Monday, 20 February 2017

Thorpy's Guide to Rigid Forks

Thorpy's Guide to Rigid Forks - Intro

A great many of the features on modern mountain bikes are aimed at racing, and going really fast and doing stunts and stuff. People used to do all this stuff on rigid forks, and probably break their wrists in the process, but now we have entered the marvelous age of suspension forks.

BUT if you are a luddite, troglodyte or hairier than average you may want to stick with a rigid fork. But where does this leave you? Can you buy a frame that is design for massive 150mm travel suspension forks, and run rigid forks on it?

Exec. Summary - If you are running a high travel 26er or 650b, you will likley be able to find a long rigid fork to replace, but for a long travel 29er, you will struggle to find a mass produced fork to cover forks above 120mm travel.

I have produced the below table to help you decide method used is to follow (click to enlarge).

Thorpy's Guide to Rigid Forks - Length Matters

Obviously you will need a fork that fits your wheel, but there is also the ride characteristic of the bike to think about. 

An over long fork will give a "slack" ride. Some people actually like this as it makes for a relaxed steering feel. However, and fork that is too short will result in a steepening of the head angle of the bike, and the steering will become "twitchy". Unless you a slightly delusional and masochistic this is not a very nice thing. 

As you change the height of your bike front, it pivots around the rear axle. This means that sever changes in height at the front end will change the position of the pedals relative to the saddle, the taller you have the saddle set, the more difference this will make. 

Thorpy's Guide to Rigid Forks - Travel

The bike below has 150mm travel suspension forks. The frame was designed (i should think) to run with forks of this sort of size, because it looks really cool. . . and some other reasons.

There are a lot of reasons why you might want to run the above frame with rigid forks. The list is long, and might includes mounting panniers or fenders / mud guards for a practical use, whilst still allowing for the use of high volume (fat) tires and a low step over height.

But can you really put rigid fork on a bike like this? Of course you can but do some maths first.

Thorpy's Guide to Rigid Forks - Maths for 27.5"

The above bike takes a 150mm fork. We need to find the "axle to crown" measurement for that fork, or any other 150mm travel fork, of the same wheel size, which in this case is 27.5" or 650b.

If we look at Axle to Crown Lengths for 150mm travel forks is we can see that axle to crown length is typically around the 550mm mark.

So we will need to find a rigid fork the same length?  NO

We also have to account for "Sag". This is when you sit on a suspension bike and it squashes down, under your weight. For different travel forks you would apply a different sag:

  • 100mm = 20%
  • 120mm = 25%
  • 150mm = 30%

So for a 100m travel fork, you would subtract 20mm. And for the above bike we would subtract 30% of 150mm . . . .  45mm . . . which give a rigid fork length of 505mm (for the above bike).

What sorts of forks are that length?. . . , not many:

There are some high end fork with 500mm axle to crown measurements such as the:

  • 3tcycling rigid-500-mm-team (tapered headset - see below)
  • Carbon Cycles Exotic 49cm

Thorpy's Guide to Rigid Forks - Headset

So you can get a fork the right length, you now need to attach it to the bike.

Some of these bikes designed for high travel forks have a 44mm headset in which case you may already to running a straight steerer tube (Commencal El Camino), but a 44m headtube can also accept tapered forks, the above list are not tapered forks (although you may find some which are).

If you have a tapered headtube, then you may need to fit an adaptor to allow for the 30mm crown race on the above forks.

Thorpy's Guide to Rigid Forks - Mega Table

Based on the above method please find below table showing required fork lengths.

Travel 100 120 150
Wheel  Sag % 20% 25% 30%
Axle to C
26" 440 490 540
Sag 20 30 45
26" w. Sag 420 460 495
27.5" 470 510 550
Sag 20 30 45
27.5" w. Sag 450 480 505
29" 480 520 560
Sag 20 30 45
29" w. Sag 460 490 515

In general I would recommend choosing a slightly longer fork (10mm - 20 mm), rather than a shorter one. A shorter fork will result in twitchy steering. Feel free to browse for forks of the correct length, but you might opt for:

Yellow = Gusset Jury 29er or Surly Karate Monkey

Green =  Thorn Mount Tura (26")

Orange = Surly Krampus (483 mm) Salsa Fire Starter (483 mm)

Blue = Exotic 29er

Thorpy's Guide to Rigid Forks - Conclusions

Is you are riding a high travel 29er then you may well have to suffer a drop at the front end (shorter  forks) in order to swap over to rigid. However, for most other bikes with 120mm travel or smaller wheels, you will no doubt find an off the peg solution. 


  1. Thanks PP, I spent a bit of time on this. I doubt many would commit so much time to a topic such as this!