Suspension upgrades, 2008 Mk 1

[zone5]

Hi all, although generally happy with the comfort and handling of my newly acquired, 45,000 km Mk 1, as is my habit I have embarked on some suspension mods. I though I might document what I do as it may be of assistance to others.

I am generally happy with only minor engine mods (sometimes none) to the bikes I buy, but do like to chase the best comfort and handling. I ride at fairly brisk pace on “often repaired” rural roads, so that sets a solid challenge for OEM suspension on most bikes. The main issue I felt with my bike was being a little under-sprung front and rear (hardly breaking news to anyone here..) and the damping having faded away on the rear shock.

I have now modded the front. Now, as a rule, I am not a fan of progressively wound springs (I’ll put a footnote below with my reasons) so initially was considering the 0.85 kg/mm linear Ohlins springs. However, I could only obtain them from overseas, with 50% of the value of the purchase in shipping and a longer wait. YSS Australia had a suitable alternative available, the YSS progressive 0.75/0.90 kg/mm, available straight away. As this was only a small of progressiveness I decided to give it a try.

Forks came off as per manual, oil was a bit dirty but I’ve seen worse. Pleased to see it was a cartridge fork. Just for interest, I measured maximum fork travel to be exactly 110 mm, not 120 mm.

The OEM spring had not sagged at all, and was right on spec @ 358mm long. The YSS was only 350 mm. First issue (one of the difficulties of a progressive spring), was how much to preload..? Not in instructions.. After much head scratching and some calculations based on assumptions about the “average” rate, I decided to make new spacers (pvc) 112 mm long to replace the OEM 100 mm long ones, then added a 3 mm thick washer for a total of 15 mm extra length.  The fork caps then sat 35 mm above the top of the tubes before being screwed in. Assuming an average spring rate of approx 0.85kg/mm in the early compression (shrug) that’s roughly 30 kg of preload for each side. It felt like about 15 kg of force to get the threads started (carefully!) so seemed about right…

I used the Honda recommended oil height of 129 mm, which seemed a little high, and used 10W. Everything felt good until the latter part of the stroke, where I feel it firmed up a bit too much on the big hits. I was only using just  over 100 mm of travel, based on my cable tie, on a really big bump at speed. IMHO the air gap/air spring was too small and thus hard, and also possibly there was a bit much preload. I removed 15 ml of oil each side, (very roughly 1 ml = 1 mm of air gap for that diameter fork tube) and removed the 3 mm thick washer that was under the fork cap, and it is now really good.  The aforementioned big bump uses 108 mm of my available 110 mm of travel, fulfilling my general rule that you should very nearly bottom your suspension on the biggest bump at the highest speed you’re likely to deal with. Rider sag is now 30-35 mm, a good number for 110 mm of travel.

Next is a Nitron R3 rear shock, which is on order, and I should be able to report on that in about a month. Expensive, yes, but the bike itself wasn’t, and I’ve decided to sell the Kawasaki Z900RS Cafe, so the bikky jar should stretch to it. I’m really liking this bike, and want it spot on for comfort and handling.

Below is comparison of YSS and OEM spring. Similar wire but OEM has more coils and closer spacing, clearly a lesser rate. YSS is almost a “dual rate” rather than progressive, with the low rate section being small, IMHO good!

[zone5]

The reason I’m not a fan of progressive springs.

The concept of progressiveness in spring action is fine, but winding springs that way is basing assumptions on static spring deflection (compression), not the sole factor in our suspension. Motorcycles hitting bumps is a dynamic event, with the moving, unsprung components such as tyres, wheels, brakes, swingarm and fork sliders suddenly acquiring velocity and thus kinetic energy. One of the main functions of your springs is to absorb that energy in the best way possible (assisted by compression damping), so then the rebound damping can then dissipate even more of the energy, without too much disturbance to the rest of the bike.

In the same physics lesson that described the basic Force/deflection equation F = kx (for a linear wound spring), we would have also learned that a spring absorbs energy in a squared relationship, E = 1/2 kx^2. So this means it takes four times as much energy to compress your springs twice as much… So, in terms of energy absorption, a linear spring is already steeply progressive. And remember, it’s the energy punched into your very suddenly accelerated suspension components by a bump when riding that compresses your springs further than the rider sag, not a static load sitting on them in your shed.

So yeah, if you start winding springs progressively, you add progressiveness on top of progressiveness, and can end up with the suspension falling thru a too soft initial wind under static load, then leaving too little travel (with too high a rate) to deal comfortably with big hits on the road. Look at the pic above, I think this is exactly the situation on the OEM fork spring out of the CBF1000. I went thru this issue modifying suspension in the 1990’s, when progressive springs were all the rage, once I figured it out I went back to linear winds, which clearly gave a smoother, better handling ride over the travel. Looking back, I’m not sure that even the spring manufactures could exactly figure the rate over the useful travel of their product, yet still seemed to offer a progressive spring for just about any motorcycle…. Hmmm….

Anyway, that’s my take on it, I know many others will say they’ve had improvement with progressive springs and I don’t doubt that’s true, but when you change springs you change many other things as well, particularly overall rate, so it may be hard to break out the definitive reason for an improvement. Nonetheless, I will admit my “gently” progressive YSS springs have improved things a lot, after tweaking other things as well.

Just my $0.02. Worth what it cost you.

chrs, jc

[Art]

Happy days

There has been suggestion on here in the past to stiffen the front end by means of adding an extra spacer and washer, from memory something like plus 1" was the favoured mod. There was much interest at the time, how many carried out the mod, their reasons and results were never clear. For my two penneth on acquiring my SC58 I felt the front end was a little soft, keeping it simple I replaced the fork oil as per the Honda specification but using a 15W oil instead of the recommended 10W, that worked for me.

The rear shock absorber has always felt right on the 4th notch whether solo riding or fully loaded on tour.

I've recently added 1L of fork oil on the shed shelf, the idea is to a get a round tuit before Big Ben rings in the New Year. Not because the fork oil needs replacing, the plan is to strip the front end for a hands on inspection, greasing and possibly replacing the headstock bearings. While I've got the forks off they'll be stripped to inspect their internal components for wear. I might even give the fork legs a lick of paint.

[zone5]

Thanks for that info Art.  I did consider just upping the preload as some have done, but for the AU$200 thought installing the correct rate springs was the go, given I was pulling the forks down anyway.

Update on the oil level, I just found the screenshot I took of the YSS specifications web page (that I took when I purchased them), and it actually does state to use an oil level of 140 mm. This info not included in the box.. I just forgot all about that when installing, but am glad that my trial and error finally gave good results, landing on a near identical value.

It also suggests that YSS have done some genuine testing and are offering good tech advice.

Hanging out for my Nitron R3 to round out the update, particularly the ability to adjust the high speed compression damping, important for ironing out the big bumps.

chrs, jc