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This was Pinkbikes take from a Sentinel review....
I've been answering all sorts of questions about offset lately, many from riders who are worried about getting left behind by some sort of new “standard.” Should you rush out and buy a fork with the least amount of offset you can find? Well, no. The amount of offset does make a noticeable handling difference, but it's not as cut and dry as saying that X amount of offset is bad and Y amount of offset is good – there's more to it than that, and installing a fork with the least amount of offset possible isn't going to automatically turn your bike into a magical shred sled.
I spent a day in the bike park switching back and forth between two Fox 36 forks, the one that came on the Sentinel, which has 44mm of offset, and one with 51mm of offset, which is what the majority of 29ers are currently spec'd with. I started off by taking three laps on the stock fork, and then made the switch to the fork with 51mm of offset. The difference is very noticeable – the increased offset felt more like what I'm used to, and the bike felt livelier, but it was also easier to oversteer and wash out the front wheel – the feeling of unlimited front wheel traction that the 44mm offset fork delivered wasn't there anymore.
I timed all of my runs, but the numbers didn't end up indicating any statistically significant difference between the two offsets; I felt like I was able to adapt my riding style fairly quickly to both forks. After swapping back and forth between the two offsets it was clear that there are benefits to the stock, reduced offset fork on the Sentinel – namely better front wheel grip and more stability – but the bike works just fine with a 'regular' 51mm offset fork as well.
Is there any reason you would want a different offset on a fork, other than to adjust the trail?
I was thinking about this, and unless I'm missing something, it's like this...
Steering speed is really just a function of trail (?)
Trail is a function of head angle (slacker = more trail), rake (more rake = less trail) and wheel size (bigger wheel = more trail)
The affect of a small change in rake on trail is pretty small compared to the other two factors...
e.g. The Sentinel has a 64 degree head angle, and with a 2.4 on the front, has an overall radius of 372mm (using BikeCalc's handy table https://www.bikecalc.com/wheel_size_math). Some basic trigonometry tells you that's a trail of 181mm with zero rake [372/tan(64)].
The rake reduces the trail by rake/sin(head angle), so a 51mm rake reduces the trail by about 73mm - OK, about 40% of the above. But reducing the rake by 7mm increases the trail by just 7.8mm, or 2% of the trail.
So I guess the question is why not just reduce the head angle to have the same effect? I make it almost exactly a degree of head angle to balance it out. Is there another limit on head angle? Stresses on the frame/headset? As far as I can see it's going to have exactly the same effect on handling.
I guess you could it's easier to reduce the offset than slacken the head tube (in which case why didn't we start there?) but it does seem like the bike industry changing something else to make it more complicated for everyone - and how do we make it less complicated? Easy, buy a new frame/fork/bike 😀
Steering feel isn’t just a function of trail. The trail dominates at high speed. At low speed the way the front of the bike drops as you turn the bars has a big effect. With high steering angles the movement of the front contact patch to the inside of the turning arc destabilises things. I’m sure there’s more from the wheel/fork end of things.
And there’s wheelbase, reach, front-rear centre ratio, bars and stem to consider.
But if all you’re thinking about is trail, front centre and reach, to get the same trail with 51mm and 42mm offset, the 51mm bike needs a 1.3 deg slacker head angle. That change adds 23mm of front centre length. Do you then shorten the reach by that much whilst increasing the stem by the same? But that’s going to change the steering feel too.
Can anyone tell me in basic terms what running this wrong ofset on a sentinel is likely to feel like at low and high speeds?
I think the word you’re looking for is: “fine”.
The quote above from that Pinkbike review sums it up pretty neatly.
to get the same trail with 51mm and 42mm offset, the 51mm bike needs a 1.3 deg slacker head angle. That change adds 23mm of front centre length. Do you then shorten the reach by that much whilst increasing the stem by the same?
Dafuq you on abah't NAO Chief?
You can't quantify what trail increase/decrease will result from H/A or offset change without tyre diameter (axle height from ground).
And you can't quantify the front centre increase/decrease from any of the above without first quantifying axle to crown length.
Wihout these quantities your post above is just number vomit
Stick with "fine" and ride your bike moar
“Dafuq you on abah’t NAO Chief?
You can’t quantify what trail increase/decrease will result from H/A or offset change without tyre diameter (axle height from ground).
And you can’t quantify the front centre increase/decrease from any of the above without first quantifying axle to crown length.”
Sorry, that was for a Sentinel, or indeed my Turbo Levo. So 29” tyre and 570mm A2C (160mm fork). Obviously head tube length has a small effect too.
“Stick with “fine” and ride your bike moar”
Certainly doing that with electric power to speed and extend the commute!
And there’s wheelbase, reach, front-rear centre ratio, bars and stem to consider.
I guess you're right. My thinking was that reach can be adjusted quite easily with bars and stems.
Thinking about whether there's an effect on front-centre and wheelbase is making my head hurt. If the axle's in the same place there isn't, and you could increase the trail by slackening the head angle and keep the axle in the same place.
What head tube length, A2C length, wheel size and reach is that for?
Thing is, if you're changing the frame design to change the head angle, you could also move it back (shorten the top tube) to give the same front centre and wheelbase.
"Thing is, if you’re changing the frame design to change the head angle, you could also move it back (shorten the top tube) to give the same front centre and wheelbase."
Yes, but then you shorten the reach, which changes the fit. And if you add stem length to get the effective reach back then you change the steering feel.
Head tube 120 including allowance for headset
A2C 497
Wheel diameter 770 (29er)
Reach 507
I drew it up on CAD just to get a picture of how trail changes with head angle change, and how that compared to offset change.
Yes, but then you shorten the reach, which changes the fit. And if you add stem length to get the effective reach back then you change the steering feel.
Yeah, true.
Why look for like for like when you could have had something that actually worked?
Yes look for a better performing product at the same time if you're setting up the best bike you can.
What I was talking about was changing the offset of a rigid or sus fork and keeping all else constant, or close to, to see what offset changes alone does for a bike - or looking at offset and HA combos etc. Purely geo experimentation.