In recent years there's been a plethora of new sizes and variations. One of these is 27.5+, AKA 650b+. These "plus" sized tyres are roughly halfway between a traditional trail tyre and fat bike tyres with widths somewhere in the 2.8" - 3.2" region. The idea is to give some of the benefits of a fat tyre without the drawbacks. In order to use these tyres a wider rim is required. As ever with something "new" there isn't a huge choice when it comes to either rims or tyres but this is slowly changing. By a happy coincidence the overall diameter of a 27.5+ wheel and tyre combo is close to that of a normal 29er wheel and tyre which means that many 29er frames can run with the plus sizes, the limiting factor is the width of the tyre since the frames and forks weren't designed with them in mind. Again this is changing, the Mk2 Solaris for example has slightly wider chainstays to allow for the plus tyres though the Mk1 (which I have) is compatible if a little "close".
Until recently if you wanted to go "plus" then the only real option was to build your own wheels using one of the rims available but Alpkit now sell a set of 650b+ wheels called the Rumpus for the bargain sum of £199. Definitely worth a punt.
One of the problems with plus tyres is figuring out if your rim/tyre combination will fit in your frame or forks, some 2.8" tyres come up wider than tyres marked as 3.0" for example, generally unless someone has actually tried the combination you are looking at using and has come up with figures then it's just a guess. After some searching around I settled on a couple of WTB tyres: a Bridger 3.0" for the front and a 2.8" Trailblazer for the rear. I knew that the Trailblazer was likely to fit as it's the tyre that Cy at Cotic used when trying out plus tyres on a similar width rim to the Rumpus (45mm) but the Bridger was a bit of a punt as I hadn't come across anyone using it with my forks.
First impressions of the Rumpus wheels: sturdy but not that heavy given that they've obviously been built to a price. Front and rear were set up for 15mm axles whereas I have a 9mm QR on the rear so had to swap out the provided axle converters which is pretty easy. It's worth noting that the QR converter on the drive side does not have the flats for a cone spanner to work with which may make converting the other way a little tricky as it is a threaded interface.
Running tyres in tubeless mode means using an air-tight rim tape to prevent air loss. Finding 45mm wide tape isn't easy(!) In the end I took the Gorilla Tape option, this is a cheaper method than the branded tapes (which are just generic industry tapes repackaged) but potentially a bit messy as it has contact adhesive on one side but since I've no intent of changing tyres on a regular basis this shouldn't be a problem. The rims are quite shallow so I only needed a single wrap of tape.
The Bridger comes up about 12mm less in diameter than a Bontrager XR4 on a Hope XC rim. The Trailblazer is about 23mm less overall diameter, again on a Hope XC rim. The front weighs 2805g (with disk rotor) compared to the Bontrager XR4/Hope combo weighing 1960g, it does add noticeable weight to the front of the bike. The Trailblazer weighs 2580g (inc rotor but not the cassette). I checked the Bridger in the forks and there's a couple of mm clearance to either side. Something to note is that with the rims being approximately 40mm less in diameter means that the widest part of the wheel may not be at the same point as with a 29er. The widths were as follows:
The XR4 (@ 25psi) measured 59mm at the tread
The XR3 (@ 27psi) measured 56mm at the tread.
Given the quoted sizes this difference isn't too far from what's expected and are spot on (to within 0.5mm) of the stated width.
The Bridger (@12psi) is 74mm at the tread, the sidewall is very roughly the same, possibly slightly less.
The Trailblazer (@13psi) is 69mm at the sidewall and 58mm at the tread.
So despite being quoted as a 2.8" tyre, the Trailblazer is only 2mm bigger than the XR3 and is narrower at the tread than the XR4! Of course it's a lot bigger in volume.
|Bridger 3.0" on left, Bontrager XR4 2.35" on right|
The smaller diameters of the wheels has two effects. Firstly the gearing is slightly lower by about 4% which is roughly half of the difference between two gears. Secondly, the bottom bracket is lower: take the average radius difference of the above values: Front radius difference = 12/2 = 6; rear = 23/2 = 11.5. So each axle is lowered by that amount. With the BB being halfway between these it's roughly halfway between the two: (6 + 11.5)/2 = 7.75mm, call it 8mm.
