Saturday, November 6, 2010

WTB fork part 2 - Solved?

My MTBR conversation continues with the guy that says I need a 440mm fork, so in an effort to be a little more definitive, I'm breaking out the digital angle finder and taking some scientific accurate measurements here in the Utahdog World Headquarters, Congress of the Americas' research lab, (aka, the laundry room).

So, with the 410mm fork, bike facing to the right, seat tube angle is 73.2 and to the left, 73.1

410mm, headtube is 72 and 72.1, again facing east west respectively.

Using my certified accurate fork length adjustment gauge, (a copy of "Field Guide to American Houses") which measures at 30mm, I simulate the length of a 440mm fork, as prescribed by MTBR self proclaimed WTB specialists.

And the results...

70.1 headtube

71.3 seat tube.

The WTB catalog page posted below says the numbers we are looking for are 71-72 head tube and seat tube angles. So actually, my 410 is a wee short, but the 440 would be a wee long, by nearly the same degree error margin, a full degree steeper with the 410 and almost a full degree slacker with the 440.

So what does this all mean? Well, it means that my 410 will still stay, because 72-73 is a very standard frame angle measure, and 70-71 would be pretty slack. It also means that the 410 fork stays because nobody I'm aware of makes an off the rack 425mm fork, which would split the difference between my measurements and give the numbers seen in the catalog. 425mm forks exist, though, for example my AMP F1 measures 420, and my F3 measures 430, with no sag on either counts. It also means that cold weather has come to Florida, and I wimped out on riding this morning, so that I could instead eat eggs, grits and coffee with my family, and play with my tools in the warmth of my house. I am a wimp.


Paul said...

It was 36 degrees with a feels like 28 when we started our ride in Brevard this morning. I live for this ***t.

Dan O said...

Dude - wouldn't tire sizes affect your readings? Even with identical tires mounted front and rear, if the rear was a bit worn, could change things slightly.

That and current tide levels. The magnetic pull of the moon will cause readings to differ from left to right.

You could always experiment with different tires sizes front and rear. Skinny BMX tire in the rear, like Tinker used to run in the '80s.

utahDOG! said...

Sort of my point with all these posts, Dan. The message board pestering by some that I'm running the wrong length fork on this bike doesn't take into account the other factors that would be involved. So we really are splitting hairs here. As one Retrobike user said, "(410 fork) You are fine"

For example, I'm 6'2" with a 31" inseam and I wear a 45 jacket (broad shoulders), so most of my weight is on the front of the bike. Does that mean that my weight compresses a tire more than the typical skinny cyclists guy? Should I account for that in my measurements? That is getting too weird.

My point is only that the guy that commented so much about my fork, apparently ran a Judy SL on his Phoenix (and what looks like a Marzocchi Atom on another) and wants to justify it as being correct. I'm not going to change his mind because he's a self proclaimed WTB expert, so I'm just putting the measurements here rather than argue about it on MTBR.

Somebody else out there also faced this question at some point, and somebody will again in the future, so maybe these numbers will help them at least as much as the MTBR conversation didn't help me.

Kona P2 410mm fork on a 1997 Phoenix? Works fine!