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Fork of different length
 

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April
Joined: 13 Dec 2003
Posts: 6072
Location: Westchester/NYC

8/11/17 9:24 PM

Fork of different length

The fork of my commuter bike, which is a hard tail mountain bike with an air fork, is no longer holding air. Not that I care about the suspension fork. I've been putting the air pressure high to make it as stiff as possible.

But with the fork not holding air, it's staying at the minimum height, which is uncomfortably low, especially for riding in city street, when I would prefer to sit up and keep an eye on my surroundings.

I know I could go buy a used rigid fork for not much money. Except I have another suspension fork sitting in the garage. It's even got a lock out to make it "almost" rigid. For free. Absolutely FREE.

Except this fork came off my dualie, and is quite a bit longer than the one on the hard tail commuter. A good 10cm! (by eye ball). I know it'll make the steering slack. What else? What other effect will it have that I might notice using it as a commuter? As an occasionally mountain bike in easy single track?

Or, am I better off just get a rigid fork (to replace my now too-short & too quick steering airless fork)?

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 3755
Location: Nashua, NH

8/12/17 6:03 AM

10 cm is WAY too much of a change

It will severely alter the handling of the bike and will also stress the front of the frame more.

Why don't you get your current fork fixed (or DIY)? That should be cheaper than a decent rigid fork. Most suspension forks are relatively easy to work on and instructions are available online to help you determine if it's something you can handle. That way you'll have the option of suspension should you change your riding habits or decide to try something different some weekend.

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Sparky
Joined: 08 Dec 2003
Posts: 15742
Location: Portland, OR

8/12/17 9:39 AM

If you can set up the second fork to have 10CM of sag you should be fine. It may have more sag if it is a 100mm instead of 80mm for example.

You can also put a bigger/taller tire on the rear.

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April
Joined: 13 Dec 2003
Posts: 6072
Location: Westchester/NYC

8/12/17 5:53 PM

The current fork (on the bike) is 20 year old. Newer forks all have thicker legs. I changed the inner about 15 years ago. Even then, I was having trouble finding inner for that size. I think the time it takes me to hunt down the suitable inner may not ne worth spending.

Sparky, thanks for that suggestion. I'll make some measurements and see how much travel the spare fork have. That should give me some idea if I could make up the difference with more sag?

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dddd
Joined: 11 Jan 2004
Posts: 3152
Location: NorCal

8/12/17 8:17 PM

I think if you will be running a huge amount of sag that the fork will at least need a lockout to control the massive amount of pogoing that might otherwise result.
But will the longer fork lock out in it's sagged position?

Might be easier to rebuild the old fork's air chamber seal, which is probably in the left leg.

Often just adding oil to the air chamber, via the shraeder valve, keeps a puddle of oil on top of the air piston as intended , which very greatly slows the escape of air past the piston. I would do this first, using the regular green Fox air chamber oil or equivalent HEAVY oil.

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 3755
Location: Nashua, NH

8/13/17 8:51 AM

Good point

If the fork has been neglected, it's possible that disassembly, cleaning and filling with the recommended oil may be all that's necessary to get it working again. It will depend on the design of the air spring and whether there is any internal wear or damage.

As for finding parts, you'd be amazed what you can find on Ebay. You can also find complete forks really cheap.

FWIW, I've never seen a fork that locked out at any position other than full ride height. If you lock it out in a sagged position, it will probably just pump up in use until it reaches full extension. The lockout only affects the compression damping; it doesn't prevent the fork from rebounding/extending, but it will prevent it from compressing again after it extends. Perhaps if you cranked the rebound damping all the way up it would work for a while, but that seems like a half-assed hack, rather than a solution.

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dfcas
Joined: 11 Jan 2004
Posts: 2379
Location: hillbilly heaven

8/13/17 11:13 AM

What wheel size is this for? Disc or rim brakes? Thru axle or quick release? Tapered or straight steerer? I have some forks and most bike shops have a bucket full of rigid forks they may give away or sell for a few $. If I have what you need I'll send it for free.

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April
Joined: 13 Dec 2003
Posts: 6072
Location: Westchester/NYC

8/13/17 8:47 PM

26"wheel, rim brake, qr, straight steerer (if my memory serves).

What do measure from to get length?

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dfcas
Joined: 11 Jan 2004
Posts: 2379
Location: hillbilly heaven

8/14/17 8:59 AM

from the center of the axle to the crown race seat. Its tricky with suspension forks because they plan on sag and compression so they are usually taller than a rigid fork when they are extended. i think I have your fork. I'll measure the axle-crown and let you know what mine is. its about 20 years old so may be right.

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dfcas
Joined: 11 Jan 2004
Posts: 2379
Location: hillbilly heaven

8/14/17 9:55 AM

I have 2. 1 1/8 steerer , rim brake, quick release. 1 is 395 axle to crown and the other is 405 axle to crown. I think the 405 is "suspension corrected" and would be the best for you. it should be equal to your fork compressed about 1/3 of its travel. steere is 7 1/2 inches long on both.

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Sparky
Joined: 08 Dec 2003
Posts: 15742
Location: Portland, OR

8/14/17 11:12 AM

The shorter one may just quicken up the steering and lower the BB enough but not too much. Chart shows HTA changes with adding or detracting to/from Axle/Crown stack.


Agree with the longer one assertion.


I am out for the day so will post it now. I already had for a previous thread on the subject apparently. It is road geom, but may assist conceptually.

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dfcas
Joined: 11 Jan 2004
Posts: 2379
Location: hillbilly heaven

8/14/17 3:30 PM

The rule of thumb is 1 cm fork height = .6 degrees of head and seat angle and 4mm of BB height.

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April
Joined: 13 Dec 2003
Posts: 6072
Location: Westchester/NYC

8/14/17 8:50 PM

The talks about sag just reminded me I had measured the fork length wrong!

The existing fork is sitting on the bike fully compressed (since it won't hold air). The spare fork is on the other hand freely floating about at full extension. So a good deal of the length difference is just the travel length of the old fork. Stupid me! :(

When both forks are at full extension, the difference is a lot less than 10cm, more like 2-3cm. That actually sounds about right to me since the old fork has 80mm travel and the spare fork has maybe 100 or 120mm!

So Dan, you can hold off on sending the fork for now. I'll swap the fork and see how it rides first.

Thanks for all the suggestions. It was those suggestions that helped clarify the my mind. Please forgive my senior moments of a couple days ago.

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