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Discussion Starter · #1 ·
I've been giving this a bunch of thought: converting my FS GT Marathon Sport 26" into a 69er. It is an aluminum frame, XL size, has a 71* head angle and is currently equipped with a 120mm travel fork. Through sitting on the bike and just doing some eyeball measurements, it looks like toe clearance with the larger wheel shouldn't be a problem.

I would go with a 29er 100mm fork, such as a Reba, to keep the front end raise to a bit of a minimum. I figure with a fairly steep head angle, this and the increased tire diameter will slacken it some but that it will still be acceptable. I will replace the wheel and tire, of course. I have Avid hydro brakes to swap over.

With this setup, I would have a fork that is 1.5" longer and a tire with a radius that is greater by 1.5" for a total increase of 3" but the shorter travel fork would offset this by almost 2" and the net effect would be a little more than a 1" raise. Again, it will be a little more slack but I believe this combined with the 29" roll characteristics, I may like it.

Here is my bike,



Your thoughts? Am I missing something critical? Folly?
 

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I'm of the opinion that it's worth a try. If you can get away with that 1" increase by the time you've factored in wheel radii, fork axle-crown dimensions etc. then that equates to some thing like a 2-2.5 degree slackening of the frame angles. (the rule-of-thumb is that 20mm = ~1 degree). A head angle of 68 - 69 degrees then.

To restore the effective seat tube angle you'll need to move the saddle forward on the rails by maybe three-quarters of an inch (it looks like you have room to do this) and you also have room to drop the stem down by an inch. This should get your contact points back into the same relationship. Unless you have big feet (and maybe you have? - it's a big frame you have there) toe overlap shouldn't be a problem.

FWIW - I've used longer-than recommended forks on old school frames, 29" front wheels on 26" bikes and that sort of thing and, unless the changes end up as being really ridiculous, I find that I quickly adapt to the handling changes and learn to compensate for them. For example, I'm currently using a '95 Kona Explosif (designed for 410mm a-c forks) with 425mm Salsa Cromoto forks and a 29" wheel with 2.35" tyre.
People tell you that it won't work, but it's the best handling bike I own.

At the end of the day, you just have to try these things......
 

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Okay, I just got out of bed, so perhaps I'm still a bit confused...

"the shorter travel fork would offset this by almost 2" "

Wouldn't the short travel lower it 20mm, rather than 2"?

My groggy guesstimations seem to indicate you will raise your front end by slightly over 2".

You could always lower the new fork to 80mm. And...I think the Fox forks have a lower A-to-C, in case you have extra $$ to spend...
 

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kramnnim said:
Okay, I just got out of bed, so perhaps I'm still a bit confused...

"the shorter travel fork would offset this by almost 2" "

Wouldn't the short travel lower it 20mm, rather than 2"?

My groggy guesstimations seem to indicate you will raise your front end by slightly over 2".

You could always lower the new fork to 80mm. And...I think the Fox forks have a lower A-to-C, in case you have extra $$ to spend...
I think that you're right :thumbsup: . (I should read things more closely:rolleyes: ).

1.5" extra fork length plus 1.5" extra wheel radius = 3" increase minus only 20mm (0.75") so yes, 2.25" extra height. Something like a bit over 3 degree angle change then.
 

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Discussion Starter · #5 ·
I don't know why my conversion math was so fuzzy -- for some reason I doubled the length of 10mm in my mind. So yes, such a conversion may raise the front too high.

Best thing for me to do is measure an actual 29er fork crown to axle center at my LBS and compare it to my current fork. The 80mm may work and may not but I think the 100mm would be too tall.

Thanks for the input so far and for cluing me on my errant thinking.
 

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Discussion Starter · #6 ·
I'm gonna do this

So, this afternoon, I stopped by the local shop and measured the 100mm Reba that I am considering using and my 120mm Suntour fork. From the bottom of the head tube to the axle center is 510mm for both. That means the only increase in head tube height will come from the 1.5" greater tire radius. This means about 2 degrees more slack at somewhere around 69*-- very acceptable. If I go with a fork that is adjustable from 80mm to 100mm, I think this could be a pretty slick setup--drop it for tighter trail type riding and raise it for more of the downhill/drop-in stuff.
 

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Ah, I didn't realize the A-to-C measurements would be the same between the two forks. My F120 has an A-to-C of under 495, and my Manitou 120mm 29er fork is ~530 (minus 20mm would be like the Reba you measured)
 
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