SJX426 

This could have gone in the C&V forum or maybe even the frame builders forum but it is a mechanical issues.

Well I picked up said frame with a number of parts (vintage Campagnolo) with a Giant steel fork (unicrown, CrMo). It weighs a ton is not what I think should be attached to this fine frame. My thinking was carbon but I am open. Since I don't know a lot about forks, other than material, steerer diameter and thread or threadless along with dimensions for this frame, I am here seeking learned recommendations for consideration.

Part of the parts list includes a Campy SR headset that is in really good condition but I am open to other headset options including threadless, but not too excited about that idea.

Ideally, a direct replacement fork would be good but what likelihood is there in finding one of those! Would a 610 of the same year have the right dimensions?

Since it was produced with Suntour Superbe group, I will likely hang Superbe Pro parts on it.

Andrew R Stewart 

Don't know if the 610 and the 760 share geometry. The folks at https://groups.google.com/group/class...es?hl=en?hl=en might shed some light.

But one can reverse engineer the likely fork to a degree. The steerer and crown race specs should be pretty straight forward. Next up is the axle to crown race seat. This is made of the axle to brake hole and the brake hole to crown seat. In that era the second dimension is often close to 1cm (being prior to unicrown forks). The axle to brake hole is made of the rim radius and the brake reach. 311 (1/2 of a 700c rim) + what ever the rear brake pads are set at. If you measure the rear A-BH remember that the rear caliper is positioned so the center bolt angles away from the rim and the front's tips towards the rim, about a 4mm difference to get the same pad position in the caliper arm slots. So now you have the fork's height and steerer. Last is the rake. Here, unless you find some OEM specs, you'll have to do some real measuring and math or guess. A guess would be 43mm. Or the long way is to set up the bike based on the known A-crown and guessed rake, checking for a roughly level top tube, then measure head angle. Do the math to figure out what ever trail you wish, with adjustments to the rake. Treks tend to be long trail bikes, like 60-63mm.

Next is to search for a fork that matches your reverse engineered one. Or get one built to your spec. Andy.

rccardr 

If you want a fork to use for mock-up, I'm in Burke and have a very nice chromed 4130 fork of unknown provenance (sloping crown, 26.4 race) that you are welcome to use.

ABH is 37cm, 8.75mm brake hole to crown seat. Don't know how to measure rake but it's slightly less than an '88 Centurion unicrown fork and about the same as a '92 Paramount PDG OS unicrown, which are the only two forks I have on hand with which to compare it.

HillRider

The Vintage Trek web site has scans of the Trek brochures from way back. Here is the section from 1984 that covers your 760: 1984 Trek Bicycle Brochure Part I - Racing Bikes

Scroll down to the 760 page and the last page gives the geometry and original components. Note that the OEM fork rake was 38 mm so a 40 mm rake should also work.

SJX426 

@rccardr - Nice offer! Is it threaded and do you know the steerer length? I need to measure the unicrown one I have, it needs a washer for the locking nut to NOT seat on the steerer. The frame is a 60 or close to whatever was standard. I assume it is a brazed crown and legs. We should meet in anycase, at some point in time.
@HillRider - great minds.... I thought of the site and did look it up and found the same number. The 610 has a 45(?) offset, or so. No other dimensions are provided but the HT angle of 73.5.

rccardr 

Yes, it is threaded and has a 212 steer tube. It's not a unicrown fork, has a sloping crown but is smooth, not lugged.

davidad

You won't be able to tell the difference between carbon and steel. The tonnage is not a real problem.

HillRider

As far as ride and handeling, I agree completely. As to the weight difference, it can be appreciable. A steel unicrown fork can weigh well over 3 pounds and a decent carbon fork under 1 pound. Whether it's worth the cost is another thing but the weight difference is substantial.

JohnDThompson 

Just don't get one with straight blades or I'll never forgive you.

SJX426 

Decided to start looking around for possible replacements on line. Criteria was 1" threaded and 38mm offset. Looks like that 38 dictates track fork. Soma has one for a reasonable price.
https://www.somafab.com/archives/prod...ded-track-fork

JohnDThompson 

Looks to be a decent fork (curved blades ), but make sure it's drilled for a brake or at least has enough crown material to be safely drilled.

Jason Roberts

well done!

hueyhoolihan

i bought an old carbon 1" Time fork on Ebay years ago for my '85 Elance. i found it preferable, in all respects, to the one that was on it. i never really considered geometry differences, nor noticed any in use.

and, since the day, a few weeks ago, i tried as an experiment, reversing the fork on my '97 Gary Fisher MTB and found that i could still ride it hands free, that the handling was only slightly different considering the shorted wheelbase, and the ride only slightly harsher, i haven't given much credence to fork geometry issues. YMMV.

SJX426 

@JohnDThompson - Universalcycles sell it and it shows a brake mounting hole in the crown. The example on the link has one too, though with black it is hard to see.
@hueyhoolihan - I imagine that slight differences may not be noticeable. I figure why not get as close to original as possible? What bothers me about the purchase is that it is more than I paid for the bike! The likelihood of finding an original fork to fit the frame is like finding a needle in a haystack! The bike is going to be somewhat of a frankenbike anyway.

The specs are 38 mm for the offset and the only forks with that dimension are track forks. everything else is 40+ pr straight (ugh).