Which offset shall i buy?

Sethius

Crashed out somewhere
Going the large, based off 37mm offset 150mm 27.5 fork. Question is, do I roll with a the 44mm or 51mm offset option for the 140mm manitou mezzer or do I go 150mm with 51mm offset. Rocking a 29x2.4 ardent and 27.5x2.4WT dissector. Rear travel is 140mm. Bike to be used for all styles of shangians. Has a 35mm length stem and 760mm bar.
 

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Nambra

Definitely should have gone to specsavers
Found this to be a useful clip, along with the blog article linked in the description.


I’m by no means an expert, but I believe the gist of reduced offset forks is to maintain trail as head angles get slacker (translating to better handling at slow speed). With your fairly slack 65.3 HA you’re probably going to want the 44mm offset over the 51mm.

I’ll add the usual disclaimer that I could be full of shit too. Have you hit up the OEM for a recommendation?
 
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LPG

likes thicc birds
From what I've seen from less biased/sensationalised sources the difference is fairly subtle and not really worth the big deal that's been made. I went with a reduced offset on my Bird AM9 build which has a similar ht angle. I dont know if it was important to the handling but I am definitely happy with the way my bird AM9 rides with the reduced offset. The long wheelbase and slack HT dont make it too much to manoeuvre in tight spots even with the reduced offset. Better or not I dont know but it isn't something that I would be worried about trying.
 

kten

understands stuff moorey doesn't
Found this to be a useful clip, along with the blog article linked in the description.


I’m by no means an expert, but I believe the gist of reduced offset forks is to maintain shorter trail as head angles get slacker (translating to better handling at slow speed). With your fairly slack 65.3 HA you’re probably going to want the 44mm offset over the 51mm.

I’ll add the usual disclaimer that I could be full of shit too. Have you hit up the OEM for a recommendation?
A reduced offset fork will increase the trail which calms the steering slightly. At a head angle of 65 and below I would personally go with a reduced offset fork.
 

Halo1

Likes Bikes and Dirt
I am looking to upgrade the fork on my gt sensor. It currently has a reduced offset and rides well so I will stick with it. My Hard tail has a Normal offset fork and I also like the way it rides so will stick with that on that bike. I am probably in the camp that would not notice the difference too much.
 

Sethius

Crashed out somewhere
This is the inner debate, run it at 140mm with reduced offset, or make it 150mm reducing the reach which I'm happy to do and keeping a 51mm offset, and the BB height is up there anyways if I go the later, I need the clearance for my 36t ring.
 

rowdyflat

chez le médecin
Not sure its that important but wouldnt fork offset V head angle be linked variables in terms of trail so why change both ?.
 

beeb

Dr. Beebenson, PhD HA, ST, Offset (hons)
One point worth considering is that a 140mm or even 150mm won't actually give you "matched" travel. Took me a while until I figured this out but I noticed it one day while playing around with Linkage (the suspension design computer program). While forks are designated by travel in millimeters, it's a deceptive measurement in isolation as it doesn't account for head angle and the effect that has on (reducing) vertical travel.

The rear suspension of most bikes has no (or very little) rearward axle path, so travel is essentially only vertical travel. Howevere because the fork is installed at an angle, a significant amount of its designated travel is actually used up in the rearward axle path.

Given the geo you've mentioned as an example - for a 160mm fork, you only really get ~143mm vertical travel. (I've attached a screen grab from a triangle calculator below)

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Obviously there's a whole bunch of dynamic variables in real world use that will dynamically effect head angle and therefore slightly alter the amount of vertical travel a fork will offer at any given moment, but overall the principle remains.

For reference, a 140mm fork (I guessed at it roughly steepening the bike ~1deg) would only offer ~126mm in vertical travel, meaning the front-end would probably get overwhelmed quite easily before the rear
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Aaaaand back on topic, I much prefer the reduced offset forks. I've ridden with both a standard and short offset fork (same make/model of fork too) on the same 27.5" bike at around 66 degrees and the difference was anything but subtle - I believe the difference is more pronounced the steeper the head angle though. With the reduced offset I found the steering much calmer, and a little less prone to understeer or washout. Some of this feeling is similar in concept to riding with a longer stem thoug -ie: for the same length stem, a reduced offset fork brings the wheel further in so it is a little easier to weight the front wheel. It is different to just putting a longer stem on a standard offset fork though, as it tracks straighter though corners with less minor corrections needed. The reduced offsets can be a bit more floppy at or near standstill though.

