Explain fork offset and boost?

ishaw

ishaw

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I'm looking at buying a new frame, but it's all new fangled and boost. What does this mean really? I get the fact that the rear end is 148mm hub diameter, but what does this mean for a fork? I've seen various offsets and have no clue what they mean in real terms but understand what an offset is, or at least I think I do.

I want to order the right forks and wheels for the frame so when it arrives I can throw it together but want to make sure I buy the right forks, wheels and I so assume cranks as these seem to also come with offset and or boost spacing,, of which I know nothing.

Can someone or many someone's enlighten me.

Annoyingly I have non-boost forks and wheels that I assume aren't suited for the 29er boost spec frame I'm looking to buy (mainly as I like the colour but also a way to drag me into what is currently modern spec).

Thanks
 
ishaw said:
I'm looking at buying a new frame, but it's all new fangled and boost. What does this mean really? I get the fact that the rear end is 148mm hub diameter, but what does this mean for a fork? I've seen various offsets and have no clue what they mean in real terms but understand what an offset is, or at least I think I do.

I want to order the right forks and wheels for the frame so when it arrives I can throw it together but want to make sure I buy the right forks, wheels and I so assume cranks as these seem to also come with offset and or boost spacing,, of which I know nothing.

Can someone or many someone's enlighten me.

Annoyingly I have non-boost forks and wheels that I assume aren't suited for the 29er boost spec frame I'm looking to buy (mainly as I like the colour but also a way to drag me into what is currently modern spec).

Thanks
Click to expand...

Fork and frame are totally independent. As long as the wheel matches it’s fine. You can use a non boost fork and front wheel on a boost frame no problem.

Chainring may be need or may not, it’s all about the chainline. If you have a set of 3 bolt fixing SRAM cranks you can just change the ring. Same with some Race Face I think.

Don’t get hung up about fork offset. Think of it as how far the crown points forward away from the steerer. It has been shortened in the past few years but frames will have a range that work. Eg Fox and Rock Shox don’t come in the same offset but work perfectly well on the same frame.

More importantly, what are you buying and why, when you’ve another thread up complaining you’ve too many ti frames and don’t have space? 😀
 
Thanks all. It was the various fork offsets I was trying to mostly get my head around why there were various different offset numbers and what it all meant and the impact on the potential build. If it makes little to no difference then that's fine then.

I didn't know crank boost was a thing either, assume it's just to allow for the fact that on a boost whe the cassette is slightly further out so a boost crank corrects this.
 
Er….not quite in the same camp as people who say ‘offset doesn’t matter’….and boost crank requirement quite an important thing.

Here’s my take. 29ers went with 51mm offset when they were originally designed, to reproduce 26 inch wheel handling characteristics with the larger hoops. The 51mm DECREASED the TRAIL figure on the bike and increased the speed and responsiveness of steering. This is good on where 51mm came from:

https://www.theproscloset.com/blogs/news/why-mountain-bike-use-reduced-offset-forks
The problem with 29 was that people thought everything would feel slow and awful. There was a complex relation between offset, trail and head angle.

Offset affects TRAIL - that is, the distance of a line drawn on the ground between where a vertical line through the axle hits the ground and where a diagonal line drawn through the fork legs hits the ground. See the drawing in the article below. Changed OFFSET also affects WHEELBASE - ie the distance between the contact patches. All this affects steering characteristics and the centre of mass of the bike. A short offset (eg 42mm) on a bike designed for 51mm shortens the dimensions of the front end. If the bike has a TT you already feel is short - ie you are running a 50mm stem - then this will be noticeable and will affect the bike’s abilities in steep stuff. My BfE Max has a 42mm offset fork but a LONG REACH to compensate. You can definitely feel the difference when you change from 51mm offset to 44mm or vice Versa. Is it huge? No. Is it important? Yes. This is a good article:

https://www.mbr.co.uk/news/fork-offset-head-angle-trail-342679
Cy T - and he is a really top designer - explains the fact that there are objective differences, but with changes to handlebar width you can accommodate different offsets:

https://www.cotic.co.uk/news/2019/forkoffset
Nice article in my view. He rides hard and designs a lot.

Now, boost cranks… Chainline is important, but only one factor. Some frames are designed around boost cranks and although you can get non-boost on there, the various gubbins (mech cable, chain device) which go on or around the BB won’t work with a non-boost crank - found that to my cost on a Transition Sentinel frame. You can shim out the BB, but it all gets a bit messy and sub-optimal.
 
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True, but the BFe is also designed around a fork from 120-160mm which is a far bigger change in character than any difference in offset.
I’m sure you can tell a difference back to back, but I’m also pretty sure that part way through a ride it’ll be forgotten about.
 
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