Tell me about ‘suspension corrected’ frames

[lune ranger]

What exactly is suspension correction?
My Fargo is corrected for a 100mm fork. Does this just mean the rigid fork is the equivalent length of a 100mm sus fork? Or is it more complex?
I ask because I just got an XFusion fork for the Fargo. It’s 120mm but can be adjusted to 100mm or 80mm. It has a drop out to crown race length of 505mm as is. The Firestarter is 483mm. If you think that sag will be 20-25mm the sus fork will have a ride height the same as the rigid. Does that mean it won’t change the bikes geometry and will work fine at 120mm travel? Do I need to drop the height to 100mm anyway?
Cheers

[GregMay]

It means the rigid fork is set to the length a suspension fork it is specced for, with optimal sag, should rest at.

So if you ran your fork at 120mm it may feel quite high and wandery. If you ran it at 80mm, quite low and tend to oversteer somewhat.

Jameso can probably give better definition, but that's my take on it.

[benp1]

My understanding is similar to Gregs

From the description, I'd be setting the fork to 100mm and with the sag that should be a similar height to the rigid fork

But I also agree with Greg in saying that Jameso is your man!

[Mariner]

Then there is the offset.
Don't forget the offset.

[Bearbonesnorm]

If you're bothered / interested, measure the axle to crown on both forks but as mentioned, subtract 15% - 20% from the sus' fork to allow for initial sag.

Yes, off-set is also important as it will effect both trail and wheelbase ... measure that too ... or don't and just bang 'em on and see how it rides.

[benp1]

Either way you won't die

I bought a rigid 29er with rigid 26er forks. I didn't die, though I did replace the forks after a short while

[Lazarus]

two things matter
1) the recommended axle to crown length of the fork measured in mm. Much more important measure than travel. I always assumed this was measured in non sag mode but realise I have no evidence for this assumption.
2) the offset if any


for 1 as long as the a-c is roughly the same as recommended the travel is somewhat irrelevant[ ie not all 100 mm travel forks have the same a-c]. if you go too long the front end will want to lift on steep climbs*[10 mm would not bother me personally but will change geometry]. Never gone to short so dont know what happens
for example i have some 120 mm rebas and some 100 [ 29er]mm travel that are very similar a-c [ I think the 100mm are actually longer than the 120 mm iirc]so i can use either n the same frame [ I bought the forks for this reason].If you are roughly right it will ride ok
if buying new and you dont know the a-c just get whatever size travel fork they recommend it as being suited for.


* I have some 150 mm forks on a 140 mm bike and it does not climb as well as it should [ anyone got some 140 mm 26 er straight steerer forks as they are hard to find. I have ridden 100mm designed bikes [two of them]with 120 and its fine 140 is hideous [ but ok down hill] 100 mm is better for all round. I had some TALAS forks for a bit so experimented. with different travel on various bikes]

[ScotRoutes]

I'm running 140mm travel forks on a frame based around 130mm forks and 120mm travel forks on a frame designed for 100mm travel forks. Both are just fine. I'd fit what you bought and ride it, then see if you feel there's a problem before getting hung up on the numbers.

[jameso]

Jameso can probably give better definition, but that's my take on it.

Nothing more to add to your take on that tbh..

Definitely an area where trial and error is good, it's all subjective feel stuff.

I do think the commonly used suspension-correct calcs are generally a bit off though (just to be awkward). 20-25% is a common sag figure but I'd want a bike to handle best in the sort of situation when the fork is working rather than static sag ie when the mean position is further into the travel. For XC riding it's probably more like 35% unless on a rougher downhill.

On a 100mm-corrected fork this 25% to 35% change isn't much difference and the difference in feel would be subjective anyway. But it explained why the 25%-corrected rigid forks I had made for early 100-120mm Genesis 26" MTBs made the bike feel a bit more slack/high/wandery than I was used to on the sus forks (of same offset) and going to a 35% sag length spec felt better to me overall. As long as your BB isn't too low and the seat angle too steep you can go shorter still.

As a rule of thumb on offset/rake - 5mm more offset has almost the same effect on (lowering) trail as 1 degree increase in head angle, but a different effect on where the wheel ends up. If this gets to brain-fog trigonometry levels try putting some numbers into this - https://www.bikecad.ca/quickapplet

[lune ranger]

Cheers fellas. I think i’ll ‘suck it and see ‘

[redefined_cycles]

I'm running 140mm travel forks on a frame based around 130mm forks and 120mm travel forks on a frame designed for 100mm travel forks. Both are just fine. I'd fit what you bought and ride it, then see if you feel there's a problem before getting hung up on the numbers.

This^^^

I had my SC Superlight running on 130mm Magura Menjas many years ago. It was built for a 100mm fork but worked spot on (up steeps and down steeps) with that. Hence, like Jameso said there's more sense in experimenting self...

Just one important thing on the suck/see method though... Dont cut thw steerer so long that its then lost value when selling on. Try and keep it as long as possible and just use the/some spacers... Then when you're totally happy do the measure twice and cut once thing...

[lune ranger]

Doubt it’ll be cut at all. I’ve an XL Fargo. It’s as tall as a horse and the head tube is at least 200mm.