Why Choose Wide Rims?

By Michael Hanslip | 9 April 2015

Do wide rims have merit for general trail riding or are they just for downhillers and those who want to run big, burly tyres? Michael Hanslip puts them to the test.

It seems that we’ve always had new trends and technical advancements vying for our attention (and cash) in mountain biking. Early in the 90s it was suspension forks; initially they were viewed with great scepticism but now it’s rare to see a MTB without. By the mid-90s it was disc brakes and I can remember one MTB magazine editor proclaiming that, “Disc brakes are a fad”. Well we all know what happened there…

More recently we’ve all witnessed the wheel size debacle, which finally seems to be simmering down. So what’s the next big thing? Of late you may have noticed a number of brands spruiking wider rims; we’re not talking full-on fat-bike material here but 30-40mm wide rims that are designed for regular MTB tyres. Are these a passing fad or will we all be riding super-wide rims in the future?

Fat History

The funny thing about this trend is that wide rims are not new at all. Back in the very early days of our sport, guys like Gary Fisher and Joe Breeze were riding modified beach cruisers down steep gravel trails of the Marin County area in California. These bikes rolled on 26-inch wheels with 25mm wide rims and 50mm wide tyres—that’s a pretty wide rim for the tyre width.

Problem was, beach cruisers weren’t high performance machines and they used ‘single wall’ rims. Not very light and not very strong, so they didn’t measure up for rugged off-road riding.

In 1984 Keith Bontrager, another early mountain bike enthusiast, took a Mavic MA2 tandem road rim and re-formed it into the 26-inch size. The MA2s were high quality ‘double walled’ rims; they were not only lighter but also a good deal stronger than the beach cruiser rims.

Being road rims they were also a lot narrower than the cruiser rims, measuring only 14mm between the bead hooks. This single act of experimentation set the trend for mountain biking for at least two decades to follow—it didn’t have to be that way and we are only now ‘correcting’ this historical mistake.

Why would I refer to it as a mistake? Well look at virtually any other form of wheeled transport with pneumatic tyres; cars, trucks, motorcycles and even road bikes run a rim width that is only slightly narrower than the tyre. Obviously it varies but the ratio is generally around 1.5:1. With a car, a 150mm rim supports a 200mm wide tyre. By comparison, many of us have been running XC rims that measure just 17mm in between the bead hooks with 53mm wide tyres—that’s more than 3:1 and more than double the ratio that you’ll find in most applications.

When mounted on a narrow rim, your typical 53mm wide tyre assumes the profile of a light bulb and the sidewalls angle inwards away from the tread. The same tyre on a wide rim has much more vertical sidewalls that are better able to support the load of the bike. It doesn’t end there, however. Stand with your feet close together and have someone push on your shoulder; it is easy to unbalance you. Now move your feet a metre apart and have the person push again; you are much more stable. It’s the same with the tyre beads; if they are barely a centimetre apart the whole tyre will flop from side to side when cornering. Double the width of their support base and the tread will be far more stable and better able to resist side loads.

To use another analogy, think of the tyre casing as a balloon; hold it down near the narrow part and give it a flick from one side—it’ll wobble around everywhere. Now grab the balloon up near the wide part and the whole thing is far more stable, even though the air pressure inside hasn’t changed.

Once was Skinny

So that’s the theory anyway, and it’s not like this knowledge is something entirely new to the MTB world. When Keith inspired the narrow rim trend with his modified road rim, most mountain bikers were running 1.7 to 1.9 inch wide tyres (44-49mm). Back then, if you were on 2.1s you were some wild and crazy extreme downhiller dude. These days 2.1 and 2.25 tyres are commonplace in both XC race and trail applications and rim widths have grown to reflect this.

Where 17mm (internal) rims were the norm a few years back, most brands now have 21 and 23mm rims as their new ‘wide’ offerings. A few millimetres wider mightn’t sound like much but it’s a 25% increase. Some may consider a 21mm rim to be ‘wide’, but the corresponding trend to wider tyres means that these rims are just maintaining a similar rim-to-tyre width ratio as what we had in the early 90s. What we’re putting to the test here are properly wide MTB rims; those that measure 28mm or more between the bead hooks.

