Will I Notice – 4mm Fork Offset Increase ?

New 2019 27.5 RS Pike arrived today, it has a 46mm offset, my old fork had a 42mm offset. Will I notice the difference (on a Whyte T130) ?

I have searched the internet and get the impression that shorter is better, but..

4mm? No, probably not.

Nope.

Nope, one of the more recent smoke & mirrors fluff from the industry IMO.

I’ve got one bike with 51mm offset and another with 44, cant tell the difference, and actually cant even remember which is which now.

I’ve got some older 29er 100mm forks whith short offset and can feel it.

Basicly shorter offset has the same effect as slacker head angles, but without increasing wheelbase.

So if you tend to ride steeper tracks then a long bike is a benefit, go for longer offset. Or vice versa if you want a shorter bike but still want stability on faster more open tracks.

I have searched the internet and get the impression that shorter is better, but..

Internet is stupid

So if you tend to ride steeper tracks then a long bike is a benefit, go for longer offset. Or vice versa if you want a shorter bike but still want stability on faster more open tracks.

Sense

If you are sensitive to small changes then yes you will notice it for 10 minutes and then it will be forgotten as the difference is not big enough to really change anything.

This might help you  https://www.bikeradar.com/news/bikeradar-tech-talk-podcast-1-fork-offset/

I think as said above the answer is No you wont notice it…..

Yes, you’ll notice it because you know there is a difference. If you hadn’t been told the offset then you wouldn’t notice though 🙂

Worth thinking of it as a tweek to frame geometry. You cant adjust the head angle or reach ( and thus front center or wheelbase) but you can adjust an existing frame which has the wheelbase you want but has too much/not enough stability in the steering.

Bear in mind people have been making and experimenting with big offsets for rigid forks (on one fatty is 55mm Stooge mk4 is 80mm iirc). Which allow slacker angles and longer wheelbase whilst keeping the steering really sharp.

If you were starting from scratch designing a bike you have 3 variables for the front.

A)Reach
B)Head angle
C)Offset

A+b gives you the front center
B+c gives you how stable the steering will feel (aka trail)

If you dont consider c then cant adjust the other two independently. E.g. a dh bike doesnt want as long reach as an enduro bike, but does want a long front center, so short reach, slack head angle, long offset. Conversely an enduro bike wants a long reach, not a daft front center (unless you want a geometeon, chris porter likes really short offsets though so its an exception in that its designed for extra ordinary levels of stability) so a steeper head angle, so a short offset takes the steering to comparable stability with the dh bike.

There appears to be a trend at the moment toward shorter offset forks (which increase the amount of steering trail). The argument is that increased trail provides more stability and self-centring, so should be good for rough high speed/steep descents. The potential downside is a slower steering feel at lower speeds. 4 mm offset doesn’t sound much, but it does amplify the difference in trail at the tyre contact patch. The range of offset we typically see in forks is 42-51 mm and that does significantly change the amount of trail.

But will you notice any difference in real life? Probably not. The steering might “feel” a little different but I very much doubt it will affect your riding in any meaningful way. Bike manufacturers have different views on which is “better”. Some are now fitting short offset forks to 29ers and others are bucking the trend by fitting long offset forks to 27.5 wheels. In other words there is no definitive “best” and it all depends on how “trail” fits in with the rest of the bike geometry and its intended purpose.

Here’s a calculator:

http://www.kreuzotter.de/english/elenk.htm

Basicly shorter offset has the same effect as slacker head angles, but without increasing wheelbase.

Fork offset must change the wheelbase as you are moving the wheel centre relative to the steering axis. More offset adds to the wheelbase and vice-versa. Although it’s a lot less than the wheelbase change from messing with head angles, which I think was your point.

We have used this principle for many years in motorsport i.e. high caster angle with loads of offset to keep the trail low (and hence relatively light steering with plenty of stability). In that application it can make a huge difference to the steering weight.

I went from a 44mm to 51mm on a 100mm XC bike
I could immediately tell there is a difference. I never could actually make my mind up what the difference is though. And the longer I have spent on the 51mm the less I remember what the 44mm was like.

“We have used this principle for many years in motorsport i.e. high caster angle with loads of offset to keep the trail low (and hence relatively light steering with plenty of stability). In that application it can make a huge difference to the steering weight.”

This is where I get puzzled. How does the steering feel change when you go from a slack long offset design to a steep short offset design, where both geometries have the exact same amount of trail?

