MTB Wheel Size Experiment | 29er Front & 27.5+ Rear Geek Edition

MTB Wheel Size Experiment | 29er Front & 27.5+ Rear Geek Edition


– This is the tech
follow-up of the GMB video where I actually tried a
bigger wheel in my Scott Genius where I ran a 29 up front
and 27.5 plus in the rear. If you’ve not seen that video already, I would check that out
before you watch this. But also, I’ve done a video in the past and we try and add 29s
on both front and rear and 27.5s so lots of
different variations here. Today, I’m talking about running
the 29 up front, 27.5 rear and I’m going to get in some
of those stats about the bike. (jazz electronic remix) So the Scott Genius is designed
to run either size wheel and all we need to do is
flip the chip between the two to try and keep the angles about the same. I’m lucky enough to have two
pairs of wheels for this bike and they’re both identical,
the DT Swiss with an M1825 rim. Both of them are 30 mill
wide so that shouldn’t affect the tire profile too much
between the different sizes. Of course, on the plus, I’ve
actually got a 2.6 inch tire. On the 29, it’s just a 2.4. So, it’s narrower up front and also not as deep as the rear tire. It is really easy to swap the wheels the one’s that’ve got center lock rotor. So all you need to is whip
that off, change it over, and then change its set. I’ve got tires on so it’s
quite an easy thing to do. Is it practical to do all the time? I guess it is. It’s gonna be expensive if
you want two pairs of wheels. (jazz remix) So the thing you do
need to do on the Genius when you’re moving between wheel sizes is actually flip that chip
on that shock linkage. Super easy to do. All we do is undo the bolt
and then turn ’em around and put them back in. And what it does is it keeps
the geometry really similar between the wheel sizes. So it’s 65 degree head
angle on the 29 wheels and it’s 65.6 on 27.5. So when I messed around and put
a smaller wheel in the back, it actually went down to a 64.2
head angle, so pretty slack. That’s getting towards
downhill bike, that is. And the BB went to 332 millimeters. So for me, that is the biggest compromise on that setting within the low. It just felt like it was too low, I’m gonna start clipping my pedals on more technical terrain. So I flipped it over and then I rode it and it did feel much more balanced so actually the head
angle went back to a 64.8, so almost 65. The BB angle, BB height went up to 336. So it actually only
changed four millimeters but that did make enough difference for me to make the bike feel like
that’s how I’d want to ride it if I was gonna do this setup. (jazz remix) So the first geometry values I gave you are all from the Scott website
so they’re official numbers. When it comes to messing
around with the wheels, I’m finding the other numbers. I’ve actually used a tape measure and I’ve borrowed Dot E’s
Wixey digital angle gauge. I don’t have one of these because you need a nerd
certificate to get one of these and I’m afraid I don’t have one. So that does mean the numbers
are a little bit approximate but that is how it feels to me. The bike does feel a little bit
slacker, a little bit lower. Also, the ride difference does feel… Basically, you’ve got more
volume in that rear tire so that does make a difference
as I talk about in the video. It’s those little bumps. So when you’re riding over little stones and edges and things like that, that bigger volume means
it’s a bit more compliant so through my feet, I felt less bumps. And then, it’s the same thing really with the bigger wheel on the front. I’ve got thinner tires so less volume but the bigger wheel has more rollover. Like I talked about in
the last tech video, I got the two different tapes out, show them rolling over that tool. The bigger tape will actually
roll over a little bit easier. So, that does mean it makes
it a little bit easier on my wrists up front. What I’m really doing is mixing up the advantages and
disadvantages of both wheels. So, let’s talk about advantages first. Front wheel, greater rollover. Real wheel, I’ve got more compliance and I’ve got a bit more
grip in certain situations and it’s a lighter wheel
than the bigger tire. Disadvantages, heavier wheel on the front. And also, when we talk about
the actual tread of that tire, one thing I find on plus tires, I talk about this quite a lot, is in the wet, they
can float a little bit. They don’t dig in quite as
well as a thinner tire does and on really hard-packed dirt, you can find that you have
to run quite high pressures otherwise you’ll be rolling
that tire around quite a lot. So all in all, is it a
good idea to mix them up? Um, for me, jury is still out. Today the test was really wet. While it was still quite grippy, it was so wet, the mud was so thin. But I think I need to try it out again and actually, I think I need
to put some times to it. But on the trail, how much
difference did I actually feel? Surprisingly, not a huge
amount, to be honest. In the past for racing in
(mumbles) on a bike like this, I’ve actually run a heavier
weight tire on the rear or an insert like a Schwalbe ProCore, because I find that I more often than not get punctures on the rear and also I give that rear
wheel a lot of punishment so I can ding the rim. So it did feel a little bit like that. I’ve got more air volume there so it just feels like that
rear wheel is more protected. I’m not getting as much
feedback through my feet. It feels slightly heavier
even though it’s not. That rear tire and rear wheel setup is actually lighter than a 29er. So, I don’t know. It’s a hard one to tell, really. I think I need to test
it more, like I said, but is it actually worth it? I don’t know. Is it the future of mountain bikes? Probably not. The big problem here is the UCI. So when it comes to racing bikes, you’ve got to have the same
size wheels front and rear. Obviously, this isn’t a downhill bike, it doesn’t count for that. You’re not gonna race cross-country on it. And Duro isn’t UCI regulated
so we could see it, I’m just not sure if
the bike manufacturers are gonna get on board with it. So it’s been nice to test this out. Will I keep riding a
mountain bike like this? Probably not, to be honest. I like this bike on 29 inch wheels, so I’ll be chucking that back on. I also think it’s a bit
of a practicality issue of having spares, so
why are you gonna carry two spare tires everywhere? Tubes, not so much of an issue. I’ll let you into a little secret here. 26-inch tubes will fit anything. They’ll pump up and fill it
so that’s not a massive issue but I think I’m just gonna
go back to riding 29. Leave us your questions, if
you’ve got any down below. I’m sure you’ve got some good ones so what I’ll do is I’ll
try and answer those in the dirt shed next week. Also, if you’ve not
seen the original video of me riding this bike like
this, click up there flat one. Give us a thumbs up if
like it and star the video, and like I say, keep
leaving those comments.

