Mullet wheels on gravel bike

Hi All--I am seeing alot of interest in mullet wheels in MTB , especially enduro and downhill, with a 650 B in rear and 29er up front.  This gives better roll over up front with the 29 er and better stability in turns with the 650 B in rear on nasty trails. There are many pros and cons, especially with geometry.  I, for one, am riding trails with my gravel bike now  that I would never have considered in prior years including single and double track with loose gravel and  more rocks and ruts, tho admittedly more tame than many MTB riders.  More and more of my gravel friends are riding these MTB trails also and I bet this trend will become more common.  Anyone riding a mullet wheel set up with 700 C up front and 650 B in rear on their gravel bike???. 
Advantages?? More hype than real??

The application on a mountain bike is completely different, and honestly I suspect it is just a trend in mountain bikes that will soon dissappear anyways.  Its not like it wasn't done in the past, over 10 years ago people were riding 69ers and B6ers and both those trends died too.  Part of the benefit is that the frame is designed around those wheel sizes so you can shorten up the chainstays to make the bike more playful.  You also can have equal width tires front and rear.  On a gravel bike, you can't shorten your chainstays unless you have a bike with a flip chip or sliding dropouts.  Most/all gravel bikes can fit a much wider 650b in the rear than they can a 700c in the front, so on a gravel bike the rear will be more capable than the front, which is typically a bad idea.  So I don't really see anyone  wanting to do this.  I suppose a dedicated mullet gravel bike could be built, but then it would suck on gravel so I don't think I see the point.  Personally I have ridden a lot of singletrack on my gravel bike, and without suspension I am not fast enough for any of these type of changes to really make a difference anyway.

Yeah... no; what @ljsmith said. It's just another passing fad, revived like a zombie and destined to fade away until some industry hack needs to boost sales of wheels, tires, and drivetrain components.

I'll join predicting it's a passing fad. 

Since big wheel handling got good and I got used to higher axle height I don't even consider as much difference between front and rear tire anymore - just faster tread pattern on one MTB I have.

Even a pair of 650B on gravel bikes for average and taller height people seems goofy to me the more I ride a drop bar on single track and all over because the roll over and foot print of 29r has so much advantage. I get 650B as a fit option for some frames, but not where associates who are 5'10 and taller seek smaller wheels to begin with.

I guessing this idea could have come from off-road motorcycles.  A typical 250cc dirt bike might have a 90/90-21 tire on the front and a 140/80-18 tire on the rear.  Overall diameters are quite close.  Of course having a motor changes everything - traction becomes a huge issue and rolling resistance a minor consideration.

An equivalent setup on a bicycle would be something like 45-622 on the front and 70-559 on the rear.  You'd probably need a mountain bike to fit that rear tire and your creation would certainly draw a crowd.

Interesting comments.  I watch bike videos alot and see gravel riders (like SAFA) riding more and more gnarly MTB trails and pushing the limits of the gravel bike.  This is happening locally too here in Az.  I remember just a few years ago that I would never see drop bar bikes on MTB trails---now it is common.  And look at the new MTB hardtail type drop bar gravel bikes like the Salsa Cutthroat.  I agree that likely mullet wheels may not offer any advantage unless you are riding more gnarly trails.  And, Chas, I have ridden my gravel bikes with a bigger volume  47mm  650 in front and  42 mm 700 in back and kind of liked it on regular gravel.  I saw no negative and have considered doing it more especially on more MTB like trails.  I have never ridden 700 in front and 650 in rear but may give it a try too.  

Frank wrote:... I have ridden my gravel bikes with a bigger volume  47mm  650 in front and  42 mm 700 in back and kind of liked it on regular gravel.  I saw no negative and have considered doing it more especially on more MTB like trails ...

I've ridden 45-622 and 48-622 tires at both ends, those are the largest I can fit at the rear.  I've been tempted to pair one of those with a 60-584 or 65-584 at the front.  My fork can handle that, the diameter wouldn't change significantly, and the new front wheel and brake disc wouldn't be all that expensive.  However tires that size are all heavy, slow, and probably suck on pavement.  Maybe I could rig a side rack to carry a spare wheel.  🙂

Frank wrote:  And, Chas, I have ridden my gravel bikes with a bigger volume  47mm  650 in front and  42 mm 700 in back and kind of liked it on regular gravel.  I saw no negative and have considered doing it more especially on more MTB like trails.  I have never ridden 700 in front and 650 in rear but may give it a try too.  

My original plan was to put the 650b in the front (with a wide tire) to give more cushion and more bite on loose turns.

