Mixed-Wheel Puzzling – It's About More Than Clearance

Mix-wheeled mountain bikes aren’t new. Specialized's Big Hit model had a smaller rear wheel than front way back in 2001 and so did the 2003 Transition Dirt Bag. Others, including the Brodie 8-Ball, also adopted the 26-inch front, 24-inch rear wheel setup and many of these bikes were around for several years. But with racers at the time shunning the idea, mixed-wheels were largely left to ‘freeriders’ and they didn’t embrace the idea much either.

Then bike engineers started messing around with the size of our wheels with large 29-inch hoops showing up and shortly after the 650B/27.5-inch wheel was pushed hard, essentially wiping out 26-inch wheeled performance mountain bikes.* As bike shapes and technology progressed, the push to 29-inch wheels has become more palatable but there remain a few undeniable traits to the larger hoops that some riders prefer to avoid.

Without diving too deep into the differences between 29 and the slightly smaller 27.5inch wheels, some key differences to the larger hoops are, they’re heavier for the same wheel, have a wider turning radius, and take more effort to accelerate. In simple terms, they can feel less dynamic on the trail, because of their size and all the traits that come with that.

But there are benefits to the larger wheel including an improved approach angle to an obstacle, effectively ‘smoothing’ the same elements of the trail that a 27.5-inch wheel will generally find rougher. There’s also a larger contact patch which translates to more grip and people also claim it provides taller riders with a more suitable fit.

*Slopestyle, dirt jump and some freeride bikes still use 26-inch wheels but generally, it’s been left to cheaper department store bikes.

Problems With The 29er Fit Statement

As someone who is considered reasonably tall, I can’t deny the better proportional fit of 29-inch wheels in a bike suited to my size. At 191cm, I’m not the tallest but it wasn’t until jumping aboard extra large bikes with 29-inch wheels that the proportions appeared better. Before that, many of my extra-large bikes looked strange with the smaller wheels. I also don’t disagree that bigger riders can generally work the bike similarly to someone considerably shorter on a smaller bike with 27.5-inch wheels, but it’s often not so simple.

I’m not fond of people trying to pigeonhole riders to a wheel size based on their height. We’ve all heard comments along the lines of 'Jenny’s short, she should be on a small wheel’ or ‘at Johny's height, the 29er works best.’ There is merit to the idea that a rider can experience benefits with a particular wheel size, but Jenny might want the calmer ride quality of a 29-inch wheel and be riding trails that don’t make clearance a problem and Johny may want to ride down the trail like a Jack Russell terrier high on caffeine.

When my G16 was built with 27.5-inch wheels, the bike performed sensationally. Until that time, I hadn't experienced such grip and composure from a bike with smaller wheels. The shape of the G16 helped it perform better than the available size extra large bikes with 29-inch wheels at the time. For that reason, Chris Porter was reluctant to even try larger wheels, claiming the grip and stability afforded by the shape of the bike made up for it.

But as I discovered, once the bigger wheels were slapped into my G16 the bike exhibited a calmer demeanour on the trails, allowing for increased speeds with a more relaxed approach. I needed to adjust my brake points for the effect of the larger wheels and recalibrate for the change in trail feedback, too.

Overall, I found the benefits provided by the shape of the bike with the smaller wheels, translated to the bigger wheels. But it was less dynamic on the trail and that’s a ride quality I enjoy when goofing around, which is most of my time on a bike these days.

The G1 Opened More Possibilities

When Nicolai/GeoMetron announced their updated version of the G16, the G1, they claimed it to be one bike with many possibilities. I was happy with the G16 but in extra physical sections of trail, or on longer runs that required constant working of the bike to remain upright, I still found the cockpit a touch small; the wheelbase was fine, though. But the thing I wanted most was to lower the BB. The G1 achieved this while offering heaps of opportunities to adjust the geometry to best suit a rider’s preferences. Having the ability to test so many shapes and configurations was the nail in the coffin and I decided, after less than two years on ‘the last bike I plan to buy’ I was already moving on.

The G1 allows owners to adjust geometry via a chip system that Nicolai refers to as, Mutators. These mutators are located at the end of the seat stay and chainstay. The benefit of the system is the bike can switch from full 2-9 to mixed-wheel, to full 27.5* and maintain its geometry across each change.

*The 27.5-inch wheel setup requires an additional external lower headset cup to bring everything back to the correct height

Subtle effects on the suspension can be felt when changes are made, particularly at the seat stay but they're minimal and for some riders, these may be beneficial. The most notable I found was with the longest mutator settings in the seat stay, which made the suspension more progressive. The downside for me was less control when hammering deep into the travel in rough terrain and it was a bit too light off the top. But overall, the differences between changes are subtle enough that the ride quality remains good.

Moving to the G1 has helped me better understand the effects of geometry, how different bike shapes can react, and what changes to the frame versus suspension can do for ride shape. Something as simple as changing the spring rate in the rear has a large impact on dynamic bike shape but until I was able to bust open a world of possibilities on the same bike, it wasn’t as clear to me. Armed with a bike that can be set up in many different ways, I’ve spent the last 12 months testing the mix-wheel setup vs full 29, maintaining the geometric settings as I went.

