Topic: CB550 fork preload settings, advise please

[Oddjob]

A mate of mine is currently restoring a CB550F, when I say restoring I mean he's doing some work on it but in no way is it going to end up original, he's using an M-Unit for instance for wiring.

I'm currently restoring a CB500K1 (1972 for you yanks as I know you love the year shown) mine is kinda original in that it's all Honda parts but not always off the same model, I bought a written off CB550F2 many years ago and decided I liked certain parts of that much more than the CB500K parts so I fitted what I wanted and TBH it still looked stock. One of the parts I fitted was the CB550F front forks, mainly because it has the twin front disc option, which I fully intended to take advantage of. So like my mate, we both have CB550F front forks.

A couple of years ago not I spotted some nice fork caps, they have adjustable preload rods and an air valve for pressuring the front forks, they didn't list them for the 550 but I asked and they said they'd make me some, I asked my mate and he was interested so we ordered a couple of sets. He's ahead of me in the rebuild stakes so his front end is assembled, one thing that did prove a problem was clearance between the top of the fork cap and the handlebars, in short they hit with no preload selected. The preload rod has 6 grooves on it, (of which only 5 show when the rods fitted, odd) on top setting it compresses the fork spring by 12mm, 15mm if you dial it all the way down but that's not a good option as the rod starts to feel loose. He found his cleared the bars at 3 grooves from the top, which I have measured and that's 6mm of compression.

After searching the forum it looks like 4.5mm is what most members seem to have said works best. However I've also seen posts saying fit a piece of PVC piping 1" long, that's 25mm of compression, surely too much?

So my question is what are your thoughts on the about of preload, I suppose a lot depends on the spring and how worn it may be, so a lot will be trial and error when we finally get the bikes on the road, I do intend to run around 5 psi of air pressure in mine, it seems to be the optimal pressure after some advice from Mark Paris.

[seanbarney41]

Well, if you are attempting to pre-load old, worn out springs (if they are old, they are worn to some degree, at least) than there is no way to accurately compare to other old worn out springs.
Also, high performance aftermarket springs are sold in one length that is shorter than stock so that they may fit many different forks, that is when you may need a longer spacer to achieve the correct pre-load. 
     If your spring rate is right for your weight and riding style (it isn't if you are using original springs, rate is what goes away when they are worn and they were not right when new), than you adjust pre-load to achieve a certain amount of static sag...
Race Tech web site has calculators for some of these settings although it may not give you all the info unless you buy their stuff!
Also, if you are not picky about this stuff, just play around until you get the best from your forks.

[Oddjob]

I was looking for a starting figure, is that 4.5mm or 25mm?

The springs fitted are ok, they haven't done a great deal of mileage as the bike they came off was written off back in the 70s, so I doubt they've done more than 6000 miles all told. I'll measure them and see what they come out as compared to the standard length. I know how to set static sag so that's a good place to start, as and when I assemble the front forks that is, still polishing the fork sliders at the moment.

[calj737]

Preload is NOT about a specific measurement, it’s all about how the front fork reacts to the weight of the rider. Assuming X amount is foolhardy because springs, fork oil weight, and rider behavior and weight are all critical aspects of suspension calculation.

You need to measure the static sag of the bike as a first step. Take a measurement at eh rear of the bike between the centerline of the axle to the top of the shock tower bolt for instance. Record that. Then sit on the bike with full weight. Record that same measurement. For the rear, you want about 3/4” of sag.

Now turn your attention to the front. I use a zip tie on the fork above the lower. Then sit on the bike again and see how far up the forks the zip tie travels. You want the bike to remain “parallel” when sagging and recovering. If you have too much preload in the front, the rear wheel will become unweighted during braking causing traction and handling problems.

You may discover that the stock springs (as mentioned above) and stock oil are pretty pathetic. It is well worth the investment to obtain some new springs wound for your specific weight, use the correct weight and volume of fork oil, then tune your adjusters once you get close to completion.

[Oddjob]

Oddly not a single seller of fork springs for the CB550 in the UK has any available for different rider weights, it's like one size fits all. That includes K-Tech, YSS, Ohlins and Wirth. Some are progressive and some aren't.

TBH I never found the standard springs that bad and I raced a 500 back in the 70/80s, just fiddled with the oil weight and it performed very well considering the abuse it was getting.

I've done static sag on my CB1300, it's set up pretty well considering how heavy the bike is, handles very well. I'm nowhere near to putting the bike on the road as yet, my mate is a lot closer. Mine should be done by mid next year I reckon. I'll tackle the problem then, I thought some advise for my mate might help him dial his in and I can copy his initial setup and alter it to suit once the bikes on the road.

I do ride with a pillion a lot BTW so I'm guessing a compromise between the 2 sag readings is needed, I'll be using either a set of Koni shocks or my newly refurbished Marzocchi AG Strada shocks. Probably the Stradas, they look nicer 

[calj737]

It doesn’t surprise that you can’t find single rate springs for the 500. They’re 40+ years old and not really a market for them. Most folks use progressive springs and run spacers because they have to settle for subpar handling and braking. That, or they don’t know enough to set a bike up properly.

[Oddjob]

Then I might as well test the old springs first and see how they perform, IIRC the 550 used progressive springs from the factory. It's a simple job to change springs, Ill try 10w first and see how that goes.

[HondaMan]

Back in their day (1970s/80s) we would routinely preload the 500/550 forks with short pieces of PVC and a big washer on the bottom of it (so the spring wouldn't eat at the PVC). The {American?] 500 front springs lost 50% of the travel for a 150-lb rider just sitting in the bike, and the rear shocks also would frequently bottom out, causing issues when carrying loads of 2 people. In short, it was just too softly sprung, IMHO.

If the PVC piece exceeded 1.25" length then the springs could bind completely when hitting larger bumps, like railroad crossings or potholes. That was how I ended up adding air valves into the fork caps. The best approach turned out to be 1 tire valve connected to a tee that fed both forks, for equal force. This also made it easier to get a desired pressure, as 5-10 PSI isn't but a sneeze into the valve!

The easiest way to make the air forks was to have the air valve off to one side with a tee to the hoses, although cosmetically this stinks. I have alternately made brackets for the valve, installed it into the back side of the headlight bucket with very small hoses exiting the wiring hole (dubious installation, easily cracked, but he wanted it that way) and made mounts across the larger 8mm fork pinch bolts to hold the valve (again with a tee to the forks). While I haven't looked lately, there are also 45-degree and 90-degree threaded tire valves (look for big truck trailer parts) made for remote mounts, and those could work well, too.