alexk_il

An update.

I finally found a mint Shimano pro carbon xcr 445 fork on ebay for £50/$60. Installed it using Delta stem riser to keep the geometry recommended by my bike fitter.

Fork's weight is ~800g, and it works fine with my 185mm rotors (180mm max specified).

​​​​​​​Next thing to explore - seats and seat posts with less offset. Tempted to try a suspension seat post, don't really need it though, just curious how they feel.



Works great with

Ihmemies

How important is it to be close to the suspension fork's axle to crown distance?

Is there some "leeway" with the dimensions, like +- a few centimeters?

Does it make sense to replace a perfectly working suspension fork, just because I think I don't need suspension?

Thanks!

I have a rock shox reba rl 29" with 100mm travel, it works yes but I'm just wondering about a rigid fork. Some spec sheet said it has 510mm A2C and I measured roughly 500mm so it's quite close.

• https://mrpbike.com/products/rock-so...14314260168746 would probably fit as a direct replacment if I bought the QR 495mm version. That is not a cheap fork for a cheap bike though.
• https://www.salsacycles.com/gear/firestarter_fork I could buy an used Salsa Firestarter for 80€, but it has A2C of only 483mm

Some backstory:

I've been looking for a bikepacking/gravel bike. Yesterday I got an idea that I could try some gravel stuff first with my mostly unused MTB. I hauled it up to my living room and I ditched the badly cracked studded winter tyres. Then I remembered that I have a shortened rear fender somewhere which I failed to install a few years ago. I installed the fender now. I should have not cut it but it is what it is.

Saddle is temporary, I removed the steerer tube extension (looking for a new stem, probably will try an adjustable one first to find a better fit).

Bike is heavy, 16,1kg with the studded tyres. Some 40-622 terra speeds would be 750g lighter per wheel, so gravel tyres would save 1,5kg of weight and lower it to 14,6kg. A rigid fork would cut 400-800g and get it under 14kg. Not exactly lightweight, but at least durable. This bike would probably give me an idea if it's wise to sink $$$$ to a "real" gravel bike.

soyabean

There are many reasons to ditch a front suspension.

Most of the time the front suspension is a cheapie without a lockout.

The purpose of the lockout is to preserve cadence energy.

Folks that use their suspension fork bikes on paved roads lose a lot of energy that is wasted into the suspension instead of the crank.

Another reason is the weight of a suspension, but pales in comparison to explained above.

maddog34

Are dimensions from the axle to Crown race seat listed? Measure at the Half way point in your present fork's travel.. axle to crown seat.

That is the distance to maintain.. Let most of the air out of the Reba to aid in getting the measurement...you do own a fork pump, correct? Fully extended or fully collapsed are Not good dimensions to imitate.

this is what i'd do.... you'll need at least a machinist' protracter with level finder for this.. a degree finder works too.
Gravel bikes are usually in between a Road bike and Rigid MTB/Off road bike regarding Specified.steering head angles.
look up some frames made specifically for gravel use, then imitate the average angle you've found... then measure that axle to crown dimension again..


And.. you can make the measurement of your fork's Axle to crown with the fork fully extended too.. simply measure it extended, then subtract 1/2 the stated travel of your Reba.... or more

Head Tube Angles

Compared to road and cyclocross bikes, gravel bikes employ slacker head tube angles and taller head tubes. This increases stability and makes for less ‘twitchy’ steering. Your stance on a gravel bike is less aggressive and more upright than on a road racing machine, while still being quick-handling.

Standard Ranges

• Road – 73° to 74°
• Cyclocross – 71.5° and 73°
• Gravel – 70° to 72°

Some gravel bikes are more influenced by mountain bike geometry. They use a head angle between 67° and 69°. They perform well on descents and trails.

https://www.bikeride.com/best-gravel-bikes/#geometry

Ihmemies

Thanks, according to some old geometry charts the frame's seattube angle would be 73 and headtube angle 70 degrees.

I don't understand anything about suspension forks and I don't sadly own any tools to do any work with them. My only knowledge is that the fork "locks" when I press a button in handlebar and a cable transmits the command to the fork.

Why the axle to crown dimension is measured when the fork is half full? 510-50mm would be 460mm then. Would 483mm be too long?

Reba suspension fork is 1770g with the toggle button, Salsa firestarter steel fork would be 1190g uncut, so it would save 600 grams. Carbon fork would save more but for signifcantly more money for an old MTB frame..

maddog34

the idea is to make the bike have an ACTUAL steering head angle of 70 to 71 degrees... you will need to determine if the "70 deg. " spec listed is an actual, fully extended measurement, or if it was a "fork partially compressed by rider weight" spec...
As the suspension fork compresses, the head angle gets steeper...

Here's another factor to consider... it also effects the handling characteristics... TRAIL, and it's relationship to head angle and offset.(there are HUNDREDS of articles on this subject.. i just grabbed one quickly)
https://www.autoevolution.com/news/a...t-1-83482.html

SJX426 

@alexk_il - lot of technical info in this thread. I will cut to the chase. I have a 1997 RockHopper whose frame was built to support both rigid and suspension. I bought it with the rigid fork as a yearend sale.
A couple of years later, I found a used 2005 Marzzocchi SL with the correct steerer length. It has 100mm travel but is all air suspension. To get the right geometry was simply air pressure adjustment after getting on the bike so it would settle.
It was run as a commuter for a couple of years with one or the other forks. I lied the disk brake and the lock out.
The commute was 11 miles one way and I kept records of time and average speed. Between the two forks, there was no difference in performance. It may have been due to the traffic but most of the travel was on a MUP. Tires were UffDa (heavy) at full tire pressure, 2.1x26. Tyipical average speed was about 17mph.
Pics or it doesn't count.
1997 Specialized RockHopper on Flickr

P1020526 on Flickr