Hi all. I am looking to replace the fork on an older Klein Quantum I picked up. I am at altitude in Norway out in the boonies and most routes go via gravel roads and worse. When the Klein popped up for sale I figured I would try to convert from the current 40mm clearance to 45-50mm for a softer ride.
- steerer tube measures 227mm from the top of the steerer tube to the top of the headset.
- fork length measured parallel to steerer approx 372.5
- diameter 25.4 (I am guessing the diameter is 25.4 / 1", but I am measuring 25.6-25.7 on the middle non-threaded part of the steerer tube).
- 40mm thread (down from the top).
- 74 head tube angle
- 1.5. front fork rake

Anyone happen to know of some options?

Thanks in advance!

The Quantum with brakes shouldn't be able to fit a tire wider than 28mm max. Even that might be too wide. How are you getting a 40mm in there? No brakes? 650b and extended calipers? None of that makes sense to me.

If the fork isn't damaged, I don't think a new one will be an improvement.

If it is damaged, look for a 1" threaded road bike fork for 700c. Your options are limited but they do exist. Some come with disc brakes mounts, which would give you more tire clearance. It's also a crime against a vintage bike to do that.

Hi. The inside measurement on the original fork is 40mm, which doesn't allow for much more than 28 - correct. So I am looking to switch to something that does. How is that a crime?

Nobody said it’s a crime, it just sounds like you’re not going to find a fork using rim brakes that will fit a 40 or 45 mm tire. Not enough clearance. You have an option of finding a fork using disc brakes that would work, but then need to change your front lever to hydro as well as replace the front wheel to a disc wheel. Trying to do this with rim brakes isn’t going to work.

My '74 Raleigh International will accept very wide and tall tires, with room to spare for fenders.
Maybe you can hunt around for similar. Plenty of forks on eBay, but you probably need to request measurements.
I would think the rear triangle, however, will not accept very wide tires. Sounds like that won't prove a limiting factor...

Thanks to you both, I may be confused, but it looked as if Rosefart was opposed to the idea when he said it was a crime, which is all well and fine; I don't see it as such. And, again, I am not trying to fit a 40mm tire but a knobby 35 – for example w "Seven Cycles, pureroad medium reach carbon fiber fork" or "Whisky no7 mid reach fork"

The first one might be short 2 millimeters, but it's ballpark of what I am looking for, and there have to be others, and maybe more reasonably priced. Not worried about caliber reach to be honest.

Are you sure that the chain stays and seat tube clearance in the back will accommodate the larger tire?

Also, a limiting factor on one of my road bikes is the vertical clearance of the rim brake calipers.

It's probably more than just the fork.

Thanks Roadcrank and Camilo, I am pretty sure the back has 45mm clearance. I will look into the geometry on the back as well. Thanks for pointing this out.

As well, will the front derailer work with a really wide tire like a 45.

Again, I wouldn't be hoping for that much with that kind of bike – 35mm tire is the goal. I could probably squeeze that on the native SR Prism fork, but it wouldn't be optimal on a rainy day with mud, gravel, and such. Which is why I am hoping to find a fork that would allow for a little more space with medium reach calibers.

The crime I spoke of was not something that would land you in jail. Think of it as a crime of fashion. A disc fork would look awful on your bike. I apologize for the confusion, the nuance of English is easy to miss on translation.

It may not matter. The only 1" disc forks I found on Internet searches are custom made, almost exclusively for touring bikes. Your price will be between $250-$500 USD. It'll probably raise the front of your bike a little and change the handling. And, I'll say it again, it'll look funny.

If I were dead set on using this bike on chunky roads and gravel, I'd do a 650b conversion. It's a smaller wheel and a bigger tire. Tektro makes long reach caliper that will fit. I'm pretty sure your bike can use 35mm tires with this setup. This would allow to do both front and rear.

Here's what you'll need.
New 650b rims and spokes and someone to build them up onto your existing hubs.
Tektro long reach calipers
New tires.

It would work. I wouldn't do it unless I dearly loved that bike. There's better options for rough roads.