Sorting USDs

Upside down forks have been around for a while now. If like me you'd been slightly left behind by the latest developments in fork technology, i.e. fitting the buggers the wrong way up, you might like me be interested to see what goes on inside these examples of high-tech state of the art suspension thingies. So when my mate Chris decided he had to fit new bushes and seals to the forks on his Honda CRM250 enduro bike we decided to turn it into a bit of a party, and have some fun exploring his very expensive bits. If you get my drift.

So why make forks upside down in the first place? Well, one of the traditional problems with telescopic forks is flex. Take any length of metal pipe, hold one end steady and apply a load to the other end and it will flex. Not much, perhaps, but enough to make your bike wobble around in corners if you have forks made from long, thin bits of pipe. And after all, forks are by nescessity long, thin things. Of course the bigger diameter tube you make the forks from, then the less flex you get. Which is why manufacturers kept increasing fork tube diameters on big bikes as designs progressed from the seventies into the eighties.

Big butch sexy gold anodised Upside Down forks with macho tough stanchion covers with Dunlop written all over them. It seems strange that these monsters are holding up the front of a 250cc two-stroke single. Lets rip them to bits and see what's inside...

Start by unscrewing the top nut off the outer tube. The easiest way to hold the fork leg while you undo the top nut is to clamp it back into the yokes. With the top nut undone and the outer tube slid down out of the way you should be able to get a spanner onto the flats above the spring retainer. Use this to hold the damper steady while you undo the top nut from the damper assembly.

Now you can access this inner steel spacer which rides above the fork spring. To release it push the spacer down to compress the spring and slide out the split reatining washer. Make sure you watch for any spacers or washers and keep everything in oreder so you know how to put it back together. Note also that we need three hands already...

I'd like to think that one day in the early nineties some Japanese clone factory worker at Showa had a moment of revelation on the production line, and went rushing into the design office with the idea of bolting the fork legs into the yokes the wrong way up. This would, of course, result in the big diameter alloy bit we'd always called a slider getting bolted solidly into the yokes while the flexy steel bit we'd always called a stanchion now got stuck out the bottom. Cos the big bits now at the top we can only get flex at the bottom where its easier to control. Make the stanchion out of real big section tubing and here's your revolution in front suspension design guaranteed to make last season's sports bikes look instantly dated and shift more new product off the showroom floor. Oops, did marketing creep into a technical feature there? Sorry folks. Of course Upside Downies also have the heavy bit of the fork at the bottom rather than the top, which in technical terms lowers the centre of gravity and makes your bike go faster. Or something.

Turning your attention to the other end of the fork, slide the dust seal out of the way and pop the oil seal retaining circlip out with a small screwdriver.

Now the scary bit. The lower bush is a press fit into the outer tube, and will have to be driven out. Take hold of the two fork tubes with each hand and whack them apart, just as if you were using a slide hammer. You may have to get a bit rough arsed here but trust me, if you've undone the top nut and removed the circlip in the last step, the fork will separate on the fourth or fifth whack. Careful when it comes apart; you'll need to note the position of all the bits to make sure you can reassemble everything in the same order.

With the fork leg separated you can spring the old lower bush apart and slide it off the fork tube, followed by the upper bush and fork seal. Lubricate the lip on the new fork seal, slide the new bits into place and it's ready to go back together. We used five hands for this step, you could probably manage with three.

We've had Upside Downies around long enough now to have realised that they don't nescessarily represent the quantum leap in suspension design we'd been promised, and while they do tend to run a slight advantage in terms of stability and control on pure balls-out sports bikes or crossers, for most of us in the real world the main difference it makes having USDs on your bike is cosmetic. I've heard rumours of USDs wearing out bushes, and even heavy wear on stanchions, at remarkably low mileages. The shop that sold Chris the bushes for the forks in the pictures told him they were good for 120 hours use. Which doesn't sound like much to me - some mistake surely?

Now the party really starts. Fit the two halves of the fork together, then to engage the lower bush into the outer tube you'll need to spring it together and, at the same time, drive it squarely into the fork tube. This will require the attention of at least four blokes, three to attack the fork with a mixture of screwdrivers and hammers and the fourth to look on and issue helpful advice. It also helps if one bloke is at least eight feet tall. Take care here as it's easy to mark the stanchion surface; with a little patience it's not too difficult a job.

With the lower bush engaged and driven home, grease the outside of the fork seal and press it home. It should go most of the way under hand pressure, but to drive it all the way in we cut a section off the old seal and used it as a drift. When it's all the way in re-fit the circlip, then the dust cover.

Now with the fork stood upright you can add the correct quantity of super expensive racing fork oil (or super cheap automatic transmiission fluid if you're a cheapskate like me), before re-fitting the top nut and bolting everything back into the bike.

In real life we found it easier to change the bushes and seals on these forks than on conventional teles. For a start we didn't need any special tools to stop the internal dampers rotating, a problem you'll frequently encounter on older designs. The difficult bit was, as expected, engaging the lower bush into the upper tube. This has to be sprung together and driven home squarely without damaging the stanchion surface, an operation requiring at least three hands. No problem, as Chris managed to muster about six for this job. Unfortunatley they all came with brains attached, each of which sought to offer conflicting advice throughout the procedure. Chris proved himself equal to the task however; ignored everyone and got on with the job. And to his credit, not only do his forks wobble and leak no more, as you read this he's belting round sunny Spain on his annual enduro holiday on the CRM. Sorted.

Dr.Rod.