Unless you have been living in a large hole of late, there is a new wind blowing through the mountain bike world. Just as everyone got used to the idea of 29inch wheels being here to stay, suddenly the industry drops yet another wheel size on unsuspecting riders. 650B or 27.5″, depending on what time of day it is and which direction the wind is blowing, offers a middle ground between 26 and 29. The ‘best of both worlds’ it has been said.
(ed. note: And yes, we know the 27.5″ rim is actually closer to 27.1″ depending on the manufacturer but we are going to use the claimed ‘marketing’ size which actually is works to the wheel’s favour!!!!)
You can’t look at a website or a magazine these days without reading about the new wonder wheel size – it’s better for those that don’t want to go 29, it rolls faster, it clears obstacles more efficiently, it is the new standard for middle ground bikes. But can 1.5″ at the rim make that much of a difference? The thing is, from everything you read, there is no sound reasoning other than “I think”, and is “I think” enough of a justification for anyone/everyone to go spend more money, for something that may or may not be better, because someone who writes for a magazine said so?
I like facts, especially when it comes to my bikes. Two wheeled objects are driven by facts and equations. In essence they are pure physics – how the hell else does an object with no means self uprighting stay upright otherwise? What’s annoying me with this current push is that no one, anywhere, has managed to put some empirical data in front of me to factually demonstrate that there is actually a clear advantage to a wheel only 1.5″, 38.1mm, bigger. So I though it was time to pull out the books.
Since my college days I’ve done a lot of reading on the topic of two wheels; my first design job was designing motorbikes in Italy. Ever since those days, the actual dynamics of two wheeled vehicles has been of great interest, and I have done my best to collect all the texts I can on the matter; which by the way is actually a very small and hard to obtain collection. I’m no expert or physicist, but I’ve learnt enough over the years. My dissemination of what makes a bicycle go can be found here – something I wrote while GM and designer for Mountain Cycle, in response to the marketing crap continuously being spilled out, especially in regards to suspension.
So in searching for answers about what potentially could be so great about this 27.5 wheel size (I’ll use 27.5 to keep things simple), I went back to the texts to try and extract all the data I could about the physical effects of wheel sizes on a bicycle. What I found is, well, interesting and at the same time, disturbing.
First, a quick recap of what makes a bike do what it does…
The overall performance of a bike is determined by a number of different and interrelated factors. The overall wheelbase, the chainstay length, head angle, fork offset and length (when all combined result in trail) and the position of the rider, which determines much of the centre of gravity. There are many, many ways to combine these factors and each will deliver a very different result; generally small changes in geometry can make big differences in handling. Added to these factors as an afterthought are tyre choices, which and of themselves, can have a dramatic effect on handling – different tyre choices for a given frame can change the overall performance and reaction quite significantly and for 26″ wheels, there can be a remarkable range of sizes that can deliver a difference of 25mm, 1″, or more!
Within these given factors, how does wheel size affect the complete bike?
Note: I am going to look at this from the aspect of the pure wheel size, forgetting about the tyre. I am doing this, as stated previously, because there are too many variations in tyre sizes and even pressures that people run to be able to realistically include them in such an analysis.
At their most basic, wheels in rotation beyond a given rotational velocity create gyroscopic forces. While it’s generally accepted that the gyroscopic forces generated by a bicycle are nowhere near enough to keep you upright (the frame and the rider far exceed the mass of the rotating wheels, hence the gyroscopic force created), they – when combined with trail – do create what’s known as a self rightening force, meaning that when in motion (and at a certain velocity), wheels want to stay upright and moving in the same direction. Here’s an interesting article in pdf that explains this pretty well.
Further, and in general, the larger and heavier the wheel, the greater this force (source article in pdf):
“The basic science is that the bits of matter that make up a spinning wheel travel faster and farther per revolution the further they are from the center, by a factor of the radius squared—reducing the importance of weight at the hub and giving the material at the tire/rim the greatest influence.”
Using this VERY basic calculation and not taking weight into account, a 26″ wheel has a factor of 676, a 27.5″ 756.26 and a 29″ wheel 841. Given that the weight difference between wheels can be tiny, on some wheels it can be 100 grams or less, these numbers indicate the difference between a 26″ and 27.5″ wheel is relatively small. While a 29″ delivers almost a third of the force again, a 27.5″ delivers barely an additional sixth, thus only a marginal increase of the gyroscopic forces that contribute to the self righting factor.
The big BUT that comes into play here is that much of these calculations work a treat for objects moving at a relatively high velocity but a mountain bike, for a majority of the time, does not. So taking this into account, we can assume that while the self righting force generated by mountain bike wheels do play a part in its stability, overall the numbers we are talking abut are relatively small, even between a 26″ and a 29″, thus in relation to the whole package (rider + gear + bike), the net effect is nominal.
So physics goes out the window; at least trying to say that by making the mountain bike wheel larger, it will automatically become more stable, is mostly bunk. What else can we look at?
One of the common cited arguments is rolling resistance, especially over rocks and the like. The argument is that the bigger wheels make rolling over obstacles easier and on the surface, this should be true. Scratch under the surface, we find that this may not be entirely the case.
Look at the below image (from How and why: Motorcycle Design and Terminology)
In crossing a depression, a larger wheel does not sink down as much as a small one, so in effect the rider is less aware of the wheel dropping, equating to a smoother ride. Additionally, with the wheel not dropping as far, the overall speed and acceleration of the bike is less affected; the bike feels faster, which is one of the reasons riders moving to 29″ wheels, in the right conditions, will claim that the bikes are faster.
But when it comes to bumps that go up, like rocks, things change. Logically, as two different sized wheels hit the same bump, they both have to rise the same amount in order to clear it. What changes though is the effort required to roll each wheel over the bump. For an equal speed, a larger wheel has a further distance to travel to clear the bump, the net effect being that the larger wheel experiences a slow in acceleration and hence overall speed. While this effect will smooth out the bump for the rider, it actually slows the bike down – a trade off of performance vs. comfort.
Looking at the diagrams, we can see a clear difference arising in the case of a 26″ vs. 29″ wheel, where the total difference of the hight of wheel centre (radius) is 1.5″ or 38.1mm (11.54%). The 29″ wheel will be faster over depressions, slower over rises. In the case of the 27.5″ wheel though, the difference is only 0.75″ or 19.05mm (5.77%). One has to ask, discounting variations in tyre size and even tyre pressures, is a difference of 19mm actually going to make a noticeable difference? One can argue easily that a near 40mm difference at the rim is going to have a noticeable impact, made greater if you allow for the tyre, but can the same be said for a 19mm difference, especially when you account for all the other dynamic and real world variables?
From this analysis, the claims that 27.5″ wheels provide greater rolling resistance over obstacles is numerically marginal at best, in the real world there is a very slight advantage, though how much could be debated – the extra 0.75″/19mm in wheel radius is not really enough of a change to provide any form of dramatic difference. So if the rolling characteristics of the wheels themselves are not the key, what other factors could be at play to substantiate the claims that a 27.5″ wheel is superior to the 26″?
The most obvious is the actual frame set up.
You don’t have to be a genius to work out that by making the wheels bigger, the frame, wheelbase, trail etc. etc. automatically changes. Depending on where you ride, swapping from a 26 to a 29″ bike changes the game quite a bit and for those that like the tight twisty stuff, the common complaint is that a 29″ is a handful. While the 29″ wheel itself has a little to do with that, the overall difference in handling comes from the adjustments to the frame. Adding in an additional 3″ per wheel extends the chainstay length, hence the overall wheelbase. The steering characteristics – defined by head angle, fork length and wheel offset – all also change. In general, the increased size of a 29er frame slows it down unless big changes are made to say the head angle.
To look at the effects, I took an existing frame design I had production drawings for and applied the needed changes to allow it to accept a 27.5″ wheel. I did not want to ‘fine tune’ but just make the needed changes to see what would happen for the same given geometry.
As you can see in the diagram, based on simple allowances, the trail increases by 5mm, BB to front +18mm and a chainstay length increase of 36mm. In all, these numbers clearly show that given the same frame geometry, the 27.5 frame will slow down and become more stable. The slight increase in trail and 54mm in overall wheelbase combine to make a frame that feels ever so slightly more stable at the bars and at speed, at the cost of some nimbleness you’d find in the same 26″ version.
Given some of the 27.5 frame frames receiving great reviews are running slacker numbers than the frame I have used for this example, it is natural to say that the bikes will feel more stable and planted, as many claim. Looking at what happens to the frame above if we slacken the head angle to 67 degrees, as some 27.5 bikes are now running, the difference is very noticeable.
The question I have here is, given the small/nominal performance increase in performance of the 27.5″ wheel, if we build a frame to deliver exactly the same geometry (wheelbase, trail etc. etc.) as that produced by a 27.5, would we notice the difference between the bikes in a blind test?
Of course there’s a caveat to all of this; naturally, right?
As I mentioned up front, the one thing I did not take into account in any of this was the tyre. The reason I did this, in case you forgot, was that despite some semblance of a sizing system, tyre sizes fluctuate wildly between brands, even within a give size bracket. On a 26″ wheel, the tyre choice can effectively add 25mm/1″ or more to the physical radius of the wheel depending on brand and size, so currently, depending on the tyres selected for the given wheels, a 26″ wheel and a 27.5″ wheel can be a lot closer in final radius than one would think. The reverse is also true of course, and the difference can be quite large.
Another factor that can fundamentally change the handling characteristics of a bike is the pressure one runs the tyres at. Many riders elect to run fairly low pressures, meaning the radius of the wheel is effectively smaller at the contact patch than at the opposite point – they are riding on a larger flat spot. So again, a rider running a ‘medium’ sized 26″ tyre at a higher pressure will be a lot closer to the rider running a 27.5″ at a lower pressure. In both these cases, the overall performance will also be affected quite significantly. The variables in the tyres themselves are numerous enough to make comparisons, especially ones made in isolation in a rather an unscientific manner, difficult at best.
As you can see, where we are talking the final radius of a wheel, tyres can play a massive role in the rolling wheel size.
And finally, the summary…
If we look at the pure numbers, forgetting the wild variations afforded by tyres etc., despite all the claims, the actual benefits provided by a 27.5″ wheel are small. The 27.5″ wheel it seems is more a case of where being in the middle just makes it a bit average, and if anything, possibly offers the downfalls of both the 26 and 29″, without enough of the upsides to warrant the bother or expense.
If one is going to test the 26 and 27.5 side-by-side, simply switching from a 26 to a 27.5 bike is just not enough – if anything it will deliver a false and misleading result as the 27.5 frame will have had more than just simple adjustments to its geometry, chances are it will have had changes made in an attempt to create a nimble bike with larger wheels, changes which create more of an overall performance difference than the actual wheels themselves. To factually test the real world differences, the two control test bikes require adjusted geometry so as to create exactly equal wheelbases, head angles, trail etc., only then can a direct comparison can be drawn. Unlike a 29er, where the difference is substantial enough to easily compare, the 26 and 27.5 wheels are so close that the control bikes demand equal geometry and overall setup to even begin accurate testing. As far as I know, to date this has yet to happen, or at least if it has the results are being kept a little ‘mum’.
I also offer up the idea that someone test a standard 27.5″ frame with both 26 and 27.5″ wheels, over identical conditions, and see what happens.
Overall, what does this say about the raft of reviews and the industry’s push into the 27.5″ wheel? You can draw your own conclusions, I have and I don’t like what I discovered. As such, I for one will not be replacing my 26″ bikes any times soon.
How fast is each wheel size over 1km?
Not allowing for the tyre or gearing (Ed. 2.1.14: or rider power output), just how much faster, over a totally flat, smooth road is each wheel?
Let’s find out…
First, we need the Circumference = 2 pi r
2 x 3.14 x r
2 x 3.14 x ([(26″ x 2.45cm)/2]=33.02cm)
1 revolution = 2.07m
1,000m / 2.07m = 483.09 revolutions
2 x 3.14 x r
2 x 3.14 x ([(27.5″ x 2.45cm)/2]=34.92cm)
1 revolution = 2.191m
1,000m / 2.191m = 456.4 revolutions
2 x 3.14 x r
2 x 3.14 x ([(29″ x 2.45cm)/2]=36.83cm)
1 revolution = 2.32m
1,000m / 2.32m = 431.03 revolutions
So to travel 1km:
26″ to 27.5″ is a difference of 26.69 revolutions, therefore the 27.5″ wheel takes 5.5% less effort
26″ to 29″ is a difference of 52 revolutions, therefore the 29″ wheel takes 10.8% less effort.
Please Note: We have closed comments on this article as many clearly misunderstood the points trying to be made, either through missing the point of what is being said or clearly just being posted to be argumentative.
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