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Stem length vs fork offset.

ChrisRobin

Turbo Monkey
Jan 30, 2002
3,027
61
Vancouver
I've never heard of any sort of rules related to offset vs stem length, but I did hear one bike mechanic say your stength should match your offset. Anyone know anything about that and the reasons for it?
 

HAB

Chelsea from Seattle
Apr 28, 2007
10,614
923
Seattle
I've definitely found that having the stem significantly shorter than the fork offset feels weird/bad. I don't think I'd say they have to match exactly.
 

slimshady

¡Mira, una ardilla!
Dec 20, 2007
2,712
774
La Plata, Argentina
I did fiddle quite a bit with several stems and forks before it was cool, when beta testing some 35mm bar/stem combo for a fellow developing them. Wasn't even aware of the effects of one over the other, so your post got me thinking about my feeling at the time. Right now I have a 35mm stem and 38mm offset fork. Feels much more direct than my previous 50 length/44 offset or 50 length/38 offset setups. Worst case was 35 length/44 offset IIRC.
 
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ChrisRobin

Turbo Monkey
Jan 30, 2002
3,027
61
Vancouver
I'm currently on a 50mm stem with 44mm offset. Seems ok. I'm thinking of slapping on my 51mm CSU and see how that is with the same stem.
 
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kidwoo

Celebrating No-Pants Day
Aug 25, 2003
22,451
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In my pants
Equal or slightly greater. I bought a 35mm stem or two and they feel like shit.

I wouldn't worry about 1mm less.
 

HAB

Chelsea from Seattle
Apr 28, 2007
10,614
923
Seattle
Equal or slightly greater. I bought a 35mm stem or two and they feel like shit.

I wouldn't worry about 1mm less.
Yeah pretty much. My bikes are 40/37 and 44/45 right now. I wish that 45mm offset was a little shorter, but it's not bad.
 

englertracing

Monkey
Mar 5, 2012
205
57
La Verne
I wanted there to be a correlation, but there isn't.....
the offset is meaningless without head angle, and the result of head angle and offset is trail. Trail is the real number that we feel.
Offset and trail have an inverse relationship when head angle is held constant
More trail/less offset is more "stable" it has more castor or self centering
What your mechanic said makes me wonder If he has actually tested various offsets on a bike. I would say it has nothing to do with strength and everything to do with feedback.
Heres a simple breakdown
Slacker head angles create more trail, while lengthening the front center
steeper head angles create less trail, while shortening the front center
Less offset creates more trail, while shortening the front center
More offset creates less trail, while lengthening the front center
essentially Less offset is like slackening the bike, except it loads the front wheel instead of unloading it.
More offset is like steepening the bike, except it unloads the front wheel instead of loading it.
fork offset.jpg


More trail has the feeling of heavier steering and at the limits of traction tends to turn the wheel in more or tuck
less trail has the feeling of lighter steering and at the limits of traction tends to allow the wheel to push
Tucking is an unstable condition, as its tends to make itself worse,
pushing is a stable condition as it tends to self correct or at least not make itself worse.

This is the major difference between the two, at the limit when your going to loose the front the setup with more trail will give you a feeling through the bars of wanting to turn in harder, the setup with less trail will go neutral and provide you with that loose or free feeling while pushing.

If there were to be some sort of ratio it would be stem to trail ratio, and the way you gents are proposing it it would be inverse, which i cant quite figure out a reasonable relationship.

so you fellas are sayin
more stem length than offset.
lets go hypothetical here
65 degree head angle
650b wheels with a 2.5 or 63mm tire
44mm offset
that gives us 119mm trail
it is proposed in this thread that for 44mm offset that maybe a 45mm stem is required and 30 will be worse ok.
so the critical numbers here are 119mm and 45mm
so 119/45 = 2.64
or 45/119 = .378

what happens when we go to 50offset trail
65 degree head angle
650b wheels with a 2.5 or 63mm tire
44mm offset
that gives us 112mm trail
but now we aer supposed to have a 55mm offset stem to match the offset right? increase the offset increase the stem? that is whats being eluded to here. BUT increasing the offset decreased the number that matters, the trail
so now whe have 112 and 55mm
so 112/55 = 2.03
or 55/112 = .4910

im having a hard time expressing my thoughts on this but i am sure that will be apparent to whomever reads this.

