120mm fork on a 1997/8 King Kahuna - is it safe?

[Monkeeee01]

@Monkeeee01 - why not try the SID you have first?... it looks like it may be at 80mm travel atm, but by removing the internal spacer it'll be 100mm.

When I've fitted forks that are too long for a frame, the steering loses it's smoothness - it hits a point where the bar and wheel quite quickly "drop away".

It would probably be worth experimenting a bit to find what you like/are happy with.

Cheers

Thanks @wadsy - I didn’t realise I could do that/don’t know how but yeah I’d like to experiment for sure.

I would prefer the Fox as I find them less flexy when you go round corners but equally if the steering goes Pete Tong then I need to find a compromise.

Hopefully where the seller has reduced the travel on the Fox they will be the best of both worlds (but we’ll see)

 

[bikeworkshop]

The better fox fork will make a bigger difference to ride quality than the 20mm extension i bet

 

[wadsy]

Thanks @wadsy - I didn’t realise I could do that/don’t know how but yeah I’d like to experiment for sure.

I would prefer the Fox as I find them less flexy when you go round corners but equally if the steering goes Pete Tong then I need to find a compromise.

Hopefully where the seller has reduced the travel on the Fox they will be the best of both worlds (but we’ll see)

@Monkeeee01 - the 32mm stanchion SID you have is pretty good and once serviced/tuned is a nice fork. The Fox may well be more stable (esp if you use a through axle) and perhaps more advanced... I'm not that familiar with more modern forks.

I can do the travel adjustment on the SID, or you'd probably find a YouTube video and manuals online too... pm me if you'd like to discuss.

Cheers

 

[grahame]

How much sag do you plan to run on the fork?

25% sag on 120mm travel fork (30mm sag) would leave the geometry effectively the same as running 10% sag on 100mm travel fork - they would both have 90mm of travel "left" and the effective axle to crown length would be about the same, but the longer fork would probably feel much more "active".

 

[Monkeeee01]

The better fox fork will make a bigger difference to ride quality than the 20mm extension i bet

I think so too

 

[Monkeeee01]

How much sag do you plan to run on the fork?

25% sag on 120mm travel fork (30mm sag) would leave the geometry effectively the same as running 10% sag on 100mm travel fork - they would both have 90mm of travel "left" and the effective axle to crown length would be about the same, but the longer fork would probably feel much more "active".

Tbh I’m not very good with setting up forks, sag etc so I don’t know without experimenting but I’ve ordered the 120mm Fox 32’s and the seller has adjusted them to be 100mm for me so I’m hoping they will be all good!

Got to source the rest of my kit now….. brakes, cassette, chain, rotors, tyres, pedals all still to go!

Thanks again for everyone’s help

 

[Monkeeee01]

@Monkeeee01 - the 32mm stanchion SID you have is pretty good and once serviced/tuned is a nice fork. The Fox may well be more stable (esp if you use a through axle) and perhaps more advanced... I'm not that familiar with more modern forks.

I can do the travel adjustment on the SID, or you'd probably find a YouTube video and manuals online too... pm me if you'd like to discuss.

Cheers

Thanks again @wadsy I’ll drop you a line when I give it a go and if I get stuck. I’ll be holding onto the SIDs for a potential future build as V-brake models are getting harder and harder to come by!

 

[Splatter Paint]

As a rule of thumb for a starting point on the early Fox, set the pressure in psi to the equivalent to your body weight in kg. I.e. 70kg rider = 70psi, you can then tune from there the sag etc.

 

[hawkhill97]

IME the main limiting factor on increasing fork length on older bikes is the raising of the BB.

I've put a u turn 85mm-130mm rockshox reba (axle to crown 465mm-510mm) on a frame intended for a 80mm manitou fork (axle to crown 440mm). It was fine at 85mm travel (20mm longer axle to crown length), in fact maybe even an improvement, but anything above that the increased height of the BB became sufficiently negative to offset any other improvements.

This mainly manifested as a negative impact on cornering (especially noticeable when linking up a series of berms, as when you're leaning the bike the BB is having to pivot through a wider arc), and also a slacker seat tube angle and effectively a very slight reduction in top tube length.

I personally wouldn't worry massively about the frame strength side of things, unless you're doing something ridiculous like fitting some 160mm Lyriks to a pre 1997 frame.

A 40mm increase in axle to crown length is pretty significant in terms of handling, but will only be around a 10% increase in lever length. Unless you're already on the edge of the bikes intended use (IE you're heavy, riding at a decent race pace, and hitting sizeable features), I don't think that's going to make the difference between safe and unsafe to ride.

I would however, consider whether fitting a longer, better functioning fork might cause you to ride the bike outside of it's intended usage (IE hitting bigger features, drops etc).

A bigger fork will probably allow you to ride a bit faster, so it's also worth bearing in mind that since kinetic energy varies with the square of your speed, a 10% increase in riding speed (eg through a rock garden) will have a bigger effect than a 10% increase in rider weight or fork lever length - but won't effect the force/energy associated with drops as much (where rider weight and fork length are more important), so depends a bit on the trails and riding style the bike will be exposed to.

IIRC fox forks have a slightly shorter axle to crown length than rockshox for a given travel, so I'd say 100mm would almost certainly work great, and 120mm could also be ok - depending on the starting height of the bottom bracket and your tolerance for increasing this.

Let us know how you get on, I'd be interested to hear your experiences and thoughts!