Fork offset adjustment

[brians647]

I was discussing bike steering quickness vs. stability with a buddy.

He has a bike with a 73 degree head tube, with a 43mm fork offset. My current ride has a 71.5 degree head tube with a 43mm offset, and I love the way the bike handles. I was sharing this with him, and we were wondering if you could get a fork with a different offset (and I was sure to slow the steering you needed >45mm, but others said less offset slows it), to make his bike handle (to some degree, in the front end) like mine - less twitchy, and more stable.

Or, are you just messing things up more?

[Louis]

Search the forum for "trail" and "rake" and you'll find plenty of opinions.

There are lots of 40mm rake forks out there.

[Tom Matchak]

He has a bike with a 73 degree head tube, with a 43mm fork offset. My current ride has a 71.5 degree head tube with a 43mm offset, and I love the way the bike handles. .... we were wondering if you could get a fork with a different offset to make his bike handle (to some degree, in the front end) like mine.

To reproduce you bike's steering charcteristics on your friend's bike will require a fork with an offset in the low-30's. This is the simple answer, as a bunch of other parameters may not be similar when you get down to comparing the two bikes in detail. But that's the ballpark that you're talking about for a basic fork swap. Very unusual.

You didn't say what kind of bike this is, but you're talking about a heap of trail/wheel flop if it has 700C/559 wheels.

[BdaGhisallo]

A 71.5 hta and 43mm rake sound like Colnago territory. They do have a lot of trail. I can recall images of watching Michael Boogerd climb out of the saddle on his Colnago and watching the front wheel flop. I am sure that front end geo, in combination with the appropriate front center and chainstay length, can be mighty stable when descending at high speeds.

If you want the real scoop on this, page the Colnago geo expert, Mr. Jerk.

[brians647]

A 71.5 hta and 43mm rake sound like Colnago territory. They do have a lot of trail. I can recall images of watching Michael Boogerd climb out of the saddle on his Colnago and watching the front wheel flop. I am sure that front end geo, in combination with the appropriate front center and chainstay length, can be mighty stable when descending at high speeds.

If you want the real scoop on this, page the Colnago geo expert, Mr. Jerk.

Yes, it is Colnago territory, the CT2 (or any of them, really). His is a Litespeed, and I was mistaken, it's a 45mm offset. I had a Litespeed too, and wasn't a huge fan of the feel at turn-in or some headshake that I got. He's okay with it, but may need to replace his fork and that got us thinking...

Colnago's storied history is great and all, but I honestly didn't buy the bike for that. The way it handles is outstanding (for me, at least).

A fork in the low 30's offset-wise is crazy. I really just wanted to understand how this all fits together, and figure out if it was an option. I guess it's not.

Maybe the Jerk will grace me with his knowledge!

Thanks for the feedback!

[Dave]

The trail for a 71.5 HTA and 43mm ofset is 67mm, which is not that huge - it won't cause a wobble at low speed, like someone sugested, but it would take a 34mm offset to get that trail with a 73 degree HTA.

What you may be overlooking is that it's common for smaller frames to have more slack HTAs and more trail than larger frames. A 67mm trail on a large frame with a longer wheelbase would be sluggish.

The formula for trail is simply (R/tanH) - (offset/sinH), where R is the tire radius and H is the head tube angle. Notice that the first half of the equation is the trail without the effect of fork offset and the second half is the reduction in trail due solely to fork offset. More trail results in a greater tendency to stay in a straight line at high speed.

If your friend wants slower steering, you can find forks with 38-40mm of offset. A fork that has a bit longer axle to crown length would produce a bit more trail.

[Peter P.]

Let me echo the other replies: your buddy's bike isn't going to get the same trail figure without a fork with unheard of low rake i.e., @35mm. You aren't going to find that in a production carbon fork, you will with custom steel, but the question is, will the wheel clear the downtube?

I wouldn't get that radical and try to DUPLICATE your trail figure on his bike, but you could endeavor to approach it with a rational dimension. A fork with 40mm of rake, versus the 45mm you finally determined your friend has, will put the trail at 60mm, a good step in the right direction, and not so small as to be considered not worth trying.

The good thing is, you can find production carbon forks with 40mm of rake. If a custom steel fork interests your buddy, I'd recommend Walt Wehner at Waltworks. He seems to LOVE building all sorts of custom steel forks and is certainly willing to talk tech. The prices are VERY reasonable, too. He built my friend a custom, rigid fork for his ATB and it is very slick looking.

As a personal note, my main bike has a 73 degree/45mm setup, and I'll admit I'd like to slow down the steering, or at least make it so it requires less attention on my part. My other bike has a 72.5/47 pairing, and with a 25mm tire, it's just a pleasure to ride.

[brians647]

Thanks Dave and Peter.

