A guide to fitting Cartridge Emulators (particularly to a TRX850 4NX)

The forks on the TRX have been ruining my enjoyment of the beast since I got it, crashing over bumps and generally doing a terrible job of pushing the front wheel into the ground. I was seriously considering fitting a front end from something else with functioning forks but the trouble of losing the speedo drive, swapping steering stems, finding forks long enough to not have the bike falling forward all the time and the trouble with lockstops come MOT time left me looking for an alternative. Then I was pointed towards these valves that take over the job of compression (riding-over-things) damping and are supposed to be generally excellent while allowing the use of the original forks. I was sold! Here is what I had to do, some things are 4NX specific but hopefully the overall direction will help others if they decide to try this mod.

Firstly, get yer bike on some stands, the front has to come off.

Remove the front wheel, if this is already beyond you then perhaps ask someone else to fit these things. KAIS in Manchester quoted me £360 for emulators, seals, oil and labour which is an excellent price now I know what is involved. Racetech valves and springs are £250 alone..

Undo the tops of the forks at least a bit, it's much easier with the stanchion (shiny bit) clamped in place. Let off the preload as far as you can for maximum ease.

 With the top off pick out the washer, spacer, washer and spring, and then pour the oil into something you can fill with about a litre of oil. This left me with this horrible sight:

That is the top of the damper rod, which is what we really need to get to. It is bolted into the bottom of the fork with a large-ish allen bolt coming up from the bottom, but in order to undo that bolt you have to stop the damper rod from spinning. Some people manage this by sticking in the end of a broom handle, but since I have a garage of "really useful things" at my disposal it was time to make a special tool to fit those serrations (at the sides of the 3 slots round the hole in the centre).

The old 2 nuts tightened onto each other trick works again, all of this is ancient imperial stuff but it fits okay. Lowered down into the top of the rod it was successfully stopped from turning and, after removing the dust seal with the help of a screwdriver (as per the manual!) and the clip below it holding the oil seal in, some serious pulling on the stanchion pulls it out along with the top bush, its washer and the oil seal. I'll explain this better come reassembly, I was so excited to get them into bits I forgot to take pics. This leaves us with all of these gubbins:

And, in particular, the damper rod of doom. And expense. And waiting, depending on who you get to modify it. You can see the serrations around the top where the Special Tool was fitted, some weird plastic clip which no one can tell me what it does exactly (oil flow control, centring the rod, no real idea), the rebound spring (when the fork is fully extended this spring stops it gently) and the weird white plastic bush thing which is now in the wrong place, but I did not know because these forks are quite different to what is shown in the TRX manual. Bloody imports!

With the disassembly complete (try and keep the two lots separate if you can) it's time for the farkle. I was going to use Racetech Gold Valves and their springs like everyone else does, but it turns out Racetech stuff is basically unavailable in the UK. Racetech serve their American customers first and their warehouse is a slow operation, and the only people currently supplying Racetech gubbins are PDQ somewhere near London but they don't buy much in at a time. I was looking at [i]at least[/i] a 4 week wait to get my hands on the stuff, and even then it might not have been in sent that order. Some searching for alternatives suggested that the PD Valves from the competing company YSS were both available and much cheaper than the Racetech valves (£93 from [url=http://www.kamarmotorsport.co.uk/]Kamar Motorsport[/url] vs £150) and with the money saved I could buy the slightly more expensive Ohlins springs (£115 vs £100) which are also fairly easy to get hold of. Plus you get to fit [i]that[/i] sticker.

After waiting a few weeks for the springs to arrive from an ebay seller in France (d'oh, it wasn't that much faster) I had everything in my grubby mitts. The reason I didn't use the original springs is that they are a little soft even for me at 68kg and the valve sitting on top of the damper rod means the spring or the spacer needs to be shorter, 14mm to be precise. YSS recommend just slicing some off the spacer but what stickers do you get for that? The Ohlins 08608-85 were my springs of choice, these would normally replace the spring [i]and[/i] spacer in a Euro spec TRX with a longer damper rod/shorter spacer, but my crazy import rods are around 2cm shorter, 21.6cm if anyone here cares. With the valve in place this means the 08608s are just about perfect, it couldn't be easier!

The original setup is on the left, the new stuff on the right. These Ohlins springs are miles long, very strange. The only thing I wasn't sure on is how much the Ohlins should be crushed when the tops go back on, I could have ended up with too much preload as the original stuff springs out when the top of the fork is removed. Happily though it all turned out alright. It would be nice if Ohlins could give at least some instructions with their products, mind. Again, if anyone here really cares the springs that came out of my forks are 39.5cm long, and the whole washer/spacer/spring combo is 53cm. The Ohlins 08608 springs are 51cm, plus the 14mm tall valve and a 2mm thick spacer makes it all about right as long as the Ohlins springs need to be squashed as much as the original springs.

