What are boost cranks ?

rowdyflat

chez le médecin
So I understand boost wheel spacing but how does boost crank spacing and boost chainrings differ from using more spacers on a non boost crankset ?
Is it just to ideally optimise the chainline on 1x into the middle of the cassette . Is it a help if you have huge tyres and a wide frame ?
Or is it to sell more product ?
 

Oddjob

Can hench anywhere any time
It's a different chainline, and if memory serves non-boost is 48mm, and boost is 51-52mm.
Because the cassette has jumped outboard a few mm, and the chainstays now potentially take up that space, "non-boost" cranks might either contact the boost chainstay and the ring is aligned with a low gear sprocket, although this will vary from frame to frame. The idea is to get your chainring centered on the cassette to reduce the angle of attack at either extreme so that it shifts properly.

There is compatibility between them, too some extent, the BB standard and Q-factors are the same. But in my opinions it's another one of those fuckoffs of boost to begin with; because half the reason they said 148mm was that it would maintain the BB standards we'd become used to (without having to go to an 83mm DH width), but then they needed to change it anyway to be optimised.

Of course, if you go a gearbox you just push a single chainline and don't need any of that nonsense.
What he said.

If you have a direct mount crank you can get different chainrings to compensate for the change in chainline. I particularly like the Absolute Black oval ones.

In one of lifes great ironies, old school 3x 104bcd cranks work great with boost as well

Sent from my SM-G900I using Tapatalk
 

Boom King

downloaded a pic of moorey's bruised arse
No change in Q factor on boost cranks, just 3mm less offset on the chain ring, 3mm versus 6mm on non boost.
 

The Duckmeister

Has stumpy thumbs, Speciaized are so weird
So I understand boost wheel spacing but how does boost crank spacing and boost chainrings differ from using more spacers on a non boost crankset ?
Is it just to ideally optimise the chainline on 1x into the middle of the cassette . Is it a help if you have huge tyres and a wide frame ?
Or is it to sell more product ?
Basically what Zaf said.... Boost hubs are 6mm wider than standard (3mm each side of centre, slightly logically), so Boost cranks locate the chainring/s (yes, there are sensible multi-ring options) 3mm further outboard to maintain alignment with the cassette, without widening the whole stance of the crank (the Q-Factor).

Packing the bottom bracket out for a pseudo-boost setup isn't a great idea. First, it widens the Q-Factor (which you may or may not actually notice), but more crucially it can compromise the proper assembly of crank & BB; how drastically will depend on the particular format, but SRAM GXP for example will suffer very badly from an overwidth BB, as it will not allow the crank to be tightened properly, and will crush the bearings instead.
 

rowdyflat

chez le médecin
Thanks Duck I noticed while installing Shimano bbs that there is a bit of variability via the spacers.
BTW how tight or much space should the cranks be against the bearing when installing them?
Always done it just touching or should it be a paper width away.?
 

Nambra

Postmeridian
Can't see it having been said in the above posts, but Shimano has boost offset cranks with standard chain rings while SRAM has standard cranks with offset chainrings to achieve boost chainlines.

@rowdyflat are you referring to Hollowtech cranks, which have a 24mm diameter axle stepping down to a 22mm diameter inside the NDS bearing? If so, then you want that 24/22mm step on the axle up against the inside of the NDS bearing. The NDS crank arm pulls snug against the outside of the NDS BB bearing to lock it into place. That means there can be some variability on the BB face to crank arm distance on the drive side, but it's not a problem.

Spacers go on the inside of the BB, regardless of whether it is a press-fit or threaded BB, to achieve the required overall BB width - it depends on the width of the BB shell on the frame. Perhaps have a look here for more details. Too many spacers will have your drive side crank contacting the BB shell on that side before the 24/22mm step on the axle hits the NDS bearing - that wouldn't be good!

Edit: yes this is GXP not Shimano, Ducky’s since corrected me! :(
 
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SummitFever

Eats Squid
Also, if you have a 68mm threaded BB then you will have one 2.5mm spacer on one side and 2 x 2.5mm spacers on the other. Quite possible to adjust the chainline by moving those spacers around. As long as you keep the total width (eg. use 3 spacers in total) then crank will work/tighten up as per normal.

