Hi ,

I am going to save a hardtail i have and make it rigid.

It has a 130mm travel fork with A2C of 500mm

I have been offered an aluminium rigid fork for a good price at 455mm.

Does this sound about right ? The idea is to stay a little under the original suspension forks length to account for sag right ?

That'd be potentially a little long, which will decrease your head tube and seat tube angles slightly. Assuming your fork is intended to sag to 25%, you'd want a 375mm fork to match the same effective A2C.

https://bikegeo.muha.cc/ plug numbers into this if you want an idea of how this will change geometry. Generally though increasing effective fork length going from a suspension to rigid fork isn't desirable as usually an offroad bike is being repurposed for more road, or at least light offroad duty.

Yes, you have the right idea. A rigid is generally a little shorter to account for sag. For a 130mm travel fork, I would think you would be looking to go in the ballpark of 25-30 mm shorter. That would be arounf 470-475mm a2c.

But there is no hard and fast rule here. You might end up liking it a little longer or shorter.

Also, was a 130mm fork what the bike was originally spec’ed with? You might want to double check what that frame was designed around.

Also several companies make lower headset cup extenders that effectively increase your fork length. I see that Wolf Tooth makes a few models. I have one from Real World Cycling that adds 15mm under the head tube.

There is some error in your maths. 25% of 130mm is 32.5 mm, which gives the corresponding effective AC of 500-32.5=467.5 mm. So 465 to 470 would be virtually the same, but I suppose 455 should be quite close too.

The bike is recommended for a 120mm fork preferably but can handle a 130mm fork at a push according to the manufacturer (brand x hardtail ht-01) https://www.chainreactioncycles.com/.../rp-prod130452

Its actually a 27.5 frame but I run it with a 26 inch wheelset and 26 inch fox suspension fork. I am not too worried about BB height as once it is made rigid it is getting slick tyres and wont go off road anymore.

OK, there is enough geometry funkiness going on here already that normal rules don't really apply. Also, if this is going to be a street bike, then going steeper on the head angle is not much of an issue.

I'd go ahead and use the fork.

The 455 fork is a little shorter than "optimum" which might be perfect for on-road since the steering will be quicker which IMO is better (unless it's for leisurely "cruiser-like" duty). If the steering is too "fast", should be able to correct easily.

This one thing I'm struggling to understand. I agree that a more responsive steering is better for commuting in the city, track or crit racing - anywhere where sharp changes of direction might be needed. But how does it matter in road riding where you'd go for miles on end without needing to turn the handlebars more than a few degrees?

This is qualitative, but it placed me in a more comfortable cycling orientation. Difficult to quantify, but was better both off and on road. If your current setup feels great, the shorter fork could make the bike ride worse, but I doubt it. A fork change doesn't take long if you retain the same headset and just change the fork race. Maybe you could do a quick experiment

You can't just look at fork length alone, more data is needed. What is the rake, what is the wheelbase, what is the BB to ground clearance.
Steering stability and agility is not really how much you turn the wheel, imagine riding with your hands up and steering with your weight alone. Steering geometry is there even if you don't steer.
There is fork rake/angle of steerer, and there is trail. Trail affects how quick or slow the bike turns more precisely than rake. If you are to use a much shorter fork, like if you slammed the brakes over a bupm and the fork locked at bottom end, your rake may become very sharp, but your trail may be tiny, which will make steering very nervous. Track bikes have little offset so the trail is increased since they don't turn much and can track a straight line while sprinting.

You also have to worry about how low your crank will be after all. If you are planning to use it on road compare the common gap with your final. If the crank is exactly at the mid point between the two axles, 50mm less in the front may be 20-25 at the crank. If you are going to use the bike as a cyclocross or gravel bike you don't want your crank too low like a road bike.

MTB geometry from the 90s to 2020 has changed radically, road geometry hasn't, except the extreme racer short wheelbase is more dominant now that it was then. But rake on some hard tail MTBs is enormous, it can never be made to be roadworthy.

Last point, the sharper the caster/rake angle is the more bracing the frame needs to minimize flex. When you shorten the fork too much you might get into a region where the frame is too flexy while turning and be really sluggish. This on the long run may cause fatigue on the frame around the head tube. It doesn't look like it as easy but 80% of frame stiffness on a road bike is the rigidity of the frame around the head tube. This affects in corner corrections, quick change of direction, etc.

This is incorrect. 375mm AC length is too short a rigid for a bike originally spec'd at 130mm travel. A fork in the range of 500mm axle to crown is a better target.
Perhaps OP could try riding the 455mm fork. It seems short to me, I wouldn't go for that. But if the frame is super slack already and a short fork reduces the effective fork angle, maybe it's palatable to them at 455 AC.

the 455mm fork will be just about perfect.

the the desired steering angle change will be hard to measure, if there is any at all

as to the chance of the crank arms being too low... the bike was designed to have decent clearance at full suspension travel anyway.
you'll be fine.

i've done many such conversions.. a slightly steeper steering angle for a smooth trail/road combo bike is a good thing.

More than likely 5 years later the OP has already figured out this situation.