I have read (but not tried), that a standard offset is better through rockgardens though if that's important to you - though with the headangle you're looking at it shouldn't be much of a problem either way. The Ripmo was a plow when I had it setup at ~64deg HA, but it also wasn't too keen on quick changes of direction (that could be a lack of upper-body strength in my case though).
 

Sethius

Crashed out somewhere
Haha yep, my concern by increasing it to 150mm front will be the BB, granted my peak days were all about high bb so may not hurt, and I do tend to keep pedalling even in rocky sections. Aslong as the shorter offset isn't going to tuck me up too much. Seems to a fine balance.
 

beeb

Dr. Beebenson, PhD HA, ST, Offset (hons)
Lol, wait - what the f*** was I actually responding too? I must've got a couple of threads crossed up in my head. Ther I go going all triangles when the situation only dictated a Yes/No. :oops:

Take 2 - If reach fit is right, fork offset will just change steering response and not much else. But I'd still choose the longer fork. A few mm higher at the BB will make sweet FA difference and can somewhat be trimmed by the rear shock pressures/spring choice.
 

Sethius

Crashed out somewhere
Reach is good, I'm not into this super long reach stuff, I'm not racing and more interested in doing stupid playful things, just like the idea of approach angle when I get tired. Tried to get along with full 29ers, evils, hightower, lt genius etc but just weren't for me.
 

born-again-biker

Is looking for a 16" bar
Just for everyone's reference....(apologies if I am just preaching to the choir)

Reduced offset fork increases trail.
Slacker HTA increases trail.
(these two things are not an offset of each other)

and/therefore,

More trail makes steering slower/calmer because you have effectively increased the natural "lever" that returns the wheel to the straight ahead position and reduced the size of the "lever" the rider has to turn the wheel away from straight ahead.

AFAIK the new reduced offset forks have very little to do with slow speed "feel" or steering response. They do not "fix" older bikes. They are not needed if your bike is already awesome.
It was brought to market in the pursuit of high speed stability (watch how the Transition guys ride down their trails - very fast, very aggressive)
....and as a correction when significantly slacker HTA's pushed the front axle too far away.
The reduced offset forks are not a value-add improver by themselves - they are a necessary change to help offset other desirable changes to the frame geo that bring undesirable characteristics - namely mega reach and super slack HTA.

A lot of the decision is personal preference, not science or hard data. i.e. - do you prefer plow/stability/calm-in-the-rough (reduced offset).......
.....or fast, twitchy, flickable steering (regular offset)

I've read a lot of these discussions where the logic gets mixed and matched and quickly approaches mis-information.

I would be interested to now about fatigue over a long descent. I reckon a 51mm offset would sap less energy because less effort should be required to turn the bike (I'm talking about mega Euro Alps / Whistler kinda stuff). For short descents it wouldn't matter.

(I wonder if I'll ever use the word "offset" that many times at once?)
 

Paulie_AU

Likes Dirt
I run 42mm offset on a 65ha hardtail with 160mm fork (66ha at native 140mm) and it is super playful. Same hardtail was 46 offset 150mm and good too. Cannot really compare feel though because the 160mm is a Lyrik vs 34.

Have 44mm offset on 64ha 29er and it is plenty playful too just requires more aggression. I need to up the speed a bit more to go finds the limit
 

FigBo0T

Puts verniers on his headtube
I just came across this video while searching something unrelated. Thought it might be of use.

 

Sethius

Crashed out somewhere
Just gonna go 150mm and 51mm, sanction I'm riding is modded to 64deg. And I find it could be slacker, gets around corners just fine. Reach should come to mid 440s which is ideal. Sanction was 435mm, so i gain the better seat angle and a longer reach without dullness I will be stoked. Shorter CS on last vs sanction but longer wheel base. Pretty excited!! Sanction always feel abit odd, last is lower too. Should be tall front, and short arse end, gonna be a hoot!
 
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