Cost of the whole caboodle:
Rumpus Wheels £200
650b+ tyres £ 95
Disk rotors £ 33
Valves £ 9
Rim tape £ 4
So what difference does it make to the bike? Is it faster, slower, more or less responsive? The only way to find out is to ride it so I came up with a loop with a variety of surfaces and plenty of climbing and descent that should give me an idea of what to expect. First though I needed a baseline so headed round it one afternoon. Whereas many wheel/tyre comparisons concern themselves purely with speed I was more concerned with overall efficiency so wore my HRM and logged that data as well as speed. On subsequent loops I'd try and keep my heart rate similar so that I'd see if the wheels really did make a difference.
So using Strava as the comparison tool and wearing a HRM so that there's a second metric to compare other than just speed this was the ride using the 29er wheels -
There was a strong SouthWesterly and due to recent heavy rain the ground was wet with lying water but underneath the surface dampness it was pretty solid, not muddy at all. I think most of the PRs are due to being on my own rather than in a group and having to wait at gates etc. I didn't really push it - at most times I would have been able to hold a conversation, but equally I wasn't dawdling.
With the new wheels fitted I headed out again. I'd had the bank holiday weekend to get a little familiar with them but I wouldn't say I'm completely up to speed with them, though ... In the intervening eleven days things have dried out completely, now rather than compliant damp turf you had rock hard ridged soil. This time the wind was from the NE so into my face on the early climbs. Again I was keeping things at about the same perceived level of exertion and the HR figures bear this out, some are slightly higher some a little lower. This is the B+ ride
and I'm somewhat shocked to get so many PRs. Remember that the B+ wheels and tyres make the bike over 1.5Kg heavier.
I expected it on one or two segments - "Boss Moor techy climb" for example is ideal for fatter tyres being stony with a few larger loose rocks lying on the surface leading to a rutted section for example. On the Mastiles Lane climb it was just a case of sitting down and spinning your legs, very little back wheel spin/slip. On the descent from Weets Top there is a very short and steep climb up from a beck, slightly loose, I have never managed to get up it. This time it just wasn't a problem (the parade of cows and their calves heading up the track shortly after was a different matter). There is a similar loose section at the start of the last climb which since it got washed out I've never managed but again nearly got it. So the grip is amazing - keep your weight just forward of the back wheel and so long as you can pedal you'll get up things.
The tyres make the bike feel "plush" for want of a better word - on ground where with normal tyres you would get a lot of high frequency chatter it's just all smoothed out. Some of the descents that normally have my eyeballs shaking in their sockets were silky smooth.
So if those are the positives, what are the negatives? Well on the descent from the top of Mastiles I hit a rollercoaster of bumps and hollows and the undamped suspension of the rear tyre set me pogoing and threatened to bounce me over the front. This made me a bit wary on some of the similar stuff later on. The little bit of road on the ride was again hard work - the largest section had the wind on my back and it felt harder than riding in to the wind on the 29er wheels. A few days later Cath and I went out for a bivvy ride, part of the ride out was along a gently rolling road. Whereas normally I'm having to coast a bit to let her keep in touch that night I had to pedal quite hard just to keep up.
Another area where they aren't as precise as normal tyres is in mud - you get a weird combination of wallowing and twitchiness, you just have to make sure you are putting pressure on the front of the bike otherwise the front wheel has a mind of its own.
It's early days. I haven't got the pressures dialed in yet, particularly the rear tyre, and when you are running at 12-15psi just one psi up or down can make a big difference. I think that for routes/rides that are predominantly off-road and composed of tracks of packed hardcore or a little rougher then they are well worth considering. For any route with a significant amount of road normal tyres would be preferable.
One thing to watch out for is the new gear "halo effect", basically being ultra keen because you are using new gear. Since I needed to get used to the tyres I ran with them for a few weeks before revisiting the loop. I also needed to get the pressure sorted, at the values in use on these large volume tyres a single PSI change can have a massive impact on handling. The starting point for standard MTB tubeless tyres is (weight in stone x 2) then subtract 1 for the front and add 2 for the rear. For me this is about right, certainly for the back tyre. For plus tyres the advice seems to be "just try pressures until you're happy", but generally start at 15psi and keep dropping 1psi until it feels wrong.
What's important to note about these "results" is that they are specific to me and how I ride that bike so they may not be applicable to anyone else.
Since writing the majority of the above I've fitted rigid carbon forks to the bike. With standard 29er wheels there's quite a bit of chatter but the "plushness" of the plus sized tyres mentioned earlier is a distinct advantage and I've not really noticed the rigid forks on subsequent rides but this might be that I've also adjusted my riding technique to handle the forks better. I used the Plus tyres on the Peak 200 last week and only really had a problem with those sections that had a thin layer of mud overlying harder substrate where you just get a wallowing effect, other than that they were fine.