I tried the fat tire on the front (50-54mm) on the 700c as an intermediate step, and liked the more relaxed steering (more trail) so much - that when I bought the permanent wheel I stayed with 700c rather than 650b.  

Besides, there is more selection with 700c.  I'm kinda liking the Furious Fred - which has a lot of volume and a lot of speed.

stu wrote:I've ridden 45-622 and 48-622 tires at both ends, those are the largest I can fit at the rear.  I've been tempted to pair one of those with a 60-584 or 65-584 at the front.  My fork can handle that, the diameter wouldn't change significantly, and the new front wheel and brake disc wouldn't be all that expensive.  However tires that size are all heavy, slow, and probably suck on pavement.  Maybe I could rig a side rack to carry a spare wheel.  🙂

I've used a Schwalbe G-One in that size (60mm)  Super fast, not too heavy, but it does really suck on mud (rather like riding on ice).  If its dry, its great.

chas wrote:I've used a Schwalbe G-One in that size (60mm)  Super fast, not too heavy, but it does really suck on mud (rather like riding on ice).  If its dry, its great.

Currently they're offering the G-One Speed in 60-622 but only 50-584 and that uses the questionable Addix compound.  The Alround comes in 57-584 and 70-584, both with the same Addix, and they're 725 and 845 g.

While looking these up I noticed the new G-One R (race).  These come in 40-622 and 45-622, have deeper tread than the Alround, and use an Addix Race compound.  The rated load is just 70 kg so these may be delicate and/or short-lived race day tires.

A review: https://bikerumor.com/2021/06/15/first-ride-schwalbe-g-one-r-gravel-tires/

I do see a rationale for the mullet--but agree that it would be a niche at best. I was just thinking about putting a larger size tire on the rear than the front after busting a spoke for reasons that apply to the mullet.   On the flat, a road bike typically has at least 60% of the weight on the rear wheel (it seems every time I search to double check something, I find Leonard Zinn https://www.velonews.com/gear/technical-faq-bike-fitting-and-setup/).  When I have slipped on the road or on moderate MTB stuff (I'm a roadie who has learned the hard way to prize avoiding the second knee surgery over momentary thrills ) or when I think about famous Tour crashes, its usually the rear slipping out in a turn.  Same tires and pressure with same max centripetal force to make the turn, but more mass and inertia on the rear wanting to go straight.

On gravel bikes, I would assume weight is even more to the rear due to more fork rake, slacker head and seat tube angles, especially on bikes without long chain stays. In going over something like a tree root, the larger diameter 700c wheel will rollover it easier, and simultaneously need less cush to absorb the blow because of the weight differential on the front.  The rear wheel, with the extra tension on drive side spokes, needs more cushion, and the rear hitting an angled bump doesn't deflect your steering like the front does.  

If you are bikepacking/touring/shopping, especially with a rear rack, an even bigger weight imbalance may happen depending on loading fore or aft of the middle of the wheelbase, especially if wearing a hydration pack instead of using bottles carried towards the front Then there is drive traction uphill on loose stuff, which isn't an issue for the front, and with more weight on the rear, getting equal rim damage protection only allows equal pressure with the front with a larger tire.

Downhill,  just like the larger 29er wheel forms a shallower angle rolling over something, it forms a a shallower angle landing.  The distance between firm tire contact on each side of an obstacle will be longer on a 29er wheel, especially if the 27.5 has a bigger tire at lower pressure that will deform over the  tree root more than a smaller 29er tire.  As down hill angles shift weight to the front, keeping the rear planted over bumps with a larger, softer rear tire could be an advantage.

As far as rolling resistance, back to the Zinn master--this made me feel so stupid for all of the discomfort I've put with riding rock hard tiny road tires over the years   https://www.velonews.com/news/gravel/optimal-tire-pressure-gravel/  Beyond the changing rolling resistance science about wider tires flattening less, this article adds the notion of sprung vs. unsprung weight and how lower pressure in the same tires can be faster on gravel.  Note that the real world roll out test has very different resistance  results than what should really be called a bicycle barrel rolling efficiency test, since it's measured by applying force and turning a small sound object, not coasting on a nearly flat one.  Also note the rider  on the bike with varying pressure thought higher pressure was faster, because we perceive frequency of vibration equals speed.  The reality was that the lower pressure turned more weight into sprung weight, and less kinetic energy was lost to bouncing rider up and down at lower pressures, which was faster, he was just feeling fewer bumps as he covered the same ground faster.