Thinking about this a little more, you would expect with the popularity of 29ers and ever slacker head angles that fork offset should really be increasing to compensate for the large increase in trail with larger diameter wheels and slacker head angles. But it seems to be the opposite trend at the moment with every geo parameter effectively increasing the amount of trail and hence increasing steering weight. The only thing balancing this out is ever wider bars, which counter the increase in steering weight with increased trail. The ratio of bar width to trail basically determines the steering weight at your hands.

This is where I get puzzled. How does the steering feel change when you go from a slack long offset design to a steep short offset design, where both geometries have the exact same amount of trail?

Because it’s not only “trail” that affects the overall steering feel. Trail primarily affects steering weight, while caster (head angle) affects how much the steering self-centres and how the dynamic trail changes as you increase steering angle. So it’s the combination of trail, head angle and bar width that give an overall “feel”.

“The ratio of bar width to trail basically determines the steering weight at your hands.”

Stem length definitely affects this too. Going from a 50 to 35mm stem made my steering feel noticeably quicker and lighter (which I didn’t like). Not exactly sure why!

“So it’s the combination of trail, head angle and bar width that give an overall “feel”.”

What does wheel diameter do? Obviously as it increases it increases trail but is that all? And there’s a wheel rotational inertia thing too, gyroscopic effects.

Stem length definitely affects this too. Going from a 50 to 35mm stem made my steering feel noticeably quicker and lighter (which I didn’t like). Not exactly sure why!

Longer stem means you are applying your steering torque further away from the steering axis, so it’s less “direct” and “slower” feeling.

What does wheel diameter do? Obviously as it increases it increases trail but is that all? And there’s a wheel rotational inertia thing too, gyroscopic effects.

Wheel diameter changes other things, but obviously it increases trail and hence why 51 mm offset was originally introduced to compensate. I haven’t given it much thought beyond that as far as steering feel goes.

Princess and the pea?

How does the steering feel change when you go from a slack long offset design to a steep short offset design, where both geometries have the exact same amount of trail?

Steering feel as in amount of wheel flop or how neutral it is/isn’t won’t change but the weight distribution or how you set up the steering (grips/bars relationship to head tube and front wheel) changes. Basically – what trail do you want, what weight distribution do you want, then set up HTA and offset (and see if it works).

What does wheel diameter do? Obviously as it increases it increases trail but is that all?

Mostly all, a larger OD wheel can also handle longer trail without the ‘flop effect’ being quite as noticeable as it is with a smaller OD wheel.

“Flop” will be greater if you go to a slacker head angle and keep the trail the same by adjusting the offset. Flop is defined as trail x sin(HA) x cos(HA) where HA – head angle. sin x cos of an angle is at a maximim at 45 degrees, so at normal head angles it will be increasing as the head angle reduces.

^ +1 maths, good point. Within the range that you might be homing in on for a given set up, say +/- a degree and a half and +/- 8mm offset you get ‘measured flop’ within a mm or 2 at the same trail. That’s enough change in where the front wheel sits compared to the head tube / bars to make a significant difference, not a ‘flop’ change I’d worry about, but 4mm or so change in flop does feel different enough to spot on a light rigid bike ime (it’s a lot when it’s ~20-25% of the total but less of note on a current MTB).

The navel gazing about fork rake amuses me as I have always thought that head angle and offset make no differences at all when the rear wheel is off the ground.
And clearly even less when the front wheel is.

“Flop” will be greater if you go to a slacker head angle and keep the trail the same by adjusting the offset.

True. If you had a 90 deg HA there would be no flop at all i.e. the wheel would just stay vertical when steered.

The navel gazing about fork rake amuses me as I have always thought that head angle and offset make no differences at all when the rear wheel is off the ground.
And clearly even less when the front wheel is.

So are you saying it doesn’t really matter what HA and offset you have on your bike? Or just that incremental changes are not so critical? I would tend to agree with the latter if that’s what you mean. But it’s still interesting to understand the affects of geometry if you are technically minded, even if the real world dynamics are far more complicated. Trail for example is a very dynamic parameter when riding over rough ground. But you still need a baseline to work from.

Fork offset must change the wheelbase as you are moving the wheel centre relative to the steering axis. More offset adds to the wheelbase and vice-versa. Although it’s a lot less than the wheelbase change from messing with head angles, which I think was your point.

