Explain fork offset and boost?

[ishaw]

Looks like the frame I have my eye on is designed for 51mm offset. Can't figure out whether cranks need boost spec or not. Any telltale signs?

I know I've listed a need more space cull type thread but reckon I can throw together a modern build with modern standards on the cheap. I've not got anything completely current and if I can get it within my limited budget then I can at least try out all this new fangled stuff that is apparently going to be 'standard' for a while and see if it is any different or worth it but on the cheap hopefully.

Plus the frame will add a bit of colour to my fairly monochrome stable.

 

[clubby]

Looks like the frame I have my eye on is designed for 51mm offset. Can't figure out whether cranks need boost spec or not. Any telltale signs?

Not really. Hardtails usually a combination of chainline and chainring clearance. 30 or 32t rings you may get away with a standard offset one. If you have cranks, just wait and see if it works. If not then you may get away with a simple ring swap. On my plus bike the “boost” crank it came with was simply an SLX double without a granny ring!

@2manyoranges is right about full suss frames though. They’re a lot more complicated, although my Sentinel just has a normal SRAM crank with a boost offset ring to give the extra clearance. My previous Santa Cruz ran best with a standard offset ring.

My only other bit of advice is just build it. You won’t really know what’s right until you do and try it out.

 

[dyna-ti]

Looks like the frame I have my eye on is designed for 51mm offset.

" 44mm offset reduces “trail” and adds turning agility, while 51mm offset adds “trail” and adds straight-line stability, especially at high speeds "

I recently got a fork to use as a spare. Fork on the bike is a fox36, and the spare was a Lyrik select. The fox has an offset of 44mm, the lyrik has an offset of 42mm. Being different i did a little research, and most of the info was the technical type above, but of the info i found concerning offset, from actual users on STW and other forums, the general gist was its unlikely you would notice really.
Every time you buy a new bike, it's probably going to be different from the one you had before and you'll quickly adapt to it.
So if its a difference of say 38mm to 51mm you I should think would notice, but you would also adapt yourself to it pretty much within the first few rides.

I think a middle ground fork's going to be 44mm. But if you find something and the price is right, and its a god quality fork, then buy that and tech differences be damned.

The spare fork i got for £230 new. Its normally £430. Do you think my primary thoughts were about the 2mm difference in offset, or the 200 quid i was saving

 

[FluffyChicken]

It is nothing new, way back when in circa 1990, Bontrager made different crowns for the rigid and rockshox forks with different offset**
Standard 44mm and a shorter rake 38mm*



(*numbers are something like that)

**offset as it's talking about angle in the crown for the legs to steerer, rake being what the effect is at the hub axle/ground.

 

[scant]

fat chance also used different offset suspension fork crown bitd

boost cranks use a concave chainring available in different offsets, 3 or 6mm is fairly normal. its basically to improve chainline when used with a 1 by 11/12 drivetrain. *certain frames can use older non boost cranks with the single chainring mounted on the inner face of the middle chainring, if theres enough chainstay clearance.

*there are adapters available to convert non boost wheels, for boost frames/ forks. basically thick washers & longer bolts that bolt to the disc hub face; but you'd need to redish the wheels accordingly.

*both these points are cheap fixes, they work okay, but arent ideal

 

[benjabbi]

I've used 42, 46 and the current 51mm offset forks on the same bike, you could probably tell the difference back to back but it's very subtle and you'd realistically only notice a difference between the two extremes. With the longer offset slightly quickening the steering. After a couple of rides you just get used to it. It won't be a disaster either way. Trail changes with head angle: steeper angle = less trail, fork offset: more offset = less trail, wheel/tyre size: bigger wheel = more trail. Less trail generally = quicker steering, more trail = slower steering but more stability at speed with greater chance of wheel flop at slower speeds. With suspension the trail decreases as the fork compresses and the head angle gets steeper, swapping to a longer or shorter fork alters it as well. Loads of variables. Just go with what's recommended for the frame if you can but don't worry too much if say a bargain fork pop's up with the wrong offset.
I've also used both Bontrager and standard Judy crowns on my Ti-Lite and couldn't tell a difference, maybe I'm just not sensitive enough?

 

[2manyoranges]

I've used 42, 46 and the current 51mm offset forks on the same bike, you could probably tell the difference back to back but it's very subtle and you'd realistically only notice a difference between the two extremes. With the longer offset slightly quickening the steering. After a couple of rides you just get used to it. It won't be a disaster either way. Trail changes with head angle: steeper angle = less trail, fork offset: more offset = less trail, wheel/tyre size: bigger wheel = more trail. Less trail generally = quicker steering, more trail = slower steering but more stability at speed with greater chance of wheel flop at slower speeds. With suspension the trail decreases as the fork compresses and the head angle gets steeper, swapping to a longer or shorter fork alters it as well. Loads of variables. Just go with what's recommended for the frame if you can but don't worry too much if say a bargain fork pop's up with the wrong offset.
I've also used both Bontrager and standard Judy crowns on my Ti-Lite and couldn't tell a difference, maybe I'm just not sensitive enough?

I think it’s the ‘holy sh+t’ moments of getting it wrong on a modern bike, and if you are on a bike which needs 42mm offset and has 42mm forks on it, then you are left with ‘how on earth did I get out of that…?’ …rather than going over the bars. I indeed have wheel flop on the bFe Max but the weirdness of that disappears after about 10 mins and when I get into big hits and drop offs I really value the wheel being WAAAY out there in front of me…..

 

[legrandefromage]

Easy:

Fork offset is what happens when the director yells 'cut' after a particularly bad take

Boost, is, well...

 

[2manyoranges]

I don’t think that bar IS boost … how wide is it?
130, 135, 142, 148, 150, 152?

 

[Nigelpa]

I don’t think that bar IS boost … how wide is it?
130, 135, 142, 148, 150, 152?

Same price but probably now 115!