Only registered users can comment.

  1. I got one question…

    I think the main benefit of 27,5 wheel are better maneuverability in corners which result in a smaller turning radius and better control.

    I think the main benefit of a 29 wheel is that leverage that allows you to get further with one turn of the pedals, the greater stability and the ability to keep momentum.

    So wouldnt it be more interesting to do 27,5 in front (for better steering) and 29 in the back (for better power-related stuff)?

    (maybe i am just wrong, but that made sence to me)

  2. Nice videos today. Could you please not crush the shadows into the blacks next time though? The grade is nice looking but just a little too hard imho

  3. For a bike that isn't that slack a 29 front & 27.5 back will slacken the head angle, offer better roll over obstacles, accelerate faster and turn better due to different wheel paths. At least this is my experience with 29×2.5 & 27.5×2.5. Next week I will also try 29×3.0 & 27.5×2.8.

  4. I was having tat idea but did not test it out . It's really great seeing GMBN trying out crazy ideas . kudos to GMBN team coming out awasome content for world 🙂

  5. I dunno about the the wheel size mixing, Neil, but you were shredding those wet trails. Great camera work too.

  6. I've been riding 29 front 26 rear for the last 10 years on my trek 69er single speed. I wouldn't change a thing as it works brilliantly.

  7. How about a 29 xc hardtail bike, changing to a trail bike? Slacker headangle more cushening out back? Bb height a bit lower. Sounds maybe like a budget quiver killer?

  8. I would think because you were running a 27.5×2.6 on the back versus the 29×2.4 up front, the overall outside diameter would be very similar. Maybe that would be why you couldn't tell much of a difference in ride feel. Maybe go with a 27.5×2.4 or smaller then see how different it handles. Trek use to have a Hardtail called the 69er, 29 front/ 26 rear.

  9. Mojo Nicolai Geometron G16 – Hybrid is meant to turn in better from what they say.
    I guess they are using far more progressive frame geometry though, so you probably won't notice it as much on the Scott.

    How about interviewing Chris Porter on this topic?

  10. It seems to me, that the greater advantage of this solution, would feel in hardtail bike. Maybe you'll test this on a hardtail, out of pure curiosity?

