Hello All,

Long time voyeur, first time poster...

I have a 1995 Bianchi Grizzly RC frame – no fork. Per BikePedia, it originally had a Marzocchi XC-600 suspension fork on it, which had a 51mm travel.
I would love to make this into a rigid fork commuter but I don’t know how to do the math to figure out what distance I need from the crown race to axel, and I’m not sure where/how a forks rake figures into the equation. Any advice/suggested forks, would be greatly appreciated.

You might find the crown to axle length for the original fork in the Marzocchi site. But when you do the conversion, make sure you use the right numbers. Suspension forks aren't ridden at full extension. You have to allow for the normal sag of about 25% of travel when calculating the desired crow/axle length of the replacement.

Many "suspension compensated" rigid fork spec the amount of travel they compensate for, so the calculation is done for you. Don't sweat rake. The rakes of most suspension forks and their replacements vary through a narrow band, so unless you're a knowledgeable high performance rider, it won't make a difference to you.

Thanks FBinNY, but the measurements are not on Marzocchi's site, I was able to find the manual, which is where I got the travel, but a las, no other measurements.

If the original fork is still on the bike, adjust it as you would for riding. Have a friend sit in the bike to settle it, and measure from the top of the crown (just under the headset) to the axle. That's the crown to axle distance that rigid forks will specify. BTW- this isn't rocket science, since the settle amounts vary anyway, so anything close is fine.

If you don't have a friend to sit on the bike, you can measure the crown to axle distance on the unloaded fork, and subtract 25% of the travel.

+1, Sit on the bike, there will be some preload sag, from your weight
have a friend measure
the axle to lower headset crown race seat distance.
Shop that blade length in the rigid fork.

of course a friend of similar weight , you can reverse the personnel placement.

Unfortinately, as stated, I do not have the fork. Frame only. I just know which fork it was from BikePedia.

Why don't you find a similar bike with a rigid fork and the same wheel size (probably 26") and measure that? It shouldn't be too far different. They don't change the fork length much if at all for different size frames.

Well...
There can be a fairly big difference between a bike designed as a rigid and one designed for a sus fork.
If you stick a rigid fork that's not sus-corrected on a frame designed for a sus fork you run the risk of ending up with a rather steep fork angle. Ride may get "lively" to a fault.

51 mm is very short travel, with todays measurements. Subtract 25% sag and you're left with 38 mm or thereabouts.

IME ATC for bikes definitely designed as rigids is between 395-415 mm, so unless the designer has opted for some extra crown clearance an ATC between 433-453 sounds like a decent guess.

It may not be advertised as a sus corrected fork.

Dabac, Thank you. This is the type of infomation that I'm looking for. I have a couple of follow up questions: 1- What does IME ATC mean? 2- On my lunch break I ran over to REI and peeked in their QBP book and found that the deminsion non corrected fork is 395 (you're right on) but their 80mm corrected fork is 425mm, which is shorter than your suggest range... would you still go with something longer?

IME is a good ol' internet abbreviation of In My Experience. Which may be greater than some, but smaller than others.
ATC is maybe my own shorthand for Axle-To-Crown, which isn't that fun to keep tapping out. I can be lazy in things like that.

Really hard to tell what I'd go for. And it probably doesn't matter in terms of rideability.
Since you haven't got a fork for it now, you have no frame of reference to how the bike "should" ride. A new bike is a new experience anyhow, so you'd probably soak up a 425 mm fork as part of the whole experience w/o second thoughts.

Seeing that you want to use it as a commuter, I think the 425 will work out fine. In city traffic, I like a bike that feels nimble and manouverable. Off-road, a bit more casual response can be nice.

But the math is interesting. I might try to track down a few other sus-corrected forks, see what they are recommending. If one were to use the fairly well established sag value recommendation for a an 80 mm fork, one should be looking at a 455 mm rigid fork to mimic ride characteristics at casual riding. At 425, it's almost 2/3 of travel used up already.

I still think a slightly longer fork would bring the bike closer to its original handling, but unless you have an explicit wish to match that as close as possible, the 425 fork will in all probability do just fine.

"But the math is interesting"...exactly

Great info. I just posted the same ? on the framebuilde'sr forum, came here and found the thread, very helpful.

Brian

Dabac, thank you. Does anyone out there have what I'm looking for that would be willing to let it go a reseasonable price? Apparently I cannot post to the WTB forum until I'm an upgraded member, which I'm not sure how to go about that.

1" fork
Canti Bosses
CTA 420-440ish
CroMo
I'll have to measure for steerer if yours has been cut.

I have some brand new (old stock) Vicious Cycles forks, I can sell for close to 1/2 retail. But even there they're not cheap @ $125-140. Anyone interested can email me through my website below.

Thanks but that's a little out of my price range. They look wonderful though.