Demystifying Fork Offset and Trail

Words Dave Tolnai
Date Feb 6, 2017

Unkle Dave,

It’s winter and I’m snowed in so naturally I have too much time to ponder first world issues such as fork offset, the virtue of carbon rims, and how large to go for my front tire next season.

Anyway, I’m swapping out my 150mm Revelation (which came stock at 140mm) with 46mm offset for a 150mm Pike with 51mm offset on my 29er trail bike.

Axle to crown is about the same for each fork, but will I notice a handling difference due to the  different offset? Is there anything I can do to negate any handling quirks due to the change such as installing an offset bushing or 160mm air shaft to slacken her out half a degree?

Sincerely yours,
Cabin Fever

Dear Cabfab:

I appreciate your question. Very occasionally, Uncle Dave feels the need to talk about something technical, such that he can maintain his bona fides as a “bicycle expert”. This simple geometry problem that you have presented is perfect. It seems complex, but a precocious child with a protractor and a steady hand could probably figure it out if you promised him or her enough Fruit Loops.

The first step is to plot out your stock bicycle as a base case. After swapping a few e-mails, we determined that your bicycle is a Specialized Enduro 29er. I assumed size Large. That gives us the following (with super, duper fancy Autocad screen caps):

Stock Stumpjumper Stock Specialized Stumpjumper 29er Large Wheelbase 1179mm BB Height 338mm Revelation 29er – 140mm – Axle to Crown 548mm Revelation 29er – Offset 46mm HA 67 degrees SA 74 degrees Trail 107.08 mm

The important number here is the trail measurement. Here is a pretty good explanation, but simply put, more trail means that a bike will be more stable and require more steering input to turn. Slackening your head angle increases trail and increasing your offset decreases trail. Make sense?

The next step was to see what the change in fork height did to your bike. That gives us the following:

Stumpjumper +10 Specialized Stumpjumper 29er Large – +10mm Fork Wheelbase 1182.9mm BB Height 341.4 mm Revelation 29er – 150mm – Axle to Crown 558mm Revelation 29er – Offset 46mm HA 66.6 degrees SA 73.6 degrees Trail 110.33 mm

So adding 10mm of travel slackened your bike out by almost (but not quite) a half degree, which increased your trail by a bit less than 3mm. As well, a few other things happened. Your seat tube angle slackened a corresponding amount, your BB raised a tiny bit, and your wheelbase got a bit longer. What does this all mean as to how your bike is going to handle?  I have no idea, but I’m guessing it probably didn’t feel all that much different after you made that change. You’ve got things that make it more stable (longer wheelbase, more trail) and you’ve got things that make it a bit tippier (higher BB).

Our last step was to see what a change in offset would do for your bike. To minimize the number of variables, I kept the axle-to-crown identical, and only changed the offset (as we already showed what a change in axle-to-crown does).  This gives us:

Stumpjumper +10 +Offset Specialized Stumpjumper 29er Large – +10mm Fork, +5mm Offset Wheelbase 1187.5mm BB Height 340.6mm 150mm – Axle to Crown 558mm 150mm 29er – Offset 51mm HA 66.6 degrees SA 73.6 degrees Trail 104.24 mm

Strange things happen when you change your fork offset. The front end of your bike drops a tiny bit, but not enough to change the head or seat angle measurements appreciably (although your BB does drop a mm or so). As well, your wheelbase lengthens (a bit less than 5mm) and your trail shrinks (a bit more than 5mm). Compared to your base case you have less trail, more wheelbase and a higher bottom bracket. What does this mean?  Beats the hell out of me. It’s interesting to see that a change in offset has a much larger impact on trail than one would intuitively assume. You’re probably looking at 20mm of additional travel or a full degree in head angle change to bring it back to where it was before you changed your offset, but that’s also going to take your BB quite a bit higher.

The amount of money required to “fix” all of this seems excessive.  And each time you target something for a positive change, there’s inevitably some negative that creeps in as well.  The offset bushings may help, but I think you should spend your money on beer instead.

Sorry,
Uncle Dave


Uncle Dave’s Music Club

I really need to start a list of what music I’ve talked about. I’m totally losing track. But I know that I’ve been meaning to talk about Parquet Courts for a long time, so I’m going to do that. If you’re not in to white kids playing weird music on guitars, please stop reading now.

