I’m looking to go put a rigid fork on a 29er and was just wondering what effect this will have on handling and the fork trail compared to a RS Reba RL 100mm fork.

Frame head angle is 70
RS Reba RL 100mm 29 a/c 506 and rake 46 (i think)
Exotic fork Rigid a/c 465 and 42 rake
Chinese Carbon Rigid a/c 450 and 38 rake

All suposedly for 29er frames but all prety different specs.
Its an xc bike so the lower a/c should raise the head angle which is ok but dont know how much?
i dont know how much the lower rake affects the forks trail and handling combined with the shorter a/c?
i probably have ocd and non of this make much difference but dont want to buy the chinese carbon to find out my xc bike now steers like a barge.

http://www.kreuzotter.de/english/elenk.htm

450 with 38 rake will give you a steeper bike with a short front centre. Trail results aside, your weight distribution may be a bit odd.

Both will be a bit sharper than the reba. The exotic will be closest though and the Chinese carbon being very quick in comparison.

Go for around 465/470 for a 100mm equivalent on a 29er. So out of your choice, I would go for the exotic. It’s a quality fork anyway at a good price and rides well.

That website is actually really helpfull, a quick google suggest 57 might be a good trail number.
With the exotic fork my trail comes out at 84.3 with a 70 deg head angle and 709 as the tyre width.
so a bit high maybe.
however what i dont know is how much the shorter a/c of the exotic will adjust the head angle.
If it changes it from 70 to 73 then then that brings the trail down to 64.5 which sounds better.

So how many mm shorter a/c will change 1 degree of head angle?

thanks for the other comments as well.

20mm iirc

yep, the cotic website confirms that.
Checking some other decent frames out there trail figures are also higher than this 57 figure so either theres more to it than this or it doesnt make much difference.
looks like the exotic fork could be a goer then and probably steepens the headangle by 1-1.5 degres
Seems to be preferable to have trail higher than 57 than lower so all good and also would be lower trail than with the original Reba, probably.

Checking some other decent frames out there trail figures are also higher than this 57 figure so either theres more to it than this or it doesnt make much difference.

Like most things, it’s complicated! My hardtail has 96mm trail, my full-sus 113mm. Incidentally my BMX (and thus almost all BMXs because they don’t tend to vary much at all) has 40mm trail.

A heavier tyre makes it feel like you have more trail due to the gyroscopic effect. A larger tyre makes it feel like you have more trail because you have more contact patch pulling on the steering, particularly behind the steering axis. And a stickier and slower rolling tyre makes it feel like you have more trail because the extra drag creates a greater self-centring force due to the trail.

With your current fork, at ride height (20mm sag), the trail is about 80mm. Those rigid forks seem far too short.

a quick google suggest 57 might be a good trail number.

for a road bike, yes.

With your current fork, at ride height (20mm sag), the trail is about 80mm

This is normal for a hardtail. 75-90mm range maybe.
Where your bars are affects trail feel – ie you can have 100mm trail with a short stem and wide bar and it’ll feel fine (better at speed than slow though), you can also have 70mm trail with some bikes and it’ll feel great but again a lot of it is down to where your weight is on the bike and how it interacts with the steering. It’s also how the trail number is arrived at – different head angle and fork offset combos can give the same trail but quite different weight distribution. The feel of a given trail number can vary with tyre size and radius (mainly due to change in contact patch) too, just to complicate things.

Try this – http://www.bikecad.ca/quickapplet
model your bike angles, lock frame then adjust the forks to see what happens.