851 A WORK IN PROGRESS 5 – Bits Other Than The Engine

Next up was some suspension mods. I had replaced the std Marzocchi Duo Shock with an Ohlins when I was reassembling the bike from bits after I had the frame painted, just on principle that the Marzocchi would be crap. However, I had never really played with either it or the std Marzocchi M1R front end. These front ends have the little adjustable rebound-damping valve on the front of the right leg. I had been told that these have way too much rebound damping on any other setting than the least (1?), and found this out with a pretty wild, off a bump, second gear tank slapper one night.

With the motor as responsive as it now is, hitting it hard in second will almost, but not quite, lift the front. With too much rebound damping however (and too much rear spring preload), this will unload the front just enough to get a wobble up. I had also raised the rear of the bike about 8 mm the night before, which wouldn’t have helped.

When it happened, it took me very much by surprise (I was on my way home from work, riding in a straight line), but it didn’t last too long or upset the bike. Much like an unexpected slap across the face, it really woke me up. I learnt a few years ago that the best response to a tank slapper is to relax and let the front end sort itself out, which it did in no time. Just shocked me at the time.

Add to this the fact that the front end had never felt that nice, and I was looking for some changes. I was toying with the idea of fitting some cartridges from a later, USD fork, but this would involve lots of machining to just get them to fit, then modifying whatever cartridges I got to suit the 851. Time and expertise I don’t really have.

I then tried the sag test, to see if the springs themselves were an issue. The table below shows what I got, compared to what I wanted to see. For your info, I’m currently about 85 kg, and the sag is in mm.

Load Front end under weight of bike Front end under weight of bike + Brad Rear end under weight of bike Rear end under weight of bike + Brad
Desired Sag 10 — 15 30 — 35 10 — 15 30 - 35
"As was" Sag 35 45 5 25
Final Sag 21 34 7 35

As you can see, the back had too little sag under the bikes own weight, but dropped about the right amount due to my weight, meaning the spring should be ok. The front, however, was just a bit wacky. I checked the front springs, which turned out to be progressive. I would imagine the preload on the front was too little also, adding to the initial drop. The final results show the sag under the bike’s weight only (not hugely important) to be a little off on both from what is accepted as the desired figure. But the sag with me on the bike – the most important figure – was good at both ends, so I was quite happy. For a much better explanation of this concept and procedure there of, check out:


This article also includes a consideration for stiction, which the M1R front end has an absolute shit load of (amazed me, actually), compared to the later Showa and Marzocchi USD forks I normally deal with. In reality, I’d be far better off ditching the M1Rs for something better, like the latest USD Kayabas with 25mm damping cartridges, as suggested by a local specialist, but I’m trying not to get that carried away. Did I really say that?

The solution to the front end problems was new constant rate springs with the correct preload and some internal mods. Because these old style, right way up forks have no internal valving cartridge, and not being that much more sophisticated than an old ‘75 SS is, there really isn’t much you can do without changing things substantially. The LH leg on the M1R forks has only a compression damping cone fitted, which is a mechanical way of increasing the compression damping as the forks get closer to bottoming out. The RH leg has both compression and rebound valving, and the rebound adjuster. So it got nearly all the attention.

They also only have 100mm of travel, whereas most modern forks have around 120 – 130mm. Which reduces the sag settings aimed for a little compared to forks with more travel. As long as the “bike with rider” sag is within the range of 25 – 33% of total travel, that’s good. So, I put the bike on a stand, pulled the forks and dropped them off at a local suspension specialist. Not that I wouldn’t mind being a local suspension specialist, apart from the fact I don’t have anywhere near the time to become one. It’s one of those things that’s really worth farming out to those who know it well.

The damping circuits in the RH leg were modified, with holes filled and drilled as required - the external rebound adjuster becoming redundant in the process. Then a Gold Valve Emulator was fitted - this takes care of the compression damping side of things. For rebound, a new sliding valve assembly was machined up and fitted to the RH leg.

The initial result was a much plusher ride around town. However, on the twisty roads I like to ride on I found it very bouncy over bumps, with quite severe bottoming out and pattering under brakes. It just felt under damped to me and too soft in general. The man who did the work felt there were two reasons for these problems:

  1. My braking package (see below) was a bit too savage.
  2. I was expecting too much from an outdated front end design.

Both of which I can accept to some extent (they really aren’t flash forks). He suggested a couple of things, but was convinced I wasn’t going to get what I wanted. I wasn’t prepared to live with it as was though. So, I figured I’d have a bit of a ring round (like you do) to see what some other people thought. The table below shows the different set ups recommended, and my trial and final settings.

