The chances are that you’ll have seen GCN, Cycling Weekly or BikeRadar’s videos on the relative advantages and disadvantages of 650B vs 700C wheels for gravel.
With variations upon a theme, they cover unscientific testing and comparisons of the technical details, and they have done a good job explaining what the key differences are.
*650B wheels and tyres offer wider tyre options for 700C bikes
*In all but the most technical of gravel riding, the 700C wheels will be faster
*Extra grip, especially on steep climbs and the option to run lower tyre pressures, with increased comfort, are the main 650B advantages
*Carbon 650B rims can offer weight savings compared to their 700C versions, with their lay up having being adjusted to the smaller rim size, were as an alloy 650B version of the same 700C rim is only about 7% lighter
Take a look at the CGN ones below if you haven’t seen them yet, there are links to BikeRadar and Cycling Weekly at the bottom too.
WHAT ELSE DO I NEED TO KNOW?
If you are choosing a 650B wheelset to fit in your 700C bike, then you should use higher volume tyres to size up as close as you can to the diameter the bike was designed for, otherwise the handling could be adversely affected. (700C x 38mm tyre = c. 697mm diameter | 650B x 47mm tyre = c. 665mm diameter)
Smaller diameter tyres will save weight, right? Wrong! A wide 650B tyre is heavier than its narrower 700C equivalent. A 700C 40mm Maxxis Rambler is the same weight as a 650B Continental Terra Speed. Saving significant weight with 650B wheels is not really a possibility.
Tread choice; 650B tyres are available with more aggressive tread (such as the WTB Sendero), but more rubber = heavier tyres. If you are thinking of 650B wheels specifically to fit aggressive treaded tyres for winter muddy conditions, there are loads of 700C full mud cyclo cross tyres that might be better option, their generally narrower width cuts down through the mud to the grippier layer below better than a wide tyre. The 700C 33mm Donnelly PDX comes up at 36mm on a 21mm internal width rim and is super grippy but rolls well on harder surfaces.
There is no doubt that higher volume tyres at lower pressure delivers noticeable benefits, but there is a strong argument that most riders are riding their 700C gravel tyres too hard. A tubeless 700C x 40mm tyre with a nice wide (25mm internal) rim can be ridden by many riders at 20-30 psi without problems.
The BikeRadar link at the bottom of the page has a great explanation of tyre pressure, explained by tyre tension.
Take a look at ACCU-GAGE tyre pressure gauges, these are the best tyre pressure gauges for accurately dialling in your tyre pressure.
Many 650B rims are MTB rims repurposed for gravel, and whilst you can get nice light wide aluminium 410g rims which are suitable for gravel, many will be overbuilt MTB trail/enduro rims at nearer 590g.
For riding off-road, rim weight rather than aerodynamics makes the biggest difference to a wheel's riding performance. A light rim will save you energy every time you accelerate after braking or on short steep uphill sections. Even if you are not racing, saving energy is as important as comfort for long days in the saddle.
Given that 650B rims are for wider tyres, then wider rims make sense too. Our LOWMASS GA25 rims are wider than most. With a 25mm internal width, they are a great wide base for wide 650B tyres.
650B’s strength is its greatest strength!
Talk to anyone who builds wheels and they will tell you that a 650B wheel is a physical work out to build. Not in terms of turning the nipples, but in terms of stressing the wheel.
The shorter spokes and wider bracing angle make the wheel much less flexible and harder work to stress (than the equivalent 700C) during the build process. These are the features that makes a wheel stronger.
The other thing that makes a wheel stronger is the number of spokes, build with 32 spokes and a 650B wheel is a very strong structure.
So should I be on 650B?
If you are answering yes to these points then yes!
*I am looking for comfort and don’t mind it being at the cost of speed
*I am looking for the gnarliest wide tyre tread
*I want more of the wheel’s diameter to be tyre compared to a 700C wheel
*I am up against wheel weight limits and am looking for the strongest wheelset
*I understand the potential geometry changes and their effect on my bike
*I have clearance for 50mm tyres
If you are answering yes to these points, without a yes in the above, then think again!
*I want to save weight
*I am going to run smooth tyres on a bike designed for 700C
*I am going to run narrower than 50mm tyres and don’t want to alter my 700C bike’s handling
*I haven't spent time dialling in my 700C tyre pressure
*I haven't tried fitting the widest 700C tyres that will fit in my frame
*I haven't looked at how wide 700C rims could work for me
650B has some very specific benefits over 700C, if they match your needs then they make sense. If not then they could be an expensive experiment.
We have avoided the complex subject of changes to geometry in this article, but will cover it in a future one. The chances are though, your 700C bike will have a lower bottom bracket with 650B wheels.
Contact us if you’d like to talk about how 650B wheels might work for you or if you would like to find out more about LOWMASS Gravel Wheels with wide 25mm + internal width rims in 650B & 700C.
There is a very good chance that a wide 700C wheelset will be better suited to you, your bike and your riding...
Take a look at our other articles - https://gravelrider.co.uk/blogs/gravel-rider-ideas-reviews-and-advice