  11. Another benefit of smaller back wheel is less torque to get it to turn …so it theory you can accelerate faster…

  12. Downloaded bubble level onto my phone. As well as a spirit level there is an angle gauge, not sure how accurate it is though.

  13. I think Cam Zink had a dh bike that was 26” rear and 27.5” front could you guys do a video on something like that?

  14. 27.5+ is NOT the same diameter as 29 once you sit on it -> geometry change inevtiable. e.g. new IBIS mojo is either 27.5 regular or plus.
    German MTB News website put it to the test and says (like IBIS) that 27.5 plus are usually run with less pressure (which makes sense I guess). In that state though, 27.5 plus tires are actually much closer to the diameter of a regular 27.5 once you sit on the bike. We see a lot of bikes that say they can either be fitted with 29 or 27.5+. However, fitting 27.5+ on a bike that is actually designed for 29er wheels, brings the BB height too far down (given that you run low pressure and sit on the bike). If you fit a 27.5er bike with normal wheels at let's say 1.5 bar and compare this with the same bike set up with plus tires at 1.05 bar (like the testers are riding it) you get the same BB height once the rider's weight is on the bike. See for yourself:

    https://www.mtb-news.de/news/2017/05/08/der-275-plus-irrtum-oder-womit-ibis-recht-hat/

  15. I think the rules of racing says You cant go different wheel size… in tour de france people would go smaler at the front to Get lower, and They made a rule and That rule is in all the different racing types now. I dont think They wil change the rules

  16. Neil/Doddy – I've settled on 2.6" 29's on my trail bike (Stumpjumper). The feel and grip is phenomenal on the surfaces I ride here in New Zealand. My question: what sort of pressures do you recommend in these tires? I'm 80kg, not a particularly heavy or aggressive style of rider, and I've found very low 20's to feel the best. For my 60kg partner, also on 2.6" 29's, we are down in mid-teens for that sweet spot. Is this getting too low? Keen to hear your thoughts.

  17. Nerd certificate!🤣 I’ve never heard that one before. I’m going to start referring to my Professional Engineering license as my nerd certificate now!

  18. Can’t believe you are allowed to ride on trails that wet and muddy. That sort of shit is highly frowned upon where I ride. Not good for the trails or for your bike components.

  19. Seems like a lot of the evaluation was based on having a plus tire rear. How about same width tires on both ends? Or even narrower rear? Most people I know run a narrower rear (though matched size wheels). Interesting question to explore though, thumbs up

  20. HI Neil, i heard an interview with Chris Porter the other day. He tried a similar setup: 29 front, 27,5 in the back, but no plus size tire. He had the following logic: 29 has better rollover, this is especially an advantage for the front wheel. however, the rear wheel is pulled over obstacles by the moving weight of the bike and rider. The rollover of the rear wheel is less important. Therefore it makes more sence to use a smaller, lighter wheel in the back to improve the suspension, (less unsprung weight). This all makes sence to me, but how does it feel in practice?

  21. As many others have said, the father taller tyre should be on the front for more front end grip and a slimmer faster rolling tyre on the rear

  22. Hi Neil & Doddy, need your advice on tyre pressure. I am 1,74cm tall, weight 90kg, riding santa cruz hightower 29" 140mm travel fork 135mm rear shock. Kindly your advice how much pressure do I have for my tyre if the trail is muddy and how much pressure if the trail is dry and rocky. Thanks.

  23. Foes makes mountain bikes that are mixed wheel sizes and have done for years. The main difference is they run a "standard" 27.5 on the rear instead of the "semi-plus" that Neil is riding.

  24. Really good you can come to a null conclusion… not try and force something you didn’t really feel. It will be interesting to revisit when it’s drier but your interim conclusions are good enough not to rush out and get a new wheel!
    What if you don’t have a flipchip though?

