Nothing influences the performance of a mountain bike as much as the wheel/tyre system. Only 80 to a maximum of 120 square centimetres connect the front and rear wheels to the ground. But it is precisely on this surface that all the forces that influence the bike and biker's movement come together: Drive, steering, braking and even impact forces. Nevertheless, wheels and tyres have not really been the focus of development work in recent years. However, this has changed with the emergence of the discussion about wheel formats. Suddenly, more fundamental questions were being asked again: How does the wheel/tyre system actually work off-road? Why do 29ers and fat bikes ride so differently? Do 29er wheels inevitably have to compromise on stiffness? Why were tyre pressures below 2 bar previously dismissed as unrideable? When fundamental questions are asked seriously, dogmas are always shaken. Hub standards are called into question, as are tyre widths and rim designs. This often results in completely new approaches. Some of them are even a little older, but have not been able to prevail in the past. Syntace, for example, initiated the discussion about wider rims five years ago - only now does the time seem ripe for such ideas to meet with wider acceptance. Other concepts such as fat bikes or the Plus formats, i.e. 2.8 to 3.0 inch wide tyres (see BIKE 10/2014), are still in their infancy, but are already producing amazing results - as our fat bike test in this issue shows. But completely new ideas are also allowed. Schwalbe and Syntace have just introduced Procore, a dual-chamber system for MTB tyres - a concept whose potential we take a closer look at in the innovation check on page 110.
Whether we will all be riding with 50 millimetre wide, single-walled carbon rims in a few years' time, with double-chamber tyres with a width of 2.8 inches - who wants to know today? What is clear, however, is that the industry is in a state of optimism and that designers and product managers are prepared to question the paradigms of the past. The following pages provide proof of this.
The three wheel trends of the current season
The developments that could point the way for the near future.
1. PLUS formats
Be it in 26", 27.5" or 29" - there are already products for each format that seek a "happy medium" in terms of tyre volume. Initially introduced as a completely separate standard with special frames and forks, the Plus formats are now also designed to be compatible with existing components, making them a genuine retrofit option.
2. fatbikes light
The name says it all: fat bikes are heavy. Whilst the geometries and handling are often already moving in the direction of trail bikes, the "acceleration" and "rotating mass" factor is still a real stumbling block. However, tubeless and lightweight construction offer initial solutions.
3. ever wider rims
Whereas a few years ago, the inner width of most rims was still 17 to 21 millimetres, the trend is currently pointing towards 30 millimetres. The majority of current models range between 21 and 26 millimetres - the more extreme models are already over 30 millimetres.
Interview Lutz Scheffer (bike developer): "Think about tyres again!"
BIKE: Where do you see blind spots in bike development?
Lutz Scheffer: Up to now, the focus has primarily been on the bike chassis and the contact points with the rider. Tyres and wheels have often been regarded as purely marginal phenomena. However, the tyre is not just the only point of contact between the bike and the ground. It can also do a lot as a suspension component. Precisely because the tyre suspension works almost without inertia and unsprung mass, it is superior to all normal suspension elements in terms of sensitivity.
Where do you think the road is going?
I see a solution in fat bikes, which fulfil many of the requirements better than conventional mountain bikes. Take traction, for example: with low tyre pressure, you have a much larger contact area. Such tyres are also much more forgiving in terms of line choice and limit range. And I am even very positively surprised by their smooth-running properties.
But what about the rotating mass?
Yes, the weights still have to come down considerably to ensure the handling is right. For the near future, I believe that lightweight 3-inch tyres with a pressure of around 1 bar on 27.5 mm rims have a good chance of success in all MTB categories.
Trend 1: Ever wider rims
"I'm convinced that in a few years, what we call extremely wide rims today will be the norm." Scott Nicol (Ibis)
No trend is so new that it won't be repeated at some point in history. In the early days of mountain biking, tyres were rarely wider than 2.0 inches (approx. 50 mm), but the rims often had rim widths of around 25 millimetres. In other words, a width ratio of 1:2. Then the call for lighter wheels caused the rims to become narrower and narrower. Somewhat later, tyres also began to increase in width - first to 2.1 inches, then to 2.2 inches, and nowadays tyres are hardly ever less than 2.3 inches wide. With a rim with an inner width of 17 millimetres, this already results in a size ratio of 1:3. As long as you were riding with stabilising tubes and high pressures, this imbalance remained relatively unproblematic. But since the advent of tubeless technologies, the trend has been towards ever lower pressures. As a result, the width ratio of tyre to rim became critical. The advantages of low pressures had to be paid for with a spongy ride feel and sometimes even with a sudden loss of pressure - or by continuing to bump along the trails at 2.5 bar. Jo Klieber, the mastermind behind Syntace, was one of the first to recognise this problem and put the solution on his agenda. He was already riding on rims with an inner width of 33.5 millimetres in 2009. However, it took time for this realisation to become a recognised trend. Only recently have other manufacturers followed suit and now offer rims in these dimensions.
Wide tyres on narrow rims result in a kind of "light bulb cross-section" (pictured right). The tyres need high operating pressures in order not to be deflected to the side when leaning or under lateral load. Under even greater loads, the tyre buckles sideways or the tyre bead is levered out of the bed - very often the cause of sudden air loss (burping) in tubeless operation. In the worst case, the tyre may even jump off the rim. Wide rims (pictured left) support the tyre carcass much better and keep the tyre stable even at low pressures.
Caution: Not every tyre harmonises with wide rims! While wider rims generally improve traction in the normal position, cornering behaviour depends heavily on the tyre cross-section. Round cross-sections usually become even more good-natured with wide rims, while already angular tyre profiles can sometimes become quite "unconventional" with very wide rims.