A Couple of House-Keeping Bits

To begin with, I should share where I landed for the 29er settings on the G1. With the bigger wheels, I eventually settled on the shortest of the three chainstay mutators, with the 3.5mm seat stay mutators. This change brings the rear-centre down from 453mm to 445mm and pitches the seat tube and head tube angles forward less than half a degree; STA of 79.4º, HTA of 62.9º. The bike can also change travel with a switch to the position of the rear shock eyelet in the rocker link. Throughout this experiment, I’ve remained in the 162mm travel option.

The above has been my preferred setting for the G1 with 29-inch wheels. I didn’t want to lengthen the front centre of the bike by removing the 3.5mm seat stay mutator (slackening out the bike) and running it taller wasn’t of interest after trying it with the stock 6.5mm seat stay mutator and shorter rear centre. The reach is more complex to calculate but with my setup including 17.5mm of spacers beneath a 35mm stem, the actual static reach was ~558mm.

It’s also worth mentioning that I ride with flat pedals almost exclusively these days. Because of this, I often find myself looking to sit a bit deeper into the bike than when I rode clips. With clips, I found myself able to relax in a slightly taller position up over the bike because my feet were connected. Flat pedals result in me pushing the bike differently through the terrain and having to be deeper to remain well connected.

Mixed-Wheel vs Full 29er

There are positives and negatives to every bike setup and despite being able to maintain close to identical bike shape when making changes during this experiment, there were elements I didn’t like from each. Yes, as a taller rider I generally have no issues with clearance, though I’ve still had some good rubber burn on the back of my legs from excessive body English. How the bike behaves between each wheel setup is significant, though. I found the 29-inch wheeled bike great in most situations but there were times where I wanted to ‘back in’ the rear of the bike more. When entering a corner, the full 29er wants to carve a more planned, well-executed path, and that path is often longer.

I noticed this quality most when visiting my favourite place to ride, Pemberton, B.C. The trails there are fast, often steep and still rough like the neighbouring towns to the south. There’s a more raw feel to Pemby trails and when making direction changes, they often require a longer approach than equivalent trails in my home network of Squamish. Despite the generally more open corners of Pemberton, I found myself wanting the rear of the bike to complete the turns more quickly than was possible with the 29er setup.

I don’t recall once noticing it to such an extent on my home trails, perhaps because the speeds are slower. I wanted the bike to complete turns more quickly when riding at home but that was purely for my preferred ride dynamics. In Pemberton, the longer turning radius was so pronounced in multiple sections that it hindered me. Understanding that a smaller rear wheel should effectively complete a turn in a shorter amount of space, it was the perfect time to dig into the experiment.

Initially, all I changed was the wheelsize and the seat stay mutator to bring the STA, HTA and bottom bracket height back to the bike’s stock static numbers, which I was no longer riding with 29-inch wheels. With the 10mm seat stay mutator installed, the bike sat back a bit more than my preferred 29er setup and ever so slightly more than the stock 29er; the HTA slackened off 0.6º (0.1º from stock 29) to 62.3 degrees and the STA 0.9º (0.4º from stock 29) to 78.5º.

The changes to the front centre of the bike worked against what I was seeking with my 29er setup – it grew longer – but despite this, it was clear I wasn't fond of the smaller rear wheel with the shorter rear centre. Across cambers especially, I found the rear wheel struggled for the level of grip I wanted. It felt skittish and I felt the difference between the front and rear centres was too dramatic. Adequately weighting the front wheel became more difficult.

Although the negative was a big one for me, the bike certainly cornered with the character I was after. It could now pivot through an arc rather than take the longer, quieter route and if I mucked up and got the line into a corner wrong, approaching a little tight, I could ‘back it in’ by loading aggressively and changing the arc more quickly. This shifted the ride dynamic of the bike in the direction I wanted and despite the negatives with the short and small rear setup, the potential was clear.

I rode home already plotting the next move. Not thrilled with the short chainstay, I knew the rear-centre needed to be lengthened, bringing back a better balance between the front and rear of the bike but hopefully not adversely affecting the more dynamic cornering traits the smaller wheel had uncovered. I’ve heard riders claim the small rear wheel with a longer chainstay to be the same as a large rear wheel with a shorter stay. In my experience, this isn't an accurate statement.

I pulled the 33mm chainstay mutators off the bike and bolted the stock 41mm option back on, switching the rear centre from 443mm to 450mm. This change lowers the bike, slackening the seat and head angles.* To counter these effects, I lengthened the seat stay 5mm. With the longer chainstay, traction improved but it was still much easier to lift into a manual, hold in a wheelie, and the bike exhibited a greater dynamic in corners or whenever switching direction.

*Chainstays are rarely level to the ground, especially when the bike is loaded. Moving the rear axle further back can lower the bike while slackening the HTA and STA. Shortening the rear raises it and steepens the angles.