Parquet Courts sort of came out of nowhere for me. I was listening to Light Up Gold a bit, and figured they’d be worth checking out when they swung through town. They seemed almost novelty-act-like at this point, but I left the house a lot more in those days, so it was no big deal. The majority of their set was the yet unreleased Sunbathing Animal and it was like a punch in the face. In these times of universal access to all music, it’s not very often that your first exposure to anything is live, so this felt pretty special. Anyhow, here are some songs.

“Human Performance” – Oh my god! Puppets! You have to watch this.


“Black and White” – Oh my shit, I love this song. I’m a sucker for a clean bass line.


“You’ve Got Me Wonderin’ Now” – This is a real toe-tapper.


“Sunbathing Animal” – With an actual sunbathing animal.


“Pretty Machines” – Also good.


“Berlin Got Blurry” – I think we linked this sucker already. But it’s real good.


And a bunch more with no videos.

“Uncast Shadow of a Northern Myth” – This feels really, really deep. Very beautiful and sad.

“Instant Disassembly” – For a while, it seemed like they were going to put a slow and sad song on each album.

“Master of my Craft” and “Yonder is Closer to the Heart” – These were the novelty act days, but I’ve come to re-assess them now that I know the band.

“Always Back in Town” – I don’t know. I just like it.


Cabfab – amid fierce competition you have won this week’s prize.  You have won a pair of Race Face Chester Pedals. They serve up a nylon composite body, a fully serviceable sealed bearing and bushing system and chromo axles. And they only weigh 340 grams! Send us an email to claim your prize.
4-Chester-Pedal-Colourway

If you have a question for Uncle Dave (make it good!) send it here…


Crystal clear? Questions?

Comments

0
Feb. 9, 2017, 8:29 p.m. -  Brad_xyz

Head tube angles are routinely posted but it seems that no bike manufacturers list trail numbers. I know you can calculate these relatively easily but has anyone seen a database anywhere that someone has already done this? It would be interesting to know the trail numbers of some of the top rated mountain bikes.

Reply

0
Feb. 8, 2017, 2:49 p.m. -  reformed roadie

http://www.bikeradar.com/mtb/gear/article/pushing-the-limits-of-fork-offset- an-experiment-45343/

I'll just leave this here…

Reply

0
Feb. 8, 2017, 3:33 p.m. -  Dirk

Ya, I've seen this article before. And we all know that more trail will add stability to a bike. The point I was trying to make is that you're not changing just the trail, and your change will have an impact on some other areas of your bike. If you just look at the trail numbers between the base case 140mm Revelation and the 150mm Pike, yes, it will probably be less stable. But your BB height and your wheelbase also changed so what does that mean? And what happens to your seat angle if you drop the rear end to compensate? Is that worth the trade-off? I can't provide the answer to that, but I can show some of the actual numbers that we're talking about.

Reply

0
Feb. 9, 2017, 8:13 a.m. -  Wacek Keepshack

I find it hard to trust him, to be perfectly honest that suspension action got better by just mounting uppers with less offset. How come? If we focus solely on suspension action can get better if you adjust the head angle accordingly to the speed you ride, as the angle of most forces coming at the bike should match the head angle as closely as possible. Then the shorter the offset of the wheel axle to the plane of sliders, the smoother it will be, since the fork would work best if the axle was in line with sliders - That would mean some humongous CSU assembly. He got the fork serviced and setup properly, that's my guess… manufacturers increase offset because it allows for relatively good handling at a wider range of speeds. If you do it just with head angle with smaller offsets you have to be more precise whether you are bombing down Garbanzo or climbing a tight singletrack. Can't have both when you can't climb well. And most people can't since they are told to spin 90RPM out of the saddle at all times because some science.

Reply

0
Feb. 9, 2017, 1:33 p.m. -  reformed roadie

I often wonder about the riding ability of people posting reviews on the interwebs…I know one writer, former editor for a major publication and he is a solid rider, but who can tell otherwise. And riding ability doesn't necessarily translate into quality analysis or writing. Some riders can shred regardless of bike or set-up, nor can they break down how they do it….they just do.

0
Feb. 9, 2017, 2:50 p.m. -  Brad_xyz

I really wish someone in the mountain bike design industry with a scientific / engineering background would write a more in depth article about trail, head angle, bottom bracket height and maybe chain stay length and how they individually affect handling. I'm not sure how you could isolate these from each other in order to test them individually but it would make very interesting reading. I have a good general feel on how they affect high speed, straight line riding but not so much in other situations.
For example, what characteristic encourages your steering to flop one way during very slow, steep and tight technical switchbacks?
Is it one or more of these characteristics that make you have to ride "off the back" to get some bikes to turn while others need a more central or forward weight bias? Inquiring minds want to know!