**Removing the rebound adjustment**

TRX850 forks come with a rebound adjuster, this is the thing with 3 rectangles and a hole in the centre sitting in the top of the damper rod. We can't keep this because the thing in the top of the rod gets in the way of the valves we're going to fit, so it has to be removed. With the damper rods in hand, remove the clip holding in the adjuster at the top and then drift out the adjuster from the bottom. If you don't do this in a jar or some kind of container the detent ball (that makes it click round) in the side of the adjuster will be launched away by its spring never to be seen again. I immediately lost both. This rebound stuff is now defunct, put it to one side or use it as a keyring. All this disassembly will leave you with a couple of bare damper rods, like so:

All these holes at the top are for the rebound adjuster and need to be filled or blocked. If you don't have any rebound adjustment on your forks it will make the mod quite a lot easier as it can normally be left alone.

**Back to the normal compression stuff**

These are the compression holes, oil is forced up here but not too quickly because of these holes restricting flow. This is the crux of the problem, oil flowing through holes is easy at low speed (so the forks dive under braking, wallow at low speed and just generally have little damping when the forks aren't being squashed quickly) and difficult at high speed (so the forks crash over big bumps or even small bumps at high speed when the oil can't move fast enough through these holes). At super high speed such as hitting a pothole o even a harsh speed bump the oil speed can be so fast and the holes so limiting to flow that the oil effectively hits hydraulic lock and that's when it gets really uncomfortable. What we are doing is removing the restriction of the holes and getting a valve to allow oil flow instead - at low oil speed the oil flows through a small hole (less dive, less wallowing) but at high speed the oil can push through the valve so the fork "gives in" and as a rider you see the suspension absorb a bump rather than roll over it.

I took my rods to a motocross specialist called SPS as the suspension guy there is a properly trained Racetech fellow, one of hardly any in the country. He was full of advice but it took three weeks for them to modify my rods and he didn't know much about the modifications I had to make, it seems a lot depends on the particular bike. Personally I wouldn't bother going to a suspension specialist to get this done again, just go to an engineering shop and tell them what you want. You have to know, though! Three weeks, some ignored phone calls and a few wasted trips later the rods were done, £61 including a litre of 15w fork oil. Quite expensive but in the end it was a good job.

**Removing the rebound adjustment**

Many people braze up the rebound holes but he didn't seem keen, reckoning that the heat required could warp the top of the rod. Since a good seal is important between the top rim of the rod and the valves we're going to use, this seemed bad. I mentioned that some people fit a sleeve inside to cover the holes and he seemed much more enthusiastic about that, so that was what happened. Unfortunately getting the sleeve to be a good interference fit was also what took up all the time, ah well!

The sleeve:

The holes, nicely blocked:

Since we've now removed all the rebound holes, we kind of need some way of getting the oil back down the fork, so we must add a single rebound hole in the lower part of the rod. The European forks already have this hole, but the 4NX ones do not. a 2mm hole was added:

Honestly I don't really understand why this is necessary but it's part of the Racetech instructions for the TRX so it got done.

**Normal compression mods**

To stop the holes being restrictive, you can either enlarge them or add more holes. Since the rods take the whole weight of the front of the bike (via the spring on the top) making two massive holes is bad because it doesn't leave much supporting metal. Instead, the two existing 7.5mm holes (rather than the existing 6mm holes found in Euro spec forks, if anyone has a fetish for TRX trivia!) are enlarged to 8mm and four more 8mm holes are added, allowing oil to slosh in and out as it pleases.

With the rods done it is time for reassembly! As mentioned, I have no manual for these forks (although I now know those fitted to a mid nineties ZZR600 are incredibly similar) so I was "flying blind" as an old boss once liked to put it.

**SUPER FUN EXTRA BONUS ROUND**

Slide your lovely new seals (you got new seals, right?) and slide them down the shiny stanchion, like so:

The oil seals are 41x53x8/9.5, nice NOK items are readily available, I'm not sure what the dust seals are but they're both the same size as all other TRXs just like I was promised by a couple of members here.