I’ve had plenty of experience with narrow rims and one of my bikes still has 17mm rims, so I’m all too familiar with the results. Running a tubeless setup allows the use of lower tyre pressures without pinch flat punctures, but drop the pressure too much with a skinny rim and you’ll suffer with a host of other problems.

The main one is tyre burping; that’s when a side load on the tyre pulls the bead away from the rim and lets some of the air escape. Previously I’d assumed that this was a shortcoming of the tubeless system but experience has since taught me otherwise.

Beyond the burping, there was a general feeling of disconnect between bike and dirt. Lower pressures are great for added traction and comfort but the loss in steering precision always prevented me from dropping too much air out of my tubeless tyres. These traits aren’t as prominent on the 19mm internal rims that I also own but the feeling of disconnect is still there.

Measuring Up

For this comparison of wide and narrow we used my 100mm travel dual suspension 29er as the mule. On the skinny front I used my tubeless compatible 19mm internal wheels. The new school wide wheelset was supplied by German brand Syntace. We used their W35 wheels which measure 35mm across the outside of the rim and 29mm between the bead hooks.

For consistency and easy swapping between wide and skinny wheels, both were fitted with brand new 2.25 Nobby Nic tyres—a popular model and width for present day trail riding and XC use.

Once installed the tyres looked substantially wider on the Syntace wheels but this proved to be more of an illusion. On the narrow rims the tread blocks measured 56mm from edge-to-edge compared to 57mm with the Syntace wheels. There was a bigger difference in the volume of the tyre casing; it grew 4mm from 54mm up to 58mm. Overall the tyre did get wider but it wouldn’t be enough to create clearance issues (unless you already faced a distinct lack of clearance that is).

As it happens there was very little weight difference between the two wheelsets. Some wide rims may incur a weight penalty but in this case the similarity allowed me to focus solely on discerning the differences created by the rim width.

For my 90kg weight, I tend to run relatively low tyre pressures; generally it’s 21psi on the front and 24psi on the rear for my own 19mm wide wheels. For consistency and not introducing too many variables in this comparison, I initially chose to stick with the same tyre pressures on both wheelsets—again we’ll just be isolating the difference created by varying the rim width.

Thick vs Thin

Over a few months I alternated between wide and narrow rims and in terms of ride feel, there is a discernible difference. Despite running both at the same air pressure, the tyres on the wide setup always felt firmer. It was as if they were overinflated, yet at the same time they also offered excellent grip—two traits I didn’t know could go together. The wide rims had completely removed the feeling of disconnect; it was a bit of a revelation for me because I never knew what caused it.

Where running lower pressures on a standard setup would nearly guarantee burping or tyre squirm, the wide rims allowed me to lower my usual pressures by 3-4psi. Even when running less than 20psi there was no vagueness in the steering and the tyres didn’t feel sluggish or underinflated. However, the improvement in traction was tangible and in rough terrain the tyres felt ‘planted’ on trail with less bouncing and pin-balling off obstacles.

Speed & Efficiency

Many people associate wide rims with gravity oriented use. Cross-country riders may fear that the wide rim profile will increase rolling resistance by changing the tyre profile and how it sits on the trail.

To assess this, I did all of my hill work on the skinny wheels one week and then used the wider option for the following week. I used a power meter to keep my efforts consistent and a GPS to monitor my performance. For straight up climbing on a steep fire trail, there was no difference between the two setups.

For me the real test was putting them into action during a three-hour club race. I rode the first half on the event on the wide rims, then swapped to the skinny rims for the remainder. With the assistance of Strava and numerous pre-existing segments on the Mt Stromlo trails, I was able to assess the two wheels in climbing, descending, fast, slow, technical and smooth sections of the course.