The diagram shows how the rim width influences the tyre pressures that can still be ridden - here for an average-weight rider (80 kg) with tyres around 2.3 inches wide. In particular, very wide rims with inner widths of over 25 mm can still be ridden at pressures at which puncture protection is already critical. The potential of very wide rims must therefore remain partially unused, or you need tricks such as special tyre designs or the Procore double chamber system from Schwalbe.
Our tips for tyre/rim combinations:
For optimum performance, especially in tubeless mode, tyre width and inner rim width should roughly match. If in doubt, however, it is better to choose a rim that is slightly too wide than too narrow:
1.9" - 2.1" tyres --> 19 - 23 mm*
2.0" - 2.3" tyres --> 23 - 27 mm*
2.2" - 2.5" tyres --> 27 - 30 mm*
> 2.4" tyres --> from 29 mm*
*Due to the stabilising effect, you can also ride slightly narrower rims with a tube.
Wide rims have advantages and disadvantages in practice
+ Noticeably more comfort at the same pressure due to larger air volume (alternatively: you can run one tyre size smaller with wider rims)
+ Increased grip due to larger contact surface with the ground (especially in combination with lower pressure)
+ Better guidance of the tyre (= more precise steering) at lower operating pressures, without becoming spongy or allowing the tyre to buckle (caution: remember adequate puncture protection!)
+ Less risk of sudden air loss (burping) even at low pressures
+ The larger rim cross-sections benefit wheel rigidity
- Wide rims of the same design are either heavier than narrower rims, or they are more susceptible to dents and defects for the same weight
Two factors where the rim width has no (direct) influence:
Breakdown protection: Here too, the measurements show that the rim width has no direct influence on the puncture protection. Once again, tyre pressure is the only decisive factor. This is precisely why wide rims seem to have punctures more often, because people like to drive on them at lower pressures, at which the puncture protection is already too weak.
Rolling resistance: The existing measurements on the tyre manufacturers' test benches as well as our own rolling tests show no relevant influence of the rim width on the smooth running of a tyre - at the same air pressure, mind you. However, the influence of tyre pressure is clearly measurable and has been proven many times over. And because a tyre with low air pressure also rolls faster off-road, the rim width has an indirect influence on rolling resistance - via the possibility of running lower pressures.
Wide rims: These models are currently available
The trend away from ultra-narrow rims with an inner width of 17 to 19 millimetres, which are more suitable for tyres up to 2.0 inches, has already been completed by almost all manufacturers. However, only a few have taken the step towards really wide rims with a rim width of over 25 mm.
Trend 2: Fatbike wheels light
It is a basic principle of cycling that lightweight tyres and wheels are good for a bike's performance and riding enjoyment. But nowhere is this more true than with fat bikes. With wheel weights that almost approach the total weight of a race bike, you have to accelerate a lot of rotating mass and keep it moving. This can be a real feat of strength and not least one of the reasons why fat bikes have so far eked out a niche existence as third or fourth bikes. The example of the Kuroshiro/Tune wheels shows what is already possible - in other words, there is still a lot of development potential in fatbike wheels and tyres. However, it is often the small tuning measures that have a big impact. Lutz Scheffer, Canyon head designer and self-confessed fatbike fan, swears by 24-inch downhill tubes (approx. 200 g) in his fatbike tyres, saving a good half a kilo in one fell swoop. Tubeless-ready technology, which is also emerging on fat bikes, offers the prospect of further savings.
Road test of the Kuroshiro Enso 685
Having fun with a fatbike on the Finale Ligure 24-hour race circuit? BIKE test author Chris Artmann had the exclusive opportunity to chase the Enso 685 carbon rims (approx. 425 g!) from the Italian luxury forge Kuroshiro over the rocky course on a fatbike. Innovative: Thanks to the special design of the spoke holes, the rim can be converted to tubeless using only sealant, and you can enjoy the triple benefit of no inner tube - lower weight, higher puncture protection and lower rolling resistance. In addition to the comfort typical of fat bikes at pressures between 0.5 and 0.75 bar and a good-naturedness that makes every all-mountain bike look like a nervous racing file in comparison, the super-light wheels bring completely atypical acceleration pleasure. Blessed with sheer endless traction, you can pull up even the steepest climbs with surprising ease - whether over roots or loose gravel. Even if this is an admittedly expensive tuning measure, it clearly shows the potential that still lies dormant in fatbike wheels.
Trend 3: PLUS formats
You simply have to take your hat off to Surly for being the first to dare to introduce an intermediate size alongside the burgeoning fatbikes and normal mountain bikes. In doing so, they left all predetermined pigeonholes and thought patterns behind. The oversized format can now be found in all three wheel sizes - in other words, wide tyres between 2.75 and 3 inches, preferably mounted on wide rims (35 to 50 mm). In terms of riding feel and performance, the plus formats could also be called "semi-fat bikes". 29+ is a rather exotic phenomenon because it requires special frames and forks - currently, 29+ wheels can only be ridden rigidly, with upside-down forks such as the RS-1 or with fatbike forks. The retrofittability in existing frames clearly speaks in favour of the two smaller formats. B+ (based on 27.5-inch wheels) and 26+ can often be easily installed in frames and forks of the next larger format. Our experience shows that you can give the familiar bike completely different characteristics, which should appeal to touring and leisure bikers in particular, but also to enduro riders.
Advantages and disadvantages of PLUS format bikes
+ Clearly noticeable increase in traction & comfort compared to standard tyres
+ Very good-natured driving behaviour, especially on loose surfaces; high driving safety
+ 26+ and 27.5+ tyres are usually compatible with bikes and components of the next larger size (can be retrofitted)
- (Still) little choice of tyres and wheels
- 29+ needs special frames/forks
There are currently hardly any Parts for the Plus formatsbut there are already some exciting new products coming onto the market in 2015. We show you the most important ones:
