Mullet Curious - Geometry Question

smitho

Likes Dirt
My current bike is a 2017 Evil Insurgent. It's a 150mm rear/160mm front 27.5 in wheeled monster, however I'm slightly mullet curious. From my barely decipherable scribbles:
  • With a 160mm Fox 36 on the front the current Axle to Crown height is 549.1mm.
  • With a 150mm Fox 36 29er on the front, allowing for the additional size of the wheel, the Axle to Crown height is 576.1mm (an extra 27mm).
  • I read on single tracks somewhere that an extra 20mm of A-C height equates to approximately 1deg different in head tube angle.
  • The Insurgent has adjustable flip-chips. In the low setting, head angle is 65.6 deg (or roughly 64.6 in 29er guise), and in the X-low setting its 64.8 (or roughly 63.8 in 29er guise).
  • My plan was to run it in X-Low to keep the BB height down (it should lower it about 11.6 mm, and then run a 1 deg Works Components headset to steepen it up to 64.8 degrees.
  • An obvious downside will be the seat tube angle which in X-low is already a pretty slack 73.4, and I'm assuming will only get slacker with the longer fork. Not too phased by this, it's going to be a winch and plummet bike. I have a 120mm trail bike for when it's necessary to go uphill for fun.
  • To deal with the 27mm higher front end, I had planned to swap from 35mm riser bars to 10mm riser bars.
Obviously bicycle geometry is a whole lot more complex than my fairly simplistic version above. My question is to the geometry geeks - is there something I'm missing here? Anyone converted their 27.5 bike to a mullet and have any feedback?
 

moorey

with a big stick
@smitho, one way to keep your ST angle and not lift the front is with angle set. I put a 170 fork on my 150 rear tranny but wanted to keep the BB and ST similar. It’s done the job perfectly with a 1deg headset, and slackened the HA a degree. It has shortened the reach by about 5mm though.
 

The Duckmeister

Has stumpy thumbs, Speciaized are so weird
AC is one aspect, but remember to factor in the axle being further off the ground with the bigger wheel. Assuming same tyre profile, axle-ground height will increase by 19mm in addition to the extra fork length.
 

creaky

XMAS Plumper
Oh god another pronoun I am going to have to learn
Google it.

@smitho i think the evils have a low BB reputation anyway (could be wrong?) which may accomodate the 29er front ‘lift’. I say go for it and slam your saddle forward on the rails (unless it is already).
 

beeb

Dr. Beebenson, PhD HA, ST, Offset (hons)
AC is one aspect, but remember to factor in the axle being further off the ground with the bigger wheel. Assuming same tyre profile, axle-ground height will increase by 19mm in addition to the extra fork length.
@smitho has done well and incorporated it in the stated 27mm difference. The A2C of the 160mm 27.5 fork (549mm) vs. the 150mm 29er fork (557mm) are surprisingly/unusually similar...

I thought the same thing at a glance earlier, but crunched the numbers and I reckon he's nailed it...

@smitho - I haven't checked every detail, but it seems like you're all over it. You've accounted for the A2C increase, mitigated the BB rise and know what's going to happen with the seat angle.

If you want to check the angles, have a play with this geometry calculator below (Just make sure to uncheck the "automatically approximate reach and stack" checkbox). You'll have to use your estimated 29er A2C you've used in the OP, but you can also input an angleset value to see how that'll raise/lower and lengthen/shorten things...
 

yuley95

Likes Bikes and Dirt
I wouldn’t necessarily worry about the Angleset. My Sentinel is 64 degree as stock but I’m running a Suntour dulolux fork with a slightly longer axle to crown measurement that would put it around 63.5 or so.
It’s a short offset fork and feels fine.
 

leitch

Feelin' a bit rrranty
I wouldn’t necessarily worry about the Angleset.
Steepening with an angle set will also raise the BB and slacken the seat angle a touch so paradoxically may act against some of the other things you’re trying to address.
 

Isildur

The Real Pedant
Alas I can't reaaly help, as I went the other way on my Ripmo, drpiing the rear wheel to 27.5 to get fishy.

I has been running the Ripmo with a -2 headset (64 deg), so when I put the Mullet on the back I also:
  • Changed the headset to -1.5, keeping the same 64 deg HA
  • Shortened cranks to 165mm, as my BB was now around 325mm.
TBH, I'm really loving it for the gravity fuelled riding I do. The low BB makes it hug the ground and corner on rails and, while it may sound low and certainly is compared to the 340mm or so it came from, the Stumpy Evo runs about 325mm so it's nor unheard of.

I haven't buzzed my arse once since I put it in and I actually appreciate the slightly easier gearing I now have (yes, I'm lazy!). And the angleset kind of negates the seat tube slackening so it still has a really nice climbing position.

The only negative is on longer firetrail or XC style riding, where I notice the lack of rolling at the back. But, as much as I could (and have) swap the wheel out for those rides, in reality it's not worth the hassle.

So for those considering The Mullet on an existing 29'er, if you can keep your BB height and your seat angle reasonable, go for it!
 

ozzybmx

call me Cáitín
I have had 2 massive crashes in the past year and both were 100% pedal strikes.

There a few really good number crunchers here and it sounds like you have it under control anyway, just watch that actual BB height with sag.
 

TheAzza

Likes Bikes and Dirt
I run my insurgent as a mullet with a 150mm Mezzer (51 offset) and in X-low.
It is definitely more gravity oriented, but when I put the 27.5 wheel back on for a back to back test I found it basically un-rideable. It pedals fine for what I want it for.

I would like to run a 44mm offset fork just because it is closer to the intended offset.

I do find that small changes now, like bar height and travel seem to have a more profound effect on handling. The geometry might be right on the edge of what will work well.

Are you in Melbourne? You are welcome to come have a pedal.




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Isildur

The Real Pedant
I have had 2 massive crashes in the past year and both were 100% pedal strikes.

There a few really good number crunchers here and it sounds like you have it under control anyway, just watch that actual BB height with sag.
Yup, definitely something to consider, but with the 165mm cranks I'm getting about the same amount of hits as I did with 175mm on the bigger hits. Which isn't that many really, and only on climbing.

I do have a tendency to try and avoid pedalling in rock gardens though, as back in the old DH days I had plenty of ejections due to exactly that!
 

Petero

Likes Dirt
The other thing to consider, if you wanted to keep seat tube angle, would be offset shock bushings, but of course these would cause the BB to go even higher.

With the 27mm increase in front end, you would assume that the BB is say 1/3 of wheelbase, so a BB rise of 9mm?So if the geo chip drops it 11.6mm you're pretty close to the standard setting?

The only other thing I can think of is the lifting the front end will also reduce the reach.
 

smitho

Likes Dirt
I run my insurgent as a mullet with a 150mm Mezzer (51 offset) and in X-low.
It is definitely more gravity oriented, but when I put the 27.5 wheel back on for a back to back test I found it basically un-rideable. It pedals fine for what I want it for.

I would like to run a 44mm offset fork just because it is closer to the intended offset.

I do find that small changes now, like bar height and travel seem to have a more profound effect on handling. The geometry might be right on the edge of what will work well.

Are you in Melbourne? You are welcome to come have a pedal.




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Cheers, yep I’m Melbourne based so that would be ideal. I’ll shoot you a PM.


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TheAzza

Likes Bikes and Dirt
Cheers, yep I’m Melbourne based so that would be ideal. I’ll shoot you a PM.


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No worries.
Glad to answer any questions. I headfucked myself for ages over this mullet business.


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