tension, so a 1200 Newton right side spokes are paired with a 600 Newton left side spokes. The left side spokes can become slack and nipples can unscrew, which will cause a catastrophical loss of spoke tension on the wheel -- the right side spokes will be affected too once the left side spokes are too loose.

Therefore, road bikes need more equal spoke tension than MTBs, and to make that possible, road bikes need wide hubs.

I'm surprised why it took so long for road bikes to start adopting 135mm hubs. Road bikes should have adopted them sooner than MTBs.

Today also the equipment freaks are discovering 6 sprockets 7 sprockets 8 sprockets 9 sprockets 10 sprockets aren't enough and it is vital to have at least 11 sprockets, but the sprocket sizes typically used in road riding aren't so large they can overhang the right hub flange, so 11-speed road bike wheels need more space for the cassette than 11-speed MTB wheels, and the hub width becomes even more critical than it used to be.