magg wrote:Increased preload will increase static ride height and has no effect on spring rate. The fork will move the same distance for the same load.

Should we not start by measuring the unloaded and loaded sag to establish whether the current spring rate is correct. Then having selected the correct springs adjust the oil level to achieve the appropriate dive under breaking and finally adjust the compression and rebound damping to match the spring rate.

You too are correct about it not increasing the spring rate but that is not the only way to get the forks off the bottom.

eg: If adding preload reduces rider sag from 40mm to 36mm then this will give you an extra 4mm of compression travel. It doesn't change the spring rate but it mans you need to put a greater force on the forks to bottom them cause before you had 4mm less of travel to use up.

In practice you will have more than 4mm of extra travel because when you add preload you lengthen the fork due to a greater load on the top out spring. but that is another thing all together.

True but you have not changed the spring rate, only increased the distance that can be traveled before all the travel is gone. If this new travel distance is still too short then adding more preload adds more static ride height which might have a detrimental effect on the chassis geometry.

Is it not better to start with a spring rate that gives the correct unloaded/loaded sag for the specific rider weight and then set the oil level and damping rates to match..

How do you know it will not be enough extra travel until you try?

I disagree with you. You add preload to give you more support, when you are using too much travel. What other reason do you think the preload adjustor is there for.

Springs might be the answer, but preload is an easy quick change and may solve the problem. If it doesnt and feels good everywhere else then I would add oil.

If it on the bottom when you are about to turn in or during a turn then a heavier spring is the way forard. But under braking, if this is the only problem. Preload and oil height is the place to start.

I consider preload to be a fine tune just like compression and rebound damping adjusters. If the basic spring rate and damping are significantly outside optimum then the adjustment range will be insufficient to optimize the setup.

Would it not be better to start by checking the sag to make sure the spring rate suits the rider/bike combination and then the preload ajuster can be used to fine tune. This also applies to the rear suspension.

John21z, having high-jacked your original thread, I should get back to the original question.

Check volume of oil in the fork legs and if already set to OEM spec try increasing and also upping the preload if bottoming under breaking is your only complaint.

If still unhappy suggest measuring the free length of the original springs and check against OEM spec. If sagged could replace with stiffer ones from RO, also might be useful to follow one of the suspension setup processes available if you google the net.

Would still be interested to know what was done during the "service".

Hi yes all that went way over my head
any way back to my thread
I serviced them myself making a good set out of two old sets.
one the orignals had knackerd stantions.
the other differant caliper brackets on the lowers due to them being of a K model , nc21 my bike is a Zh model
every thing is the same apart from the caliper brackets.
so then I picked the longer of the two sets of springs.because I was told that they would be the better of the two
and new seals and 10w fork oil rebult them to the Zh spec ie cc of fork oil same as a nc24.
now im going to swap the springs back over and see if that solves it. i have never road it on the original springs
without issues ie not alot ov fork oil due two the seals being knackerd.
plus i might use 15w oil this time.
ps thay bottermed out on the road just before a bend not good
i live in the high peak got some great roads around hear

IF they are the progressive springs throw them in the bin. New springs are cheap.

Don't use 15W fork oil as fade will be more evident. 10W is fine.

IF they are not cartridge forks, you will be battling to get a good balance without the use of emulators.

Magg, I believe springs and preload to carry the same weight on the tuning scale. Ride height comes before it and re-valving below, finally is turning the knobs.

John21z, something to try before you pull the forks apart to change springs etc, measure the unloaded and loaded sag. The results will give you a guide as to whether the spring is appropriate for you weight.

As 999Gomerz said, 15wt oil is unlikely to fix the problem and will probably make the ride harsher.

IMO, dive under brakes is a low speed compression issue, provided the spring rate is high enough to support the weight of the rider and bike. Too much sag can be overcome by lots of compression damping I suppose but other things suffer, poor traction on bumpy surfaces for one.

Once the spring rate is correct then other factors can be tuned to reach optimum performance.