Frame and Fork compatibility

[Krieges hund]

Hey. I'm new here, but have been riding for a few years now. Been riding a 2000 Specialized Rockhopper, 17 inch to my 6'1 height. Cramps galore at times. But just a week ago I got a hold of a 2006 Specialized Hardrock, 21 inch frame. So, I'm slowly building this guy up to 1x9 XC/Light Freeride setup.

My main concern I have is fork height. Though the steerer tube on my RH is smaller, I wanted to do a geometry check on the HR. The 100mm Rock Shox fork seems to small for such a big frame, as the head tube sits lower than expected. When I finally get the extra cash to buy a new fork what is the max height I can put on the HR? I want to go 130mm if possible as that raises the head tube enough that it would be more comfortable for my size and arm length.

I've been reading mixed reviews about fork length on the HR so I wanted some feedback.

[Bill]

Ugh this is a question for ummm trying to think of the member who does all the math real well

[Krieges hund]

Sorry about that. Got a bit carried away, so I took some pictures with the 100mm Rock Shox from the RH.


And a mock up of the 130mm


I see no real dangers of using a 130mm fork. And with the 30mm riser bars, and the 60mm length stem I have, this fits comfortable.

Just have read about a lot of "Don't go 130mm, keep it 100mm". But they are mostly talking about the 15 or 17inch HR. Curious if going to 130mm on such a large frame as mine would cause any real problems.

[RobAR]

Hi Krieges. Welcome aboard!
To begin with, you need to research what travel fork that the '06 HardRock was designed around. Not being a Spesh dealer, I don't know off the top of my head. 80 - 100mm is pretty standard fare for a XC bike. A bikes geometry is designed around it's intended purpose and the current forks available for the same purpose.

Fork travel and frame size have nothing to do with each other. To add a longer travel fork to a frame in order to increase height of the steerer tube / top tube is counter-productive.
Let's say that a bikes geometry is designed around an 80. If one were to install a 120 on that frame, two things happen -

(1) The extra travel (120 - 80 = 40mm = 1.5 inches) is reflected by the A-C (axle to crown) distance, effectively slackening (or relaxing) the frames geometry. The head-tube and seat-tube are now farther away from 90 degrees by whatever 40mm translates to over the radius of the wheel-base (I'm not the Math Guy that Bill is looking for - where IS he? ).
The wheel-base just increased a small amount as well, so is a multiplyer.
This may noticeably alter the steering of the bike to be slower. It's not dangerous unless you're used to the bike and suddenly find yourself head-butting a cedar tree at 15 MPH because you overshot the corner in a tight turn on familiar singletrack. That's not good, but at least is was a rider injury and the bike is okay!

(2) By relaxing the geometry, more stress (due to more leverage) is placed at the intersections of the front triangle tubes. Notice the Under hard use, this may eventually cause a 'tear' (looks like a 'crack' but not to a welder) in the frame material at some location. And that is NOT a Warranty Issue.
In this instance, we might ride upon a fellow biker, all bruised up, sitting at the base of a big cedar tree, tears falling on a frame. That's not good. At all.

Now, back to Your bike ... 30mm is less than the scenario above but, same eventual outcome is probable. This throws me, too ---
"I see no real dangers of using a 130mm fork. And with the 30mm riser bars, and the 60mm length stem I have, this fits comfortable."
--- You are comfortable with bar height and stem length as is?
Did I miss something??

[Krieges hund]

Thanks to your information, I understand where I went wrong. The Spec HR was built around a 100mm fork, and the bar height and stem length are comfortable at that level. But more so at a 130mm-ish height. But I got what you were saying, and I don't want to strain the head tube because I wanted a bigger fork. I'll stick to the 100mm fork it was designed for.

Thanks for your help.

Also. Any ideas what kind of fork? Something that can handle heavy XC? I was thinking maybe a Marz DJ 1. I don't care about weight, my concern is function.

[Jordan300]

I've used a DJ 1 and they rock. I think it was on a jumping bike that someone let me ride. I think this model was purple. It was a while ago.

[Krieges hund]

Yeah, keeping my eye on a 2010 model DJ 1, so well see what happens.