  Gold Valve preload Spring rate
Spring preload
Oil weight Oil height
Initial Setting 1 1/2 turns .85 22 5 140
Alternative Recom. 2 1/2 turns .90 - .95 As required 15 130
Revision 1 2 turns .90 20 15 130
Revision 2 2 turns .90 20 15 LH, 10 RH 130

The "Gold Valve preload" refers to the loading of the spring on the gold valve that gives the high speed compression damping adjustment. Basically a small spring that controls the movement of a plate off the seat of a port in the valve.

Gold Valves

Given the rebound damping valving in the RH leg had been modified, I was a bit concerned about how increasing the oil weight would affect the rebound damping. I had serviced a ’90 900SS the week before (with non adj. M1R front end), and had swapped the initially filled 10 weight oil in the RH leg with 5 weight, just to reduce the excessive rebound damping. So I was hoping I could run 15 weight oil with this new rebound valve. There was another internal mod the second specialist had suggested, but I thought I’d try the basics first before I got too tricky.

One of the variables you can see is oil height. This is the height the oil is filled to, with the fork fully compressed without springs or preload spacers fitted. What this does is set the amount of free air volume in the fork once the springs, etc and fitted and the caps screwed on. As the forks compress, this free volume reduces, meaning the air in the free volume when the fork was assembled is now compressed somewhat. This increased air pressure inside the fork acts as an extra spring, which is progressive in nature with fork travel (compression). Increasing the height decreases the free volume of air inside the forks, leading to a higher compression ratio. Which leads to more “spring at full travel” or bottom out resistance. Good for the brake induced bottoming out problem. It has no effect at all on damping, either rebound or compression, which is why it confuses some people.

So, I removed the 0.85 kg/mm rate springs initially fitted and swapped them for some 0.90 kg/mm rate springs. I then removed the Gold Valve and added _ turn of preload for a total of 2 turns, then refilled the forks with 15 wt oil to 130mm. Quite a bit of change in spec, really. I was a bit apprehensive, as, frankly, I was getting a bit over pulling the front end apart and didn’t really want to have to do it again (not having external knobs to twiddle can be a real pain sometimes). But, on the ride home from the shop, it felt not that different compared to the initial setup. The heavier oil certainly hadn’t increased the rebound much at all - bouncing the front end with the brakes on in the workshop had shown me this, much to my surprise. And the heavier springs and increased compression damping hadn’t made any dramatic difference to the plush feel over Melbourne’s often crappy suburban road surfaces. I ended up running over all the bumps and pot holes, etc, I could see just to convince myself that the fork action was really as nice as it was. A situation similar to that on the first ride after fitting the Ohlins to the front of the BMW R1100S, and just as smile inducing. I arrived home much less stressed than when I left work 15 minutes earlier.

The real acid test, so to speak, came the next weekend with a ride over the same roads I had ridden a couple of weeks previously, where the problems with the initial setup had shown themselves. Of course, I had broken the golden rule – make only one change at a time – but was curious to see what would happen. The bottoming out under brakes was gone, replaced with a nice controlled feeling that was still using nearly all of the travel, but without the thumping. So that was good. The bouncing through corners was also gone, so good again. I had introduced another couple of problems, however. It would now tankslap quite badly coming out of bumpy corners, and was quite harsh on really hard-edged bumps.

For really hard-edged bumps, the centre section of the Black Spur is an ideal place to test for this. With some of the crappiest tar surfacing I have come across (the first and last ‘thirds’ are far better), the run back down the hill really shows up poor front end behavior, especially under brakes. So, I assumed that, even though the front still felt quite nice over the sort of hard edged bumps around town, the 15wt oil and increased Gold Valve preload had increased the high speed compression damping, showing up when really put to the test. Also, given the tank slapping under acceleration, I’d expect the 15wt oil had also increased the high speed rebound damping a little too much. I could fit a steering damper, but I’ve never really liked the way a tight damper feels in day to day use. Plus there’s the issue of actually fitting one, often not an easy thing to do on a fully faired bike like the 851. I do have some small BMW R1100R ones laying around at work, plus a leaky one from the 900M, so maybe I’ll try one of them in the future.