I mean what the above indicates is that perhaps there is a ratio between trail and stem length, maybe YOU or rather someone wants the stem at least 1/3rd of the trail? like the .378 you see when you use 44mm offset and a 45mm stem.
but using this, the lower offset you use the LONGER the stem you need.

but Im not convinced you see.
outsider bikes 2.jpg

I have a set of these for my 40s
You can mount the stem in like 6 or 8mm increments,
oh and you can change the offset trailside with an allen wrench.....
With my experimenation I find
changing stem length reguardless of offset position, changes stem length feelings
changing offset reguardless of stem position changes trail feel.......o_O

For my mx bike I have a couple sets of triple clamps, and a rekluse E axle I use 22-24mm offset vs the stock 20mm to reduce the bikes tucking tendency.

e axle.jpg

This little gizmo let me dial the offset trail or track side on the 450

thats all it takes is 2mm to totally change the front end feedback.
I also have a top clamp that provieds 8 mounting positions.
for years i ran it full forward.
then i ended up with it only 1 click forward of centered think going form a 45mm stem to a 35.
in the mean time i went from 22mm offset to 23.5mm offset and was really digging it, could push the front end vus tucking at 20, and a little tuckey at 22mm

decided to give the longer "stem lenght" a try to spread my feet and hands apart further to provede some more support to my hands which have to deal with a 50hp barky 450 yanking them for hours on end not to mention the pumbling they take from plowing rock gardens full of baby head and cinder block shaped rocks. Moving back to the longer "stem" length was horrifying on the day I rode it like that. Longer stems make the bars swing in a wider arc, and with the pull of the motorcycle, it was constantly taking steering inputs when none were meant to be given. It was mentally tiring and confidence sapping. In my delirious dehydrated ran ragged day dreaming desert mind I tried to create a link between offset and "stem length" I though i had come up with something solid. I even emailed Aaron Bland (outsider bikes) about it, but in the end the idea faded and quit making sense. It was something along the lines that the more offset you had the less stem length you needed as you don't want too much leverage with too little trail . I Don't think there anything too it in reality, bar width is the main determining factor in the leverage you have on the steering. The stem length just controls the arc which it swings in.

I do make note that on the motorcyce a longer "stem" is destabilizing. While on a bike a longer stem is actually stabilizing and slows steering response in some ways. On a bicycle your more often pushing on the bars, or applying weight to them, where on the motorcycle, well the motorycle wants to leave you behind, your legs and body should be primarily taking the acceleration but jerks to the arms do happen. Back to the bike, yes we manual, and bunny hop, but even in a bunnyhop we end up pushing forward, lots and lots of forward inputs, the stem wants to straighten when we push on it and the longer it is the more this is true.

now at this point im out of ideas....
well, im still stuck here at work for a half hour i so here have some more
I think if we were able to isolate stem length which is not precisely possible we would find that the preference to stem length has to do with how much pushing the outside of the bar vs pulling the inside of the bar a rider prefers to do. Which will prefer which? I have to think about that more


If longer stems are stabilizing, perhaps they should be used with more offset/less trail to counteract the light wandering feeling?
If shorter stems are less stabilizing, perhaps they can be used with less offset/more trail.
ah we have gone in a circle now!:busted:

last bit now
if you run in hard with a bike that pushes neutrally, you feel it in which hand?
for me I believe its the out side hand the one pushing the bar, the bar pushes back slightly into your palm
if you run in hard with a bike that tucks, which hand do you feel it in?
I feel it in my inside hand as the bar applies pressure to my palm
now where i was going with this?
something about stem length and arc....... and then i got distracted by this dam job thing and lost idea.
would love to hear any disagreements, agreements, sarcastic comments, confusion etc.....
 

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kidwoo

Celebrating No-Pants Day
Aug 25, 2003
22,451
2,127
In my pants
If longer stems are stabilizing, perhaps they should be used with more offset/less trail to counteract the light wandering feeling?
If shorter stems are less stabilizing, perhaps they can be used with less offset/more trail.
That's probably why stems have gotten progressively shorter over the last 30 years. Less stability. It's what we're all after.

I actually have ridden lots of different fork offsets. Headangle has nothing to do with the correlation of offset to stem length.
 
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englertracing

Monkey
Mar 5, 2012
205
57
La Verne
Let me simpilize it for you
Dispite what many laymen imagine how nice and important the arc of the front wheel moves in matches the arc the stem swings in.
News flash, the Contact patch is behind the steering axis and swings in an arc behind the steering axis opposite of the arc the steering stem swings in in front of the steering axis.
Less offset actually makes the wheel more behind the steering axis....

Wow so the effects offset doesn't change with head angle really now?
A 650 b bike with 64ha and 44mm offset has 119mm trail
Slack it mondraker style to 61 and you gain 30mm trail to 149mm

Quite a change to the dimension that is actually creating the feedback your feeling through your handlebars, attached to various length stem
 

ChrisRobin

Turbo Monkey
Jan 30, 2002
3,027
61
Vancouver
Just a bit of an update. I'm still on my 50mm stem which feels good on the Megatower and I had my 44mm offset fork. Overall everything felt fine. I ended up doing a fork service and just to see what would happen, I installed my 51mm CSU. Overall I found the front end a tiny bit less stable in the turns when things got steeper and tighter, as in it felt like i had less room for error between making the turn and the front wheel tucking in.