You're right, his bike is a couple cm's larger than mine. A 40mm rake would help in that regard, but I'm trying to understand the whole picture too. For instance, I was playing on bikecad, and as you decrease the offset, the front-center measurement comes down too. I'm not clear as to, of even if, that relates to handling, but I'm curious as to how those two numbers affect handling and if they work in concert.

Anyway, thanks for your responses! :beer:

[brians647]

Actually, I was just looking through Serotta's geometry, and the larger bikes there (56 and up) run 43mm forks, w/only 56mm of layback/offset at the ground (?). I haven't heard of Serotta's having twitchy handling, so I wonder what the differnce is. If anybody wants to chime in about stock Ottrott geometry, please do!

[Dave]

Thanks Dave and Peter.

You're right, his bike is a couple cm's larger than mine. A 40mm rake would help in that regard, but I'm trying to understand the whole picture too. For instance, I was playing on bikecad, and as you decrease the offset, the front-center measurement comes down too. I'm not clear as to, of even if, that relates to handling, but I'm curious as to how those two numbers affect handling and if they work in concert.

Anyway, thanks for your responses! :beer:

Yes, the F-C and wheelbase decrease by a small amount when the fork offset is reduced, but the change is very small compared to the percentage change in trail. Bikes with a longer wheelbase won't be quite as nimble as shorter ones, but the frame has to fit the rider. Those who ride smaller frames know that this can be overdone. Cervelo put an incredibly short F-C on the 49 and 51cm sizes of their R3 and Soloist models by combining a 73 degree STA with a relatively slack STA of 73 degrees with a relatively short TT length. The result was a F-C that's 2cm shorter than a lot of other brands, with a huge amount of toe overlap to go with it. I owned one, but sold it after only 200 miles. It wasn't what I wanted for mountain descents.

[jerk]

you can't just look at trail. changing the fork to get a bike with a steepish headangle into colnago'esque trail dimensions is going to make the bike ride worse not better. the colnago works because it also has a longish front center and shortish chainstays and an appropriate amount of setback....its about the totalilty i guess....you can't look at one piece of the design without looking at the whole.

beyond this is the fact that colnago's balance the rider's weight between th wheels a bit differently. they tend to suit riders with less developed upper bodies/narrower shoulders who might otherwise consider say, a pinarello a bit to nervous upfront.

anyway,

jerk

[e-RICHIE]

you can't look at one piece of the design without looking at the whole.
live it
learn it
gestaltmo
atmo -


http://www.atpm.com/9.10/images/design-gestalt.gif

[FMS_rider]

you can't just look at trail. changing the fork to get a bike with a steepish headangle into colnago'esque trail dimensions is going to make the bike ride worse not better. the colnago works because it also has a longish front center and shortish chainstays and an appropriate amount of setback....its about the totalilty i guess....you can't look at one piece of the design without looking at the whole.

beyond this is the fact that colnago's balance the rider's weight between th wheels a bit differently. they tend to suit riders with less developed upper bodies/narrower shoulders who might otherwise consider say, a pinarello a bit to nervous upfront.

anyway,

jerkWho the hell are you and will you let me implant a direct interface into your brain?

[jerk]

Who the hell are you and will you let me implant a direct interface into your brain?

are you threatening me or hitting on me?

jerk

[FMS_rider]

are you threatening me or hitting on me?

jerkI only want your brain.

[jerk]

I only want your brain.


so its like a danzig thing.

http://youtube.com/watch?v=r-7pTQx6d3U

jerk

[FMS_rider]

so its like a danzig thing.

http://youtube.com/watch?v=r-7pTQx6d3U

jerkSkulls are fun to look at but not very useful.

I just want to download all of your bike knowledge --the implant would only be temporary and wouldn’t hurt at all, trust me.

[columbusslx]

Storck produces a carbon fork with 37mm offset.

[brians647]

you can't just look at trail. changing the fork to get a bike with a steepish headangle into colnago'esque trail dimensions is going to make the bike ride worse not better. the colnago works because it also has a longish front center and shortish chainstays and an appropriate amount of setback....its about the totalilty i guess....you can't look at one piece of the design without looking at the whole.

beyond this is the fact that colnago's balance the rider's weight between th wheels a bit differently. they tend to suit riders with less developed upper bodies/narrower shoulders who might otherwise consider say, a pinarello a bit to nervous upfront.

anyway,

jerk


Interesting. I love the Colnago's and I'm the exact opposite (which, makes me a poor candidate for a cyclist). Regardless, I see your point - thanks Guru of 'Nagos.

live it
learn it
gestaltmo
atmo -


http://www.atpm.com/9.10/images/design-gestalt.gif

That's the bottom line, isn't it?

[superunleaded]

FWIW
another fork Q&A thread (http://forums.thepaceline.net/showthread.php?t=30636)

[brians647]

FWIW
another fork Q&A thread (http://forums.thepaceline.net/showthread.php?t=30636)

Thanks, Gas. Great link.