Yes but that is by about the same ammount as the change in offset. 4mm on the wheelbase really is going to be unnoticeable. Whats the tolerance at the factory? Iirc there was an article about trek sessions in dirt that quoted half a degree, which is near enough 10mm on a dh fork.

I was pretty sure I noticed a difference when I went from my pikes to my G2 34s with their custom offset. Then it turned out they were both the same offset, it’s just that the Foxes had a big sticker on which added performance.

Whats the tolerance at the factory? Iirc there was an article about trek sessions in dirt that quoted half a degree

About right – +/- 0.5 degree. Usually closer, but chat on angles when you might not know the actual angles you’re riding is also a bit pointless. A lot more noise in the ride/terrain and difficulty to measure a complex-shaped FS MTB than either with a trad frame tube road bike.

Yes but that is by about the same ammount as the change in offset. 4mm on the wheelbase really is going to be unnoticeable

I agree. The effect of 4 mm offset on trail would be a bit more significant, but not something I would personally care about. Both my current bikes (29ers) have “standard” 51 mm offset forks, so I haven’t tried anything less in recent years. I doubt it would be any kind of revelation.

About right – +/- 0.5 degree. Usually closer, but chat on angles when you might not know the actual angles you’re riding is also a bit pointless. A lot more noise in the ride/terrain and difficulty to measure a complex-shaped FS MTB than either with a trad frame tube road bike.

It’s still useful to know what affects what though. Tolerances are the same regardless of your baseline geometry and if you fit a fork with 4 mm less offset to the same bike then the change is still 4 mm, with its corresponding change in trail. I do agree though that it’s probably all still in the noise when riding on the trail and in this case the tolerances in the damper settings between the two forks will probably be more significant than the 4 mm offset difference. Especially if they are totally different dampers or the old one was knackered.

I’m excessively interested in this stuff, I’ve always liked vehicle dynamics and neither of my bikes are set up with stock geometry. On my Levo I had the 150mm travel 51mm offset Sektor replaced with a 160mm travel 42mm offset Lyrik, whilst on my Zero AM I have a -2 deg headset with the usual 150mm Pike.

So the Levo’s at a 65 deg HA with 126mm trail vs stock at 65.5 deg HA with 113mm trail; and the Zero AM is at 65 deg HA with 119mm trail vs stock at 67 deg HA with 105mm trail.

Interesting to note that going from 51mm to 42mm offset on a 29 increases the trail by the same amount as slackening the head angle by 1.3 degrees.

So to be clear, changing offset does not affect flop at all? (Flop is the vertical position change of the headtube when turning the bars, isn’t it?)

Curiously the new breed of 29er DH bikes are running 63 deg HA with 58mm offset on a Fox 49, giving 123mm trail, so less than my Levo! But I’ve heard that quite a few WC DH riders are fitting custom offset crowns from Mojo Rising:

https://www.mojo.co.uk/mojo-morc-crowns-3-c.asp

the tolerances in the damper settings between the two forks will probably be more significant than the 4 mm offset difference.

Rock Shox air shocks show +/- 2mm tolerance on shock length. Throw that into a 3:1 linkage and wooo… 12mm potential difference!

@chiefgrooveguru according to the formula, changing offset will change flop by changing the trail, which is one of the factors. More trail=more flop. The slight head angle change you also get will operate in the same direction. And that is what I understand flop to be as well. The trigonometry is well tricky though.

I need to make a spreadsheet and some graphs!

Tried to understand this but it’s more complex than Brexit, sat on a jet, on top of a conveyer belt whilst orbiting a black hole.

Not even sure what the offset is on my 29er forks… Don’t think I will ever look after this thread either!😁

The navel gazing about fork rake amuses me as I have always thought that head angle and offset make no differences at all when the rear wheel is off the ground.
And clearly even less when the front wheel is.

Which is all well and good if you’re Rad to the power of Sick, but for everyone else…?

So are you saying it doesn’t really matter what HA and offset you have on your bike?

No… He’s just being his usual unhelpful, facetious self…

True. If you had a 90 deg HA there would be no flop at all i.e. the wheel would just stay vertical when steered.

Yes, but an incredible amount of instability! Think shopping trolley wheel with a mind of its own, and you’re about right…

I agree. The effect of 4 mm offset on trail would be a bit more significant, but not something I would personally care about. Both my current bikes (29ers) have “standard” 51 mm offset forks, so I haven’t tried anything less in recent years. I doubt it would be any kind of revelation.