Reply

0
Feb. 7, 2017, 1:57 p.m. -  Morgan Taylor

How about those trail numbers at full compression?

Reply

0
Feb. 7, 2017, 2:09 p.m. -  Dirk

Assuming the same amount of travel front and rear, it doesn't change.

Reply

0
Feb. 8, 2017, 10:24 a.m. -  Morgan Taylor

Assuming front and rear ends always compress at the same rate, that is true.

0
Feb. 8, 2017, 10:45 a.m. -  Dirk

I don't think my sub-par Autocad skills will allow me to compensate for various spring rates and differences in travel.

0
Feb. 7, 2017, 8:23 a.m. -  DJ

Dave should tackle seat tube vs effective seat tube angles, especially for tall guys. (crouches in corner with tin foil hat)

Reply

0
Feb. 7, 2017, 8:44 a.m. -  Dirk

It's difficult to discuss something that only exists in the imagination of the bike designer.

Reply

0
0
Feb. 7, 2017, 2:06 p.m. -  Dirk

Sweet! They did it for me!

The original response I had typed out mumbled through some excuses about bike companies not providing enough information to properly figure this out. That's great the Canfield goes that far.

0
Feb. 7, 2017, 8:48 a.m. -  Jerry Willows

The only angle that is relevant is the line from the bb to where the seat rail clamps at full extension.

Reply

0
Feb. 7, 2017, 4:03 a.m. -  Wacek Keepshack

Oh… I heard that discussion before…

http://www.pinkbike.com/photo/11655861/

Reply

0
Feb. 7, 2017, 7:29 a.m. -  Pete Roggeman

Haha! Great one, Waki. I'm posting the image to save people from having to click through. https://uploads.disquscdn.com/images/ea6e6b5df5106493ee3537ce94147cb9e70f5fa404b99fe95ecd2e7404f98691.jpg

Reply

0
Feb. 9, 2017, 3:58 a.m. -  Wacek Keepshack

Thank you guys for a warm welcome. Wow Pete, didn't know I can post pictures directly. That opens a sea of possibilities…

Perhaps I should move here and comment using hand drawings only. Especially that I deeply enjoy Uncle Dave's articles. He seems a bit like a functioning sociopath and I tend to stick to such people…

0
Feb. 9, 2017, 9:17 a.m. -  Pete Roggeman

Uh-oh, Dave's going to be stewing over that one for weeks…

Love your illustrations, so please drop them on us in the form of comments - or whatever - whenever you like! For anyone else reading this, just look for the little icon to the left of the "Post as" box

0
Feb. 7, 2017, 10:01 a.m. -  whatyouthink

I am so happy that Waki shows up in NSMB too.

Reply

0
Feb. 7, 2017, 12:48 a.m. -  Finn the antipodean

I swapped a 55CR for a Lyrik on my Spitfire. Both 26″ (I know, luddite).
Same travel and A-C. The 55 had a 4mm bigger offset, and the handling was appreciably twitchier…

Reply

0
Feb. 8, 2017, 8:21 a.m. -  rvoi

did you eventually switch back and have an equal perception of the increase in stability? wondering what the limit is if you pushed it on the other end… would 2-4 mm less further improve stability for this bike or would it instantly make it more difficult to maneuver? a creative tinker could fabricate micro horizontal sliding dropouts on a fork to explore the goldilocks range on different bikes. super geeky, but it could be done!

Reply

0
Feb. 8, 2017, 10:14 a.m. -  Finn the antipodean

I did switch back to the Lyrik, and prefer the slower handling.
I tried playing with stem length to balance it out (35-50mm), which helped, but isn't the same kind of twitchy.
Currently setup with Lyrik (coil 170mm) and 50mm stem.

I'd like to see those sliding fork drops.

0
Feb. 8, 2017, 11:35 a.m. -  rvoi

I am sure Waki could draw up the blueprints!

0
Feb. 6, 2017, 11:51 p.m. -  kontra

you might fancy this one uncle dave http://bikegeo.muha.cc/

Reply

Please log in to leave a comment.