Get your empty leg (the big, non-shiny bit) and the matching stanchion (shiny bit, previously mentioned) and slide it in. Then slide the top bush down and struggle to knock it down into place. Around 20 minutes tapping by hand with a tiny drift worked for me. From this:

To this:

Slide down the big washer:

Then slide down the fork oil seal. This can either be driven in with a piece of PVC pipe, a proper seal driver hammery thing or knocked in bit by bit with a drift but that's a last resort. My Dad started helping at this point and found a big piece of metal to slide down and knock the seal into place, very handy. Also cover the seal and the hole with red rubber grease, it helps. Remember during disassembly when the stanchion was used to knock out the bush and seal? I hope this explaisn how that happens - the little grey collar around the very bottom of the stanchion can't get past the bush so the bush, big washer and oil seal are pushed out along with it. Makes it easier to remove the seal at least.

Fit the clip over the seal:

And now it's time to fit the damper rod, complete with red plastic ring, rebound spring and bush.

Experienced hands will already know what's up, but as I say I was flying blind with no manual and that white plastic bush seemed very happy to sit right there. When the forks were disassembled it all just kind of fell to bits so I didn't know where it came from exactly..

The Special Tool at work again!

As an aside I would recommend using new copper washers to seal the oil into the bottom of the fork where the big allen bolt goes into it from underneath, but I just annealed (seriously heated) the old ones and they sealed just fine.

At this point we hit a snag - the damper rod would not go far enough down the inside of the stanchion to be reachable by the bolt. Closer inspection of the still disassembled fork #2 learned me the error of my ways..

The white plastic bush would not go past this ring, whatever it does. This is because the white plastic bush doesn't live on the bottom of the rod like that, oh no, where does it live..? At the very bottom of the stanchion, underneath the Ring Of Terror.

Disassemble the mostly-assembled fork and put the white plastic bush in the bottom of the stanchion, then build it all back up.

**END OF SUPER FUN EXTRA BONUS ROUND**

Now that the bottom of the forks are built properly, it's time to start fitting the snazzy farkle. We start by filling the forks with oil - I put ~250ml in each fork and left them overnight to see if any leaking happened. Thankfully the reused copper washers work and there was no seepage. Next up is to compress the forks as much as possible so that we can fill them with the rest of the oil:

They need to be compressed because this is how the air gap is measured - no springs, no valves, just compressed forks. I made a super technical 14cm dipstick and filled the forks at the angle given by the yokes until the fluid was just about touching the [s]paper[/s] dipstick. Straight up this might be a 14,5cm air gap. Standard for the TRX with valves seems to be 13-14cm but I'm looking for a plush ride rather than a harsh trackday setup, so more air is good. This used about 900-950ml. The dipstick:

Finally we get to fit the valves. These are the YSS PD Valves, 33.5mm OD is a perfect fit for the tops of the TRX's rods. If you're fitting Racetech Gold Valves then the equivalent are S4101s.

You can see that the thread of the long bolt sticks out the bottom, that is why the rebound adjuster had to be removed. I set the bolt to having two turns of preload on the little spring as is the recommended default according to YSS - with so few people having any tips on how to set these YSS valves up I wasn't about to set them to anything else. Bye bye my expensive little buddies!

It's very difficult to tell if the valves have sunk properly into the tops of the rods, but both springs ended up at the same height and lots of poking, jiggling, shaking and bouncing the springs up and down didn't make anything sink any lower. The springs come up to here with the fork fully extended:

And of course a washer has to go on as well to spread the load of the fork top, particularly the preload adjuster as that is what presses down first. These are two of the original washers, nothing fancy.

**Removing the rebound adjustment**

The final part of this is to cut off the rods from the fork tops that turn the rebound adjuster. Not only are they not needed anymore thanks to the adjuster no longer being present, the rod will hit the tall allen bolt in the valve. A hacksaw will do the trick, though it feels destructive.

**Back to normal**

Fit the fork tops and it's all done. The springs being so long means the sag is about right for me with no extra preload, though it looks strange with the adjusters sticking all the way up like that.

Initial impressions are favourable. The forks seem very hard and un-wallowy compared to stock, yet they ride through bumps instead of crashing over them. It's really quite impressive over big bumps and deliciously controlled over speed bumps. The rebound is possibly a little slow (thinner oil can fix this) and the spring at 85N/mm is perhaps a little hard despite being exactly what the Racetech site recommends for my weight - I think Racetech's site errs on the harder side, when what I want is a lazy, compliant setup. This may explain why 85 is the hardest 08608 that Ohlins sell. It is possible that the preload is right on the point of being too much for my weight too, loaded sag is around 25mm without gear - so much for the plush, lazy, compliant ride I was looking for. However, it's instantly good enough to show the rear suspension up as rubbish, gliding over imperfections that have the rear bouncing. I'd do it again.