As both the compression (Gold Valve) and rebound damping are housed in the RH leg (the LH has only the compression cone), I changed the 15wt oil in the RH leg for 10wt. Then we were off to try again. I have to admit, I was getting a little sick of “trying it again” by this stage. But then, that’s half the fun isn’t it?

This second revision went quite a long way to addressing the issues my first revision caused, without returning the initial problems, so I was happy to leave it there for the moment. Maybe when it comes to summer and I can ride the bike regularly on the rides I like I’ll play a bit more.

So, I now have a nicer front end. Not too hard, nor too soft. Firm, but plush is the best way to describe it. It seems to turn in nicer than I remember it, although it was seven months since I had ridden it. Suspension is funny stuff, but the best way to tell if it’s any good is to try to remember how it feels. Bad suspension you notice straight away. Good suspension you don’t notice. Really good suspension is quite sublime. These days, it’s the sort of thing best left to people who specialise and know what they are doing. I’d love to learn a whole lot more and do it myself, but I like engines more, and, more importantly, just don’t have that many hours in my day.

As for the rear, the first step was to reduce the spring preload. I ended up taking 2 turns out of the spring preload (increasing the fitted length from 174 to 177mm) using the lock rings to get the desired result, and am much happier for it. I only had to do this once, which was a real bonus. You need to get the shock out to make this change if you want to have any hope of not damaging the lockrings or the frame. And, on an ’89 851, that means the subframe has to be pivoted down, the breather valve/hose removed and the entire exhaust system removed to get the lower bolt out. Not a quick or happy job.

Some damping adjustments have finished it off. The fact that this is a single seat bike, with only my arse to contend with, makes it much easier to set up. Although, the damping settings I decided on for the shock surprised me a bit. Both the compression and rebound adjusters are nearly all the way in (hard), meaning the shim stack bypass passages are nearly completely closed off, and the shim stacks are doing all the work. Not that there is anything wrong with that, and I remember a comment from one of the HRT V8 Supercar suspension guys (a friend of one of our parts guys) who was surprised that the std settings for the Ohlins shock were around 15 clicks out. He expected them to be much closer to zero, based on the fact that this works the shim stack in the way it is designed to work, and not letting the oil bypass it. I guess I just assumed (not necessarily correctly) that the ‘as delivered’ settings would be about right.

The changes I made settled the back down greatly, and are similar to the changes I made to my Guzzi Sport 1100i. On that bike, I had originally thought there was too much damping, causing harshness and lots of bouncy bouncy, but it turned out to be too little, giving little resistance to the spring’s movement. More compression damping on the rear made a noticeable difference to the way the Guzzi drove out of corners too, but I didn’t really notice this with the 851. Although, I’m probably not riding the 851 as hard as I rode the Guzzi just yet, as that was pre crash when I had much more confidence in front end grip. The Guzzi’s shaft drive/swingarm arrangement may have had something to do with this also.

This suspension checking and setting procedure is the sort of thing any serious rider (and not just sports riders) should be doing to set their bike up to suit themselves. Far too many owners see suspension as an untouchable area, so just ignore it. I’ve been truly amazed at some of the things I’ve seen people riding over the years, just because they won’t play and experiment. For bikes that see lots of unusual riding – say two up, etc - that moves the suspension out of the range it was designed for, modifying to suit you and your needs can make a big difference. Both to enjoyment and safety.

Just a side note here – constant versus progressive rate springs. Those who sell either will claim theirs are better – really, I don’t know. The results of this job were very nice, but I also fitted the 888 with some Hyperpro progressive springs for a very nice result. Really, I think it comes down to a job done right being a job done right using the chosen method, and there is more than one way to do it.

Next was a brake upgrade of sorts. I had fitted 996 discs after the wheels were painted, as I used the original discs from this bike to replace some undersize discs on another bike I needed to sell. Very convenient. I was using EBC HH pads, which I have used successfully before in quite a few bikes, but they never seemed to have the bite I expected. I was talking one day to ex mechanic Mick about it, and he asked “Has it got the small master cylinder”. “Dunno” I replied, with a ‘why didn’t I think of that’ look on my face. Since we were standing near the bike at the time, we went and had a look.