This may result in the crank being not perfectly centered in the frame but it's probably not humanly possible to detect a 2.5mm shift.
 

Nambra

Postmeridian
Well said @SummitFever - the solution to cranks not being perfectly centred is to have one leg shorter than the other... something @rowdyflat might be able to help with given his profession.

Last bike I build up was a GXP BB with an AC bearing on the DS, which needed preload via a wavy washer and getting the number of spacers right to get the right tension was a major PITA. I did get it the crank centered though by juggling spacers of varying thicknesses under both BB shells - I took vernier measurements from the machined face of the pedal holes on the inside of the crank to the edge of the rear wheel rim on each side, inserting the wheel backward to ensure that the rim was L/R first.

Sure, I could have just stuffed the spacers in on one side, had offset cranks and my body wouldn't have noticed... but my mind would have!
 

SummitFever

Eats Squid
... the solution to cranks not being perfectly centred is to have one leg shorter than the other...
Funny you say that. Leg length differences are very common (I think in one scientific study it was >30%) and for those with even length legs, tilted hips/alignment is also very common. A super accurate bike fit will typically involve having your legs measured by xray...
 

rowdyflat

chez le médecin
Yep different length legs are common usually the pelvis and spine tilts to cope with it on a bike .
Greater than 10 mm may be noticeable but people who have a hip replacement or born that way may have > 20 mm and need a shoe built up.
Yeah I noticed the difference between different crank widths when I put the wrong number of spaces in and the front derailleur no longer worked correctly. 2.5 mm can make a difference.
 

The Duckmeister

Has stumpy thumbs, Speciaized are so weird
@rowdyflat are you referring to Hollowtech cranks, which have a 24mm diameter axle stepping down to a 22mm diameter inside the NDS bearing? If so, then you want that 24/22mm step on the axle up against the inside of the NDS bearing. The NDS crank arm pulls snug against the outside of the NDS BB bearing to lock it into place. That means there can be some variability on the BB face to crank arm distance on the drive side, but it's not a problem.
That's GXP, not Hollowtech (which incidentally refers to the crank arm construction, not the spindle attachment). Shimano two-piece cranks, not all of which are Hollowtech, use a straight 24mm spindle, and are designed for a total assembled bottom bracket width, screw-in outboard bearings or press-fit, of around 96mm.
 

ChrisJC

Likes Bikes and Dirt
Is there such a thing as dedicated boost cranks? Shimano M8000 do one but as just about every crankset is now direct mount, it comes down to your chainring offest. 3mm for boost; 6mm non boost.
 

Nambra

Postmeridian
I thought the Shimano cranks with spiders came in boost and non-boost models, there’s a ‘B’ n the part number to differentiate. eg. FC-M8000-1 is the standard 1x M8000 XT crank and FC-M8000-B1 is the boost offset version. SLX M7000 is the same.

@Duckmeister please feel free to correct me again!
 

Jpez

Knows which side his bread is butthurt
Is there such a thing as dedicated boost cranks? Shimano M8000 do one but as just about every crankset is now direct mount, it comes down to your chainring offest. 3mm for boost; 6mm non boost.
My bike is boost and came with a standard non boost RF Aaefect crankset. 134mm spindle from memory. It shat me though as the chainring was flipped to to get the correct chainline. 55mm
So ended up buying some RF Turbine ‘boost’ cranks at 143mm so I didn’t have to run a reversed chainring.
 

ChrisJC

Likes Bikes and Dirt
My bike is boost and came with a standard non boost RF Aaefect crankset. 134mm spindle from memory. It shat me though as the chainring was flipped to to get the correct chainline. 55mm
So ended up buying some RF Turbine ‘boost’ cranks at 143mm so I didn’t have to run a reversed chainring.
Yeah, my wife has the same cranks on a stumpy. With the standard RF ring.
 
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