Paso Tyres – the 16” wheel conundrum
I put this article together for the Ducati Owners Club GB, but thought I should also share it here.
Several years ago, long after the short-lived craze for 16” wheels had been forgotten, Pirelli quietly announced that they were to cease making the odd-sized MP7 tyres once fitted as standard to the 750 and 906 Paso. Along with first series of 750 Sports (belt driven) the 851 Tricolor, and the Bimota DB1, these were the only bikes ever to use the low profile 130/60 front and 160/60 rear tyre sizes affected. That reduced the choice of tyre supplier to one. Michelin.
That was OK until March 2006, when Michelin decided that it wasn’t worth making the A/M59 in this size any more, and ceased production too. Until this time, the only problem was that of limited availability, but at least you could get them…eventually. Now even that avenue has been closed. The seemingly simple option of fitting 17” wheels, or different profile 16” tyres, is stymied because of the sheer lack of room to fit anything else into the standard Paso swinging arm, which is blessed with a meaty brace right across the front section.
There have been several people who have advocated, and pursued, the obvious course of action, which is to convert to 17” wheels. However, this is not without its problems, not the least of which is cost. The simple solution is to graft in the rear end from a 907 or later 851, but the swinging arm, brakes, spindle sizes, chain adjusters, and everything else aft of the swinging arm spindle are all different, and the cost of buying new parts would be well in excess of £1,000 before you even think about the tyre itself…or the front end. Even going down the route of second hand bits would be pretty expensive.
The front end would also need new brakes, as the old single piston callipers would be incompatible with the bigger rotors used on 17 inchers, plus maybe even forks to keep things simple. The cost spirals, and eventually becomes completely uneconomic. Inevitably, many owners faced with a simple lack of tyres took the option of selling the bike, or even breaking it for spares, rather than shelling out more than the bike was worth to keep it running.
My own view, being of the skinflint variety, was to look for a solution that involved keeping as much of the original Paso as possible. After several months of thought, a plot was hatched, then enacted, and the results tested using a number of different tyres and sizes, and which I can now confidently attest to working. The cost would be a couple of hundred pounds, with minimal alteration to the basic structure of the machine, and could be easily converted back to standard if ever some decent tyres became available (some chance now eh!?).
The front end is dead easy, and can be dealt with on its own with very little effort. The saviour here is Honda’s decision to get clever several years ago, and equip the early Fireblades with a taller version of the 16” front tyre, in 130/70 size. The idea was to give the same overall dimensions as a 17” equivalent but with the quicker steering supposedly attributable to the 16” wheel. The extra 10% height on this aspect ratio gives a virtually identical overall rolling radius to a 120/70 tyre size on a 17” rim.
The only work needed to the Paso front end is to raise the front mudguard by about ½ “ (13mm) in order to clear the tyre. This is simply achieved by using longer bolts to hold the mudguard mount/fork brace to the top of the fork sliders, with ½” spacers under the mudguard.
The result is a much wider choice of 16” rubber, including some grippy “track formula” compounds in a commonly available size (thank you Honda!).
I tried this initially retaining the standard size rear Michelin, and using a Bridgestone Battleax front. The difference in handling is immediately apparent, with easier steering into corners, holding a line better, and no running wide on corner exit symptomatic of the Michelin 130/60. It is also very much less prone to stand up and make for the hedge when you are forced into braking mid-corner. Stability at speed is completely unaffected, and you can scrape footrests with confidence.
When I couldn’t get hold of a 130/70 front, I also tried a 120/80 Avon AV27, with excellent results. The only drawback to this option is that the slightly taller profile leads to the tyre just (and I mean “only just”) kissing against the front of the lower fairing under heavy braking. Once a few hundred miles had been covered, even the characteristic hiss of the tyre contacting the fibreglass disappeared as the tread had worn enough for the required clearance to re-appear. I stress here that this only happened under extremely heavy braking, though, and in every other respect, this tyre size produced brilliant results and a completely dependable front end.
Essentially, the front end problem is solved both cheaply and easily by using either tyre size, but the ready supply of 130/70 front tyres, and the complete lack of any negative issues makes this the most favourable option.
