Fork Set Up
RD forks are very low tech
They have no adjustable damping, no adjustable preload, but they can be tweaked to give a super ride. There's probably 2 approaches to take, the low cost easy way or the loadsa money hi tech way. I'll describe both as I see them and leave you to choose how far you want to go!
Method 1 - EASY
Basically with this method, you spend as little as possible on different grades of fork oil and hope the standard spring rate (15lbf) is enough for you. By tweaking preload and damping you can get a fairly good ride from the RD forks. You want to set the forks up with about 1" of static sag. This is to say, when you are sat on the bike, the forks use up about 1" of their 4" travel. You do this by using more or less preload spacers. In the UK, I found adding 2 pence pieces worked really well. If when you get off the bike the forks top out, you probably are of larger build and the springs have too low a rate and we are compensating with too much preload, but as this is the cheapo method, we ignore that and work around it with the fork oil.
The way I see it is, you never have a bike without a rider, so setting ride height without a rider is just a waste of time. 10wt is the standard oil. This gives a soft plush ride, so if you ride your RD hard (like it should be!) you may find your forks lacking. Using the same manufacturers oil (this is very important - never mix manufacturer - a 10wt from Silkolene may not the same as a 10wt from Red Line!) you can get inbetween grades by mixing proportions of oil. e.g. a 50/50 mix of 10wt and 20wt will give a 15wt oil. If you find 10wt too soft, try the recommended 145cc of 20wt. If this is too much, start blending lighter grades till you hit the spot. Remember we talk about adjusting the spring rate using oil? To adjust this you need to reduce the air gap left at the top of the forks. As the forks compress, this air gets compress also. Air has it's own spring rate. The less air the higher the spring rate. So to increase our overall rate, we must add oil to reduce the air gap. Be careful with this! Too much oil and you could end up being squirted in the eye as an oil seal gives out under pressure. Add 10cc at a time and try some hard braking until you are satisfied with the setup.
Method 2 - Hold your breath !!
With this you need to spend more money, you could spend loads, but we'll be realistic about it. This time we add in the spring rate to the equation and change fork springs to get this rate right. It is quite easy to get hold of heavy duty fork springs to fit in RDs. One popular make is Progressive. These are on average 10% stiffer than standard springs. At about £40 a go it's not something we want to change too often!
So on to the principles of the "hi tech" setup! If you find you are having to preload your springs by more than a few inches (that does not neccassarly mean the length of the actual spacer) to get you 1" sag, your spring rate is likely to be too soft. There are two methods we can acomplish this: 1) Buy a new spring. 2) Shorten the spring we have. Pro's and con's of each - 1) can work out expensive if we try too many springs, but it's easy to go back a step. 2) costs nothing but time, but if you go to far you can't put bits back on. You may also shorten the available travel if you hack too much off.
Ok, how to buy a spring is obvious, how do you shorten one? Well quite easy really, you need a vice, a saw or grinder, some pliers and a butane torch. Holding the spring in the vice, cut off the required length from the end with least windings on using a saw or grinder. Exactly how much, I leave to you the reader. Now you need to flatten the end over. Heat the last 2 coils with the torch until they glow red, then using the pliers, squash the end down. Use the grinder or a file to create a flat to the end of the spring. Hey presto! A shorter, stiffer spring !
If you have used method 2) you will a longer spacer to get your preload back in line. Buying a spring aga is pot luck, if it's the same length as stock, you are ok, other wise, you need to re-setup the preload spacer. Basically repeat this process until you are happy with the spring rate and sag. Tip: ride new springs around for a few miles to allow them to settle.
Now we move on to the damping. A heavier spring will in general require more damping to control. Try the 20wt again, or even 30wt. Adjust as before. As start to use heavier oils though, we run into limitations. The heavier an oil weight, the more prone it is to frothing and to fade as the oil heats up. We can use lighter oils by adapting the damping rods. One method is to weld up two of the four damping holes. The other is to use cartridge emulators such as the ones made by Racetech. These are reputed to be well worth the money and function exceedingly well. The suspension will work alot better a be more adjustable with the lighter oils. Again you can fine tune spring rate with the oil level.
Real Case Scenario
Riding the RD around London in heavy traffic started to show up the limitations of the sagging old springs. Dabbing the front brake sent the front end diving into the tarmac and set the front wheel wagging. Ok on an open road, but with cars an inch either side of the handlebars, not easy to control! Even with 20wt oil, the springs were showing they were tired and need to be replaced. I duly purchased Progressive's heavy duty springs for the RD. Progressive recommend you leave out the preload spacers when installing their springs on an RD. No shit Sherlock! With over 4" of spring sitting above the top of the fork tube, there is no way the spacer will go back in. It was a real effort to refit the fork caps without stripping the threads!I found that despite having 20wt oil, the forks would top out over ripples and bumps. The springs had way too much preload on them and something had to be done! Out came the angle grinder and butane torch and I lopped 2" off the springs. I wasn't concerned about the rise in rate as a little more wouldn't go amiss. Since putting the springs in they had settled a couple of inches so I now had the springs sitting flush with the top of the fork tubes. I put the caps back on and the stood the bike up, the springs held its weight. When I sat on it however, the forks dipped over half their travel! (what was I saying about bikes never go anywhere without a rider?!) More preload was needed. A 1" spacer was cut from alloy tubing and put in the forks. Much better! The bike handled superbly, nicely controlled under heavy braking and very stable around corners (even embarassing a few larger bikes around Donnington Park !)