Over time I got tired and my climbing speed decreased – I expected that – but there was also three really fun descents per lap with very little pedalling. In the end I was surprised to find no discernible difference based on wheel choice. Across a half-dozen timed sections (three flat and three descending) there was only one or two seconds’ difference between the segments with no pattern to suggest that either option was faster. The only variance was that I lost the front end and crashed during my first skinny-rim lap because the front tyre slid a bit more than I was used to with the wider wheels.

Prior to undertaking this test I assumed that one setup would be clearly superior in its performance against the clock. The reality is that even with GPS recording and power data there was no clear champion.

Of course for most of us, mountain biking is more about the experience than analysing Strava segments. In this regard the improved feel and feedback from the wider wheels would undoubtedly lead me in that direction. Add to this the ability to run lower tyre pressures for better traction and comfort without degrading the handling and I feel the wide rims stand as a clear winner. Knowing that there’s no real speed disadvantage only helps to cement this position. In fact there’s every chance that they may offer an against the clock advantage when faced with trails that are more technically demanding, as that’s where the traction and handling benefits are more likely to shine through.

How Wide is Too Wide?

Are there limits to rim width or should fit fatbike rims to our regular trail bikes? As mentioned earlier, most vehicles run a 1.5:1 tyre to rim ratio. That’s like mounting your 55mm wide MTB tyres to rims that measure 37mm betweenthe bead hooks. In other words, you’d need to fit an extraordinarily wide rim to do anything weird to the whole tyre/rim interface.

Some tread patterns will perform differently on wide rims. With particularly square topped tyres, it may put the cornering knobs in contact with the trail slightly earlier in the piece. There’s nothing to suggest that this is good or bad; it’s simply different and whether you like the handling with a particular rim/tyre combination will be subjective.

There’s almost always a handful of extra grams in a wider rim and this may deter weight conscious riders. On this front, extra-wide carbon rims can be very strong and stiff whilst matching the weight of a narrower alloy model—you’ll just need very deep pockets to buy them. Be aware that carbon rims tend to have very thick bead hooks, so an impressive looking 30mm wide carbon rim may only offer a fairly traditional 23mm internal width, and it’s the internal measurement that really matters.

You may encounter alloy rims that are both wide and light, but a thin-walled alloy rim structure could be more dent-prone than a narrower rim of the same weight. This is something that needs to be judged on a case by case basis. Our Syntace W35 wheels came in at 1,890g for the pair and withstood an absolute pounding, but they use a 560g rim that’s clearly very solid.

If weight is an important consideration, you can also use a narrower tyre to save some grams. The overall package weight may be the same (or potentially lighter) than a fat tyre/narrow rim combination, but you’ll gain added traction and squirm-free cornering at lower tyre pressures—a better proposition overall.

While the wide-rim advantages are pretty clear, those who ride hard on rugged and rock-strewn trails need to be wary of going too low with their tyre pressures. Dropping below 18psi with a 30mm wide rim mightn’t create handling problems, but there’s less to stop a big rock bottoming the tyre on the rim. Even if you run a tubeless setup it’s still possible to ‘pinch flat’ the tyre casing if you hit something hard enough.

As a result, going super-wide can be a case of diminishing returns. Changing from a 17mm rim to 24mm will provide a distinct improvement in stability and allow you to drop the tyre pressure by a reasonable amount. Jumping the same amount to a 31mm rim will allow you to go even lower with your tyre pressures but there comes a point where you’ll start bottoming the tyre on the rocks. This mightn’t be a consideration if your trails are soft and demand good flotation and traction, but rocky terrain may force you to keep a little extra pressure in your tyres.

My experience has made it pretty clear that wide rims are much more than a passing fad to extract cash from poor mountain bikers. It’s more a case of righting the wrongs of two decades ago that saw us all mounting balloon like off-road rubber on road bike rims. We’re not saying that you need to rush out and sell your skinny rims but given a choice, there’s clear merit in choosing the fat option when looking for a new wheelset.