The rear is the thorny problem, and required most thought and effort. A second hand swinging arm was purchased, and taken to Spondon Engineering, in Derby to be modified. The swinging arm is cut just at the point wher the welded on eccentric chain adjusters stop it tapering outwards, and from where the faces become parallel. My calculations had indicated that a 150/80 or 160/80 tyre would need around 1-1.1/4” of extra room, so it would need around 3” of extension in order to accommodate a larger tyre and leave room for chain adjustment, but I asked them for a bit more to be safe. What I got was a 6” chain adjuster extension, using one of their “drag bike” drawbolt style adjusters. That gave me more than enough room to manoeuvre!
File comment: The original chain adjusters gave plenty of room to work with.
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The standard wheel, brakes, sprocket etc are all retained, with Spondon providing a replacement spindle, drawbolts and other minor requirements. Because the wheel sits about an inch further back, the standard caliper carrier does not engage with the lug welded on to the top of the swinging arm to prevent it rotating. Accordingly, I had to make up a stop to fit, which I did using a section of 1” square alloy, with a suitable size bolt to replace the lug. This was then attached to the existing boss atop the swinging arm using a section of 90 degree angled alloy. The only drawback to this is that the standard silencer fouled the bolt, so judicious application of a 2lb lump hammer to the back of the silencer created a recess which is unnoticeable from anywhere except within the rear wheel.
The centrestand was modified to include an extra inch of length, so that the rear wheel stood clear of the ground when parked, but I didn’t change the sidestand. It just leans a bit further. Also, as the rear mudguard would have fouled the bigger tyre, I created a little extra room by cutting the mudguard (which is only plastic) from the direction of the wheel centre out towards the angle at the top of the number plate. The whole rear section can then be bent out slightly, and I used a couple of small alloy plates pop riveted to the mudguard sides to keep the position. This was then covered over with a small section of rubber glued to the outside of the mudguard to hide the spacer (in fact I used a piece of old inner tube – the right colour, flexible and waterproof).
Finally, to get rolling again, a new piece of brake pipe was required, as my old braided line wouldn’t stretch the extra inch, and a new chain with five extra links took up the required length in the drive train.
So now everything was ready for the acid test of the new rear tyre. The first thing to be aware of is that an 80 section tyre is really designed for a narrower rim, and so it needs to be “compressed” with a “tourniquet” to get the beads on to the rim and allow inflation. Get it done by a proper fitter as you’ll never handle it on your own. Or use a tube.
Initially, I tried a 160/80 Avon Azarro, in conjunction with the 120/80 front. This was immediately found to be a big mistake. The profile was all wrong, and it never gave any confidence in the back end. Just looking at the tyre in situ it seemed way to tall, and I think that the overall geometry was just too extreme. Even when switching the front end back to a 130/70 (Dunlop D207) the feeling of uneasiness didn’t disappear, so as soon as practical, it was changed again, this time for a 150/80 Dunlop D205. Now this is the business!
The slightly smaller tyre when matched to the front gives absolutely superb results. Even when the D207 was replaced with the later D208 at the sharp end, the two work outstandingly well together. I suppose this should be expected of properly matched tyres, but I have experimented with combinations of (F & R respectively) Bridgestone/Michelin, Avon/Michelin, Avon/Avon, Dunlop/Avon, Dunlop/Dunlop, and several of these have proved to be excellent combinations.
The Avon 120/80 front and standard Michelin 160/60 rear was fantastic, with an absolutely planted feel to the front end that made you feel you could put the bike anywhere, and the rest would follow. The combination of the two “matched” Avons (120/80 and 160/80) however, proved to be a disaster, inspiring no confidence at all.
Overall, though, I think that I have found the right balance with the pair of Dunlops (D208 front in 130/70 size and D205 rear in 150/80). Indeed, at the DOC’s Cadwell track day a couple of years ago I took great delight in riding inside, outside (and any other side I could find) of SS’s, 916 types, and sundry Japanese iron. During the whole day, I only got passed by about five bikes (GSXR1000, Hyabusa, Blade and a couple of 916 variants) and they all got me down the straights. Bum. OK, so I did bin it later, but that was my own silly fault getting too enthusiastic