A serious fork-length question
 

Dminor serious? I know it sounds unlikely but . . .

I 'inherited' a 2003 Diamonback XTS Moto frame from my son, which I am building up into a long-legged trailbike. The bike originally came with a Marzocchi Jr. T which had an axle-to-crown length of 555.6mm. I have my choice between a couple of 09 RST Storm fork samples to try on it: the Storm Air in 160mm-travel version or the Storm Super with 180mm travel.

Now, here's my dilemma: I would prefer to get the Storm Air -- it weighs about a pound less, it has just over 6" of travel (which is fine for this project) and it has more sophisticated damping circuitry. However the 160mm version has an axle-to-crown of 540mm. The Super is kind of overkill though: it weighs a pound more; has 10mm more travel than the original Jr. T; and has an A-to-C of 560mm.

My concern is if giving up almost 15mm in A-to-C is going to have too much of an adverse affect on the geometry. I wouldn't mind if it made it a little less sled-like (and I already have a sled if that was what I was after) but I don't want to steepen it too much or lower the BB too much.

Thanks! I know you'll all come through with good advice, some bad jokes, a funny picture-joke, some uninformed opinions and a zinger from junkyard :)

I've got a cove stiffee FR (obviously a freeride bike) that's able to take up to 160mm of travel, and currently I'm running a 90mm Manitou without any ill effects. You should be fine.

There's your good advice.

well the uninformed opinion part is taken care of...now for the bad joke:

15mm spacer under crown race.

Seriously, I don't think 15mm is gonna make much difference, and it's fun riding a quick bike - keeps you sharp.
I would go with it as is, just leave a little steerer tube in case it's too steep, then you could look into headsets with tall lower races, a la cane creek tank hit.

^^ Funny you should mention tall headsets like the CC Tank Hit - - just so happens that I have a NIB FSA Pig Pro with the deep-insertion cups that I was trying to find a home for anyway. It has a bottom stack-height of 15.4mm - - nearly the same as the CC's 16mm.

After all of your Kackling regarding a certain Frok...I decided to keep my Van and drop it down to 100mm. I really like it in the tight singletrack. It's obviously a smidgey twitchy on fast/steep stuff, but your superior skeeelz shouldn't feel anything from that, eh?

I really like how quick my bike is right now. I'm definately considering a long term U-Turn or straight up 100mm option next year.

I believe my bike was built for a 100mm fork though. The HA right now is 70-71.

What are the geo specs for the DB?

What's the rear travel?

Run the shorter fork, maybe with a shorter i2i and stroke length shock in back to regain some slackness? Suppose that depends on how much lower you want things, and all that may shorten the wheelbase more tahn you want as well.

Any way to convert the taller fork to air to drop some of the additional weight?

never - - Not sure what the specs are. Can't seem to find them anywhere any more - - just old enough I guess. '05 Moto head angle was 67º with a 13.7 BB height.

scrub - - Rear is 6½ - 7" (?) if I recall right. There is another lower shock mount hole (in the swing-link) that should shorten travel/slacken it back some. That will probably be my first move since I'm not looking to squeeze all the travel I can out of this one.

Not sure if I could convert the longer one. Some of the extra weight, besides springs, is the fact that it uses chromoly stanchions rather than than 7050 alloy ones; so internal diameters could be very different. I think I'll keep the air model coming and try it; if it's too twitchy I guess I'll just have to have 'em send the longer one.

Cane Creek makes a version of the S3 headset with a 5mm taller than normal lower cup set. And Ventana makes a 7mm tall crown race that fits King headsets. I've successfully combined that with the S3 +5 lower cup to get a net 12mm increase to lower stack height in the past. Something like that might work for you.

Otherwise, yeah, stick the shock in the other spot to slacken things. Run lotsa' sag in the back. 24" rear wheel. Nice tall profile tire up front to help as well. Don't forget to run the 3" Gazzi with that 24" rear wheel, eh? :)

Good luck/have fun/post pics!

D - I have an 07 Diamondback catalogue floating around somewhere, I'll see if i can find it. Not sure how much use it's going to be, but I'm pretty sure it has geo charts on it.

S'okay, BFG: DB has specs back to '05 via their web site so I got the numbers at least from that year's geometry chart.

Maybe I shouldn't worry about 15mm; but when I laid out the frame on the basement floor with some straight-edges and a protractor (ghetto, I know), I was coming up with possibly 1½-2º differences. And since I'm already planning a pretty tall headset it doesn't give me much more wiggle room.

I'm not worried about air springs too much, DBD. I prefer coils too; but I have an RST Aerosa HT that's still holding fine and an old Fox Alps 5 that's still holding its air.

I did some weight comparisons and found that the stock Jr. T was 7+ lbs, an 08 Boxxer is 6.89 and Fox 36s run 5-5½ depending on year/model. So at 5.94 lbs. the Super - - even though it's a pound heavier than the Air - - is not outlandish for the frame it's going on.

Gonna find out if they have a Super ready to ship and maybe give that a try. Thanks for your advice everyone; and don't think I snubbed yours if I didn't go with it. I weighed your comments thoughtfully but decided to err on the side of slack since my preferences tend to run that way anyway.

No zingers here. Just don't keep your "new" bike in the van. The heat might melt it. Oh, and it's easier to ride up hill when the moon is on the opposite side of the earth. Remember the tides. Aside from that, I have nothing useful for this discussion.

But you contributed - - that's what counts.

And the boldfaced reminds me -- if toilets swirl the other direction in the southern hemisphere, does that mean left-side pedals need to have right-hand threads down there? And do BB cups have to be threaded the opposite way there too? I'm sooo confused . . . .

Don't forget the left sided drive stuff.

Hey, on the taller fork : provision for making some spacers or something to drop the fork down, maybe with a shorter coil? You'd still have the weight gain, but maybe the ability to custom tune the ride height a bit more?

^^ I will be curious to know how the other travel lengths are achieved on it - - whether is some internal tweakings/parts and shorter springs or if they are more involved than that (different crown/stanchion sets, etc.) They are good at generating schematics and exploded views but not as good about making them readily available.