KZ750H forks topping out

Couldn't seem to find an answer to this, so let me know if I missed an existing thread.

I replaced the springs in my 750H forks with YSS progressive springs but stuck to the standard oil (10W) and level (436mm from top with fork extended). It seemed pretty stiff so I didn't put any air in them.

The problem I am having is the forks are audibly topping out when riding. I thought about putting some air in but that would only make things worse (even more pressure on the extension stroke). I'm pretty sure I just need heavier and/or more oil. Does anybody have a starting point for me? I'd prefer to just top up the oil but if I need to replace it with 15W or 20W then it would be good to know if I stick to the standard oil level.

On a side note: if I ditch the air completely which models can I pinch some top caps off to get rid of the valves?

Hi Callum, did you measure the spring length before putting them in ? I ask because I recently changed to some racetech springs in the front end I'm using and they were a different length, even though they were made for my forks, I checked their site to confirm this, they "audibly" topped out before I even put them on the bike, there was NO WAY i could get any preload on the forks as they were, to fix it I just made up some longer spacers, worked a treat... I wouldn't remove the air as it works mainly on compression, just like the springs, the oil is more for the rebound damping side of the fork action.

I agree about Spring length or they are just to big of a wire gauge and to stiff.

If you have replaced the springs with aftermarket, most aftermarket springs are not the same size - they're generic to fit on many bikes, and then need a longer spacer to make up the difference.

You don't necessarily need to take your forks apart again, just check your sag. With your weight on the bike, the front fork should sag a few cm - about an inch. An easy way to check this is to put a small cable tie on the fork, sit on the bike as is you are riding it, then put it on its centre stand and see how far up the fork the cable tie has been pushed.

If your forks are sagging too much, make up the difference with a spacer. I changed my ~8cm stock spacer on my KZ750H with a 13cm spacer to make up 5cm of sag from ditching the air and older springs. Most people seem to use PVC for the spacers, but I use a small length of galvanized pipe cut with a pipe cutter, seemed a little more robust to me.

If your sag is fine and you're happy with the spring rate, the oil grade will help with fork dive (the oil grade will not effect the sag). But really, unless you're really, really pushing it hard, it should be very difficult to bottom out the forks. Most people on this forum tend to agree that 10W is too light for the forks, but even with 10W oil and the proper spring rate it shouldn't bottom out.

Agree with checking sag. Here is a good guide for doing it... racetech.com/articles/SuspensionAndSprings.htm

Thanks guys.
Sag is OK. To be honest I don't think I could get the top cap on with a longer spacer (the new spring was slightly longer than the original one).
The problem isn't bottoming out but rather that the stanchion is slamming back up into the rebound spring on the damper on the expansion stroke (to much rebound). It just needs more damping but I don't want to risk putting too much oil in. How can I tell what the limit is (to avoid hydrolock)?

At the moment I'm thinking of putting some 20W in there with a level of about 400mm.

calum wrote: It just needs more damping but I don't want to risk putting too much oil in. How can I tell what the limit is (to avoid hydrolock)?
At the moment I'm thinking of putting some 20W in there with a level of about 400mm.

The risk of hydrolock is pretty low unless you really overfill.

But I think your suggestion of using a 20W is a better alternative. By the sounds of things your spring rate is already pretty high, and filling the oil higher will actually have the effect of increasing the spring rate further, since there is less air in there to compress.

As I mentioned before, most people here agree that 10W is too light, 15W is more common, and some use 20W and even 30W. Give 20W a go - worst case scenario is that you just change it to something else if it doesn't work for you.

Irish Yobbo wrote:

calum wrote: It just needs more damping but I don't want to risk putting too much oil in. How can I tell what the limit is (to avoid hydrolock)?
At the moment I'm thinking of putting some 20W in there with a level of about 400mm.

The risk of hydrolock is pretty low unless you really overfill.

But I think your suggestion of using a 20W is a better alternative. By the sounds of things your spring rate is already pretty high, and filling the oil higher will actually have the effect of increasing the spring rate further, since there is less air in there to compress.

As I mentioned before, most people here agree that 10W is too light, 15W is more common, and some use 20W and even 30W. Give 20W a go - worst case scenario is that you just change it to something else if it doesn't work for you.

Yep, Yobbo is onto it, My problem was slightly different, my springs were shorter, I needed longer spacers which I made out of aluminium Tube. Use heavier fork oil at the recommended height, I wouldn't overfill, it may create other problems as suggested, these forks were designed to work with a certain amount of air as part of the suspension, reducing the air by increasing oil height will harden the overall feel of the forks as oil is harder to compress than air. One small change at a time with suspension is the only way to do it. I like to use later model, multi adjustable RSU {right side up} forks for this very reason, lots of adjustment at your fingertips... Let us know how you go...

Thanks guys.
I'll order some 20W and will start with the standard height. Then I can experiment with air pressure to compensate for the 100kg on the seat

20w oil is really thick and will make the forks feel harsh on compression. I'd try 15w first. Before that though I'd make sure that your springs + spacer don't sit proud of the top of the tube with the forks fully extended. If they do you are applying too much preload which can cause your topping issue.

Be sure you stick with the same brand of fork oil, and the very same line within that brand. Fork oil weights are mostly marketing and voodoo, and have little relation to the actual viscosity. A 15 weight from one brand will likely behave quite differently from a 15 from another brand. There can even be quite a difference between different lines in the same brand. At least if you stick to exactly the same oil, you can be pretty confident that the 15 will be thicker than the 10, but not as thick as the 20.

Nessism wrote: Before that though I'd make sure that your springs + spacer don't sit proud of the top of the tube with the forks fully extended. If they do you are applying too much preload which can cause your topping issue.

Mmm... This may be my problem. I did wonder when I assemble the forks but talked to some local guys and figured it would be OK. Thanks Ed!
Guess I need to make some shorter spacers. Should they be flush with the end of the tube?