Considering most MTB’s have trail figures in the region of 100mm or so, and most modern MTB wheelbases are typically 12x that, it’s fair to say that the effect on the wheelbase is negligible, but the effect on the trail can be noticable, albeit only by a few percent. Having recently gone from 51mm offset down to 44mm offset, I’d argue that depending on your riding style and preferences, it can be very noticeable, or actually make no difference at all… For me, riding a mid travel Trail bike predominantly on fast, steep technical tracks etc, I found the increase in stability (on a bike with a 65deg HA already) caused by the increase in trail, very noticeable indeed, and for me, in a good way (though this will be subjective of course).

There appears to be a trend at the moment toward shorter offset forks (which increase the amount of steering trail). The argument is that increased trail provides more stability and self-centring, so should be good for rough high speed/steep descents. The potential downside is a slower steering feel at lower speeds. 4 mm offset doesn’t sound much, but it does amplify the difference in trail at the tyre contact patch. The range of offset we typically see in forks is 42-51 mm and that does significantly change the amount of trail.

The thing is, and don’t get me wrong my name isn’t Chris Porter (though to a degree he has a point), I have NEVER felt the need for faster steering on an MTB! EVER! I’ve got to a point on some tight, twisty singletrack on a modern LLS (Long/Low/Slack) 29er full sus with reduced offset fork where I noticed that it was a little slower steering than I was used to, but quite simply I just pushed a bit harder on the bars to compensate. I’ve done a few Enduro days now (on a KTM 450 ECX-F, rather than Enduro MTB) and these bikes have wheelbases another 250-300mm longer than a typical enduro MTB, along with significantly less offset and much less steering lock.

I can assure you, they may require a bit more effort, but you can get a 100+KG Enduro Motorbike round similarly tight ans twisty singletrack that you can get an Enduro MTB if you’re prepared to put the effort in… It may not be its forte, but it’ll do it. Of course, these bikes are designed to be stable over bumpy terrain at speeds an MTB will never reach, so we don’t need to go to those extremes in terms of LLS, but whilst I’m a firm believer that you can go too far, the likes of Porter and Mondraker etc do have a very valid point!

Curiously the new breed of 29er DH bikes are running 63 deg HA with 58mm offset on a Fox 49, giving 123mm trail, so less than my Levo! But I’ve heard that quite a few WC DH riders are fitting custom offset crowns from Mojo Rising:

Word is that Fox dropped a bollock with the super long offset on the first Fox 49’s. Worried that the bikes would suddenly be incredibly difficult to turn, but not realising the significant reason that a lot of riders were turning to 29er wheels on their DH bikes was increased stability and cornering grip, Fox over egged the pudding with the offset, and WC riders were immediately requesting shorter offset crowns.

Rock Shox air shocks show +/- 2mm tolerance on shock length.

We are talking here about a company that ships most of its forks with barely a trace of oil in the lower legs, and who believe that the best way to increase mid stroke support on a fork is to increase the volume of the negative air chamber whilst chocking the positive air chamber full of volume reducers…

As a triumph of Marketing BS over substance, Rockshox are right up there with Trek & Specialized these days!

The trigonometry is well tricky though.

Not really. We know the rear axle is a fixed point. We know the fork length. We know the reduction in offset. On a 65deg HA bike, a 4mm increase in offset is going to increase the wheelbase by 3.625mm, and decrease the trail by the same amount. Lets assume the bike had a 1200mm wheelbase and 100mm of trail already… You have increased the wheelbase by approximately 0.3%. Bugger all basically!

The reduction in trail is 3.625% though, not massive for sure, but 12x more noticeable than the effect that the increase in trail has on the wheelbase length… Put it this way… Many people didn’t really understand what they hype was in changing from 26″ to 27.5″ wheels, and when they tried it, didn’t notice much difference at all. For others, it was a total game changer! And then there’s guys like me who were on 29ers before the 27.5 thing even kicked off, because we were already after more stability/grip/rollover speed etc…

The thing to take from the fork offset debate, is there really isn’t a hard and fast answer to what is right, only what might work for you… I know given a choice, I prefer a shorter offset fork myself, but then there’s guys out there (Porter for one) pushing tboundaries and taking fork offsets to an extreme that would feel odd to me. You should also not forget that bikes are designed as a package, and whilst it might be OK to alter the fork offset by 10% or so and the bike still behave acceptably (as you might if you increased your fork travel a similar amount), if you change it by a much larger amount, the likelihood of it ruining the handling of your bike will increase drastically too!