    I have a 26 wheel off my DJ I can fit to the kids 24er if I get a converter from 20mm thru axle … I might get him to give it a go ..(. 8 yr olds bounce better than I do … ) or just wait till it’s drier and stick the 2.5 DH carcass on the front and 2.1 XC tyre on the back…

    Since the kids Norco doesn’t come with a flip chip I could drop 12mm off the front using a straight steerer instead of the external lower headset used to accommodate the tapered steerer. or I could drop 20mm off the fork with a shorter airshaft? .. but it seems a bit of faff changing the headset lowers or shortening travel if it’s only being used pointing DH and we use flyup going back up? It also seems a bit mean taking 20mm of fork travel away for Mr Rooty 😉

  25. What was it MBR used to say "Get out and ride" 24", 26" 24" and 26" as on the Big Hit, 27,5" 29" and now mixi match 27.5" and 29". Tried 29er just do not like it and does not work for me. However some people love it to death. All we need now is 26" and 27.5" and we will have done every combination. All have pros and cons. For me 26" and 27.5" are both great. Its not about the science it is about rolling on the trails and having a good ride out.

  26. Grab a 27.5+ fs bike, throw a fat tire on the front. 4.8 in the front, 3.0 in the back. Now that I want to see. I think rockshox makes a fat bike fork with 110mm of travel too. Let’s get crazy

  27. I didn't read all the comments to know if this has been brought up, but how on earth can the BB be lower in this config?

  28. Hey Neil, I have a 2011 Giant reign X and I started running 27.5 up front and 26 in the back sense the beginning of last Season. I wanted to upgrade my fork but all the good ones were all for 27.5 or 29er. I Could have stayed 26 but I wanted to take advantage of the extra rollover. It slackens out the HA about 1 degree and raises the BB height a little. I could instantly tell the difference. Thanks for putting this video together.

  29. You like the 2018 Genius on 29s? I was thinking 27.5 are more maneuverable in technical terrain. Or is that not the case?

  30. I used to do this back in early 2000s with my DH bike. I rode 26' in the front with 3' wide tires and 24' rear wheel 2.8' Gazzaloddi. Early free riding days. Lol

  31. Liteville engineered a bike to have those hybrid wheel sizes. (Discontinued, the pundits hypothesise, because of lack of public understanding). Apparently when set up with similar width tyres (i.e. the rear is actually significantly smaller diameter) the claim is that it makes the rear track the front better on cornering. I read an interesting US mag test about their 26/27.5 and 27.5/29 bikes (https://m.pinkbike.com/news/liteville-601-mk-2-review-2015.html). After a LOT of testing, your local geometry and suspension guru Chris Porter (Porter is Mr.”think out of the box” at Mojo Rising / Geometron Bikes) runs his own Geometron with “hybrid” wheels (27.5R&29F) and LOVES IT. Geometron bikes offer that variant and recommend it. I am hooked, my own 27.5 Geometron will be going that way soon. Better rollover and cornering. Easier speed. Stronger back wheel. Few worthwhile advantages there. None of the disadvantages if the rear wagon wheel either.

  32. My Liteville 101 has the possibility to shorten the chainstay to fit a 27.5” wheel without changing the geometry. I still have to test that out since I have SRAM/Shimano on my wheels and can’t really swap them as easily as you do. This who have tried it say it works pretty well though.

  33. When I raced BMX years ago, we would put a skinny tire on the rear pumped up pretty hard and a fat tire on the front with a bit less air. The rear gave us less rolling resistance=(more speed) and the front gave us more traction in the turns. What I may do on my next mtb is put a 29×2.0 (Maxxis Ikon) on the rear and a 27.5+ X 2.8 (Maxxis Minion DHF) on the front. Pump up the rear and run less air on the front. Speed on one end, control on the other. It might be a great combination of XC and DH = Enduro.

  34. Can’t wait to test this..being 5’6” on my 29er on the real steep chunky stuff my ass hits the rear tire but never did on my 27.5 ..since I have both sets of wheel size I will test the 27.5 on the rear only on the steeps 👍🏻

  35. I've been running a 29×2.5 front and 27.5×2.8 back in my Stumpjumper for a while now. Coming from riding motocross, it makes sense. No flip chip, so it's just a little slacker now, but isn't that the new thing? Rollover in front and great cushy grip in rear.

  36. I used to ride a 29+ hardtail and now ride a 27.5+ full suspension. When making the switch, I missed the easy roll over on rocky technical trails. Would you consider testing again on a rocky technical trail?

  37. I ride 26" and thinking of upgrading front wheel to 27.5", whats your suggestions, what I think is it will give me front wheel with more clearance and rear wheel with better acceleration of 26"

  38. 26" tube will fit anything! what great advice. been carrying two different sized tubes around for my son and I, no longer! Cheers.

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