longer fork length and it's effect on geometry

[MIN]

I have a bike with typical road geometry and a standard 368mm axle-to-crown (ATC) length fork. I want to put a 383mm ATC fork on in it's place because the new fork would have fender mounts, which the current fork lacks. (So to recap, I am going from a standard carbon fork to a taller, more versatile steel fork.) Both are 45mm rake.

That's a 15mm height increase in the front, relative to the hub. I imagine that would lift the bottom bracket by about half that distance. (+7mm increase.)

Stock geometry:
73.5 deg HT
73 deg ST
70mm BB Drop

Will this make my frame handle adversely? The BB is low enough at 70mm drop that I feel that it would be a reasonable change. I am curious to hear of others' experience with such a change.

[Louis]

Sheldon Brown (http://www.sheldonbrown.com/rinard/forklengths.htm) tells all.

[MIN]

Sheldon Brown (http://www.sheldonbrown.com/rinard/forklengths.htm) tells all.

thanks.

According to his formula, a 15mm ATC increase will result in a -.86 deg slackening of the HTA with a wheelbase of 993. Not much.

[change in head angle] =
arcsin [(old length - new length)/(wheel base)]

[IPH]

but what the hell.

the increase in the bb height you mentioned doesn't seem like it would be the biggest impact on your ride from this fork change. if the fork rake is the same, it seems to me that the biggest effect on handling would be the fact that you're increasing trail. so the bike will be less responsive (or more stable, depending on how you want to look at it). just how much the handling changes and whether it's acceptable is probably up to you to find out.

hopefully someone who actually knows what they are talking about will confirm or refute this!

[Tom Matchak]

Sheldon's simple trig solution is a good approximation, and certainly close enough for this purpose.

But, after plugging your scenario into my frame design model, here's what your proposed fork swap will yield.

HTA = 72.7 deg
STA = 72.2 deg
BB Drop = 64 mm
WB = +5mm (mostly in the front-center)
Trail = +5mm (w/ same tires)

You might find yourself sliding the saddle forward the better part of a centimeter to preserve your knee-to-pedal relationship. Doing so will effectively shorten the TT length.

Your original steering geometry has a trail in the low 50's, which is a nice responsive condition. The new geomertry will have a higher trail, and more wheel flop. These are small, incremental impacts on the handling, and I wouldn't think significant enough to sink your project. Just something to understand in advance of your modification.

I wouldn't fret about the slightly higher BB.

Cheers,
Tom

http://tommatchakcycles.blogspot.com/

[Peter P.]

Tom Matchak hit on it when he said the words, "wheel flop". Sure, the rest of the numbers change and the results look reasonable, but extending the fork legs the extra 15mm will cause the bike to feel like the front end wants to collapse when you want to initiate a turn.

I know this because I experienced it first hand when I installed a headset with a taller lower headset cup than my bike's original headset. To be sure, it was a subtle feeling, but it was real. I tolerated it for a year or so, then changed to a headset with a lower stack height for the bottom cup.

Of course, YOU may find the taller fork not problem. I can only tell you my experience and let you decide for yourself.

[MIN]

tom and peter-

that was great information from the both of you. this is the bike in question and I have a salsa caseroll steel fork on the way.

http://velospace.org/files/fillmoremain1.jpg

[Tom Matchak]

tom and peter-
this is the bike in question and I have a salsa caseroll steel fork on the way.



Ah, a picture says so much, and begs the question ... Why the Salsa Caseroll fork, when it's not such a great fit, and installing it is going to degrade the bike's handling?

Since you're in need of a 9/8" threadless steerer, consider the Surly Pacer fork which is 376mm axle-to-crown, 44mm offset, and has the eyelets you desire.

The reason the Salsa fork has such a large ATC measurement is that it's built around a uni-crown, which inherently produces a "taller" crown than you'll find with a cast crown like that on the Surly fork. Add to that the fact that the Salsa fork is designed for long (standard) reach calipers, and you get a fork with a 383mm ATC.

That's a quality bike you have there, so why not spend a few more $$ and get a steel fork that works with the original geometry? You're going to have to buy a new front brake to work with the Salsa fork, so there's some $$ that you could apply to a proper fork.

Just asking ...

Cheers,
Tom

[MIN]

Tom,

I intentionally got the Salsa fork due to

(1) it's fender mounts.
(2) ability to clear larger tires.

Your suggestion of the Pacer fork is good but it doesn't take anything larger than what I have already. It's a cheap fork and I won't be heartbroken if it doesn't work out. I can always sell it or use it as a fireplace poker. :)