The diameter of the master cylinder bore is stamped on the lever pivot boss generally – this one said 15mm, whereas the later ones are 16mm. The calipers on mine are the older, non-differential piston diameter ones too, with all pistons being 32mm, as opposed to 30/34 for the later calipers. The actual piston area for both styles of caliper is actually very much the same, so the master cylinder requirement should be no different. I had a later, black, remote reservoir master lying around that originally came off the Guzzi Sport 1100i and was on the KR1S for a while. This was 16mm, so on it went. Of course, it isn’t correct for the bike, but most of the front brake system now isn’t anyway.

Another problem is that the EBC pads like a nice bed in. The first set I had in the R1100S got about 3 seconds to bed in, as I went charging out the lane way and down the hill in the side street towards the lights after work the night I fitted them. Not really perceiving how much I actually rely on the brakes in normal use (quite a lot it turns out), the usual stop for the traffic lights turned into panic when the thing wouldn’t. After a big hard pull on the lever it did stop, but the tradeoff was squealing brakes from then on. Didn’t effect the way they worked though – it stopped great. The 851, however, didn’t stop great. I wasn’t sure if the pads had anything to do with it, so I pulled them out, gave them a quick rub and then stuck them back in. I was gentle with the brakes for a while, as re-doing the bed in procedure can help the HH pads.

So, with a 1mm bigger master cylinder and renewed pads, we were in business. Much better brakes, with far more urgency to the way it stopped, just like I expected it to have. Except for the fork dive.

The next item for attention was a rather dim headlight. The headlights the 851 run are the same as that in the 888 and ‘91-’98 900SS. Which are usually pretty good, even with the std 55/60W globes. I had tried one of the new style HID “white” globes without any real improvement, and then decided I was out of ideas. So, instead of thinking too hard about it, I did what I often do when I have an electrical problem that has me flummoxed - I rang City Auto Electrics. The ‘Hillbillies’ are an institution of sorts in the Melbourne bike scene, as are their Christmas parties, and are always responsive and helpful to questions over the phone. Their suggestion – as I should have thought myself, considering this is a late ‘80s Ducati, was voltage drop or bad earth.

That night I pulled out the wiring diagram, and then the fairings off the bike. I found that, with the ignition on, but the bike not running, the battery had around 12V, while the headlight globe had 8V. Quite a difference. I discovered the advantages of checking wiring looms under load with the 888 starter draw problem, so went over the 851 loom in the same fashion. Just about every lead/connector/switch in the circuit I managed to check had around 150 – 250 mV loss, which added up to the 4V difference. I did manage to find an extra volt with some general cleaning/fiddling, but the light circuit – battery to fusebox, fusebox to ignition switch, ignition switch to fusebox, fusebox to light switch, light switch to headlight plus several connectors – had just a few too many components. And a lot of wire. The power goes from the middle of the bike to the tail, then to the front, back to the tail, then back to the front for the switch and finally headlight. I got to the point where I still had a variation of 2.8V from battery to headlight globe, and had to admit defeat. The only serious option was to disassemble every connector, clean with flux and solder each crimp joint and then reassemble. Which I may very well end up doing at some stage of owning this bike, but it was getting late and I’d rather have been home with Hunnybunny and Fat Cat, so I decided that I would fit relays at some stage in the future to the headlight circuit, and go home.

“Fitting relays” means running a power wire directly from the battery to a relay (an electrically activated switch) near the headlight. The original wire going to the headlight globe is then used as the switch wire to turn the relay on, and a wire from the relay to the headlight globe delivers battery power to the globe, bypassing the wiring loom. It’s something we have done for many years with charging systems – Pasos especially – where the charge voltage is too high due to a low ‘reference’ voltage at the regulator, once again due to voltage drop through an old wiring loom.

The extra volt I found that initial night did make a noticeable difference to the headlights output (there was now a visibly lit patch on the road ahead), but with the relays fitted – one for low beam, one for high – the difference was, almost literally, night and day. Much better, and far safer when riding without the aid of streetlights.

So, a whole report with no engine work or graphs. Not too bad at all. The bike is much nicer to ride now – fast or slow - due to the upgrades and fiddling with suspension, brakes and the headlight/wiring mods. It pretty much takes anything in its stride, even commuting. I do like riding it very much. In its current state, the next thing for some work is me. Since my ‘big crash’ I’ve been riding like a bit of a nana, so maybe it’s time for the next session of rider training. Or just some good track days to get my confidence back.

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