Another element I haven’t mentioned was the noticeable effect of the smaller rear wheel when picking up momentum. It was quicker to spin up to my preferred cadence with the smaller rear, requiring less effort and shorter bursts of energy to get the bike back up to a given pace. It also required less effort to maintain a pace when climbing compared to the full 29er. But I still wanted more from the bike. I was now riding the stock geometry with the longer 450mm rear centre and I wanted to shorten the front centre.

After getting the all-clear from GeoMetron to stack the seat stay mutators, I slung the smallest 3.5mm option in with the 15mm for a total of 18.5mm. This change brought the static numbers back closer to where I preferred, shortening the front centre a touch. It raised the BB height, and the feel of the rear suspension also changed. With the longer seat stay, the suspension felt lighter off the top, dynamically sitting a touch deeper and it was more progressive through the stroke.

I was beginning to really enjoy the more light-hearted approach of the mix-wheel setup. The small rear wheel provides cornering characteristics I enjoy while the larger front keeps the grip and stability high while also smoothing out trail feedback better. The rear has hung up more than with the 29er setup and short chainstay, but it’s something I've mostly been able to resolve with rear shock adjustments. It rides less ’seriously’ with the smaller rear wheel, opening things up for more party-time in the woods.

Still not content with the setup, I kept puzzling. I wanted to see if the bike could ride more consistently and smooth the progressive energy increase in the rear suspension. The bike came with two springs for my weight – 325lb and 350lb options – and I’d been riding it with the 325lb option. With the heavier spring installed and the mutators left the same, the bike rode tall and the front became nervous in rough or steep terrain.

Although I didn't find the 15mm seat stay mutator ideal with the lighter 325lb spring, I decided to give it a crack with the stiffer spring. Increasing the spring rate raised the dynamic height of the bike and looking back at my notes from that first ride, I was immediately stoked with the change. Aside from some back and forth testing to finalize this article, I spent most of the time with mixed-wheels in this setting.

With a longer rear centre and stiffer spring, I achieved the balance I sought from the bike and the increased dynamic ride height kept the front centre in check. Despite the static numbers being slacker with this hybrid setup, the bike handles well, with a similar balance across the wheelbase to my preferred full 29er setup. It’s fun to ride, and while the large wheels have their benefits, I found them a bit too serious once on a bike that fits comfortably.

Summary of Findings

A large takeaway after almost an entire year experimenting has shown me that simply slapping a smaller wheel in the rear or a larger wheel to the front is not necessarily going to provide the best ride. Even considering axle to crown height or flip chips to raise bikes and keep the geometry close to stock doesn't seem to be enough. Will it be able to pivot through turns and change direction more quickly? Yes, but in many cases, I believe there's more to consider.

In my experience, the front centre can feel too big when a small rear wheel is put in a stock 29er without extending the rear centre to compensate. The dynamic, lighter riding characteristics of the small rear wheel transfer well to the longer rear-centre, but there's also improved balance across the bike. Not extending the rear when mixing wheel sizes hindered performance for me and it was harder to sufficiently weigh the front end.

A bonus finding from the experimentation was how large an effect the front centre has on a bike. I never once had a problem with the head angle on my roughly 15mm shorter G16 and even ran it down as low as 62 degrees without fuss. But with the XL G1, the growth to the front of my bike and the 62.5-degree head angle became too much. Lengthening the rear-centre helped correct this, lifting my feet and effectively pushing my weight forward. But the longer of the two rear centres I ran on the 29er didn’t achieve the ride I wanted.

I could go on for days about my experiments with bike shape but to summarize, I strongly feel the front centre measurement is often overlooked and focusing on head angle alone doesn’t tell enough of the story. Some manufacturers are sharing their front and rear centre measurements, which is fantastic, but the majority are sticking with the typical reach, top tube, stack and head angle numbers for the front of the bike. This ignores the reality to what I'm learning is a vital element and one that has a large effect on the ride dynamics of a bike.

But that’s a bonus from the past year experimenting. The mixed-wheel bike setup can provide riders with the best of both worlds; fun and control. The front wheel is more planted and calmer on the trail, while the smaller rear wheel allows the bike to be more dynamic. It pivots more tightly through corners and I generally find it to be more fun. Mixed-wheels are about more than clearance, they’re about having a choice over ride characteristics, because not everyone wants the more muted ride of a full 29er.

I once enjoyed a Top Gear video where they put an Audi R8 against a Porsche 911 Carrera and something Richard Hammond said caught my attention; “It’s not just about the result, it’s about the sensation along the way." The R8 handles corners sensationally, its tires gripping the surface like a dog gripping its favourite bone, but the 911 isn’t quite as calm, involving the driver more because it requires more focus and input to handle.

This can also be transferred to the difference between wheel sizes. But with a mix of 29 front and 27.5 rear, riders are no longer stuck at one end or the other and can blend elements of the two. It's time we dismiss mixed-wheels as primarily a tool for improved clearance because there’s far more to it than that. They're great fun on the trail and although at times I miss elements of riding full 29 and full 27.5, I'm generally happier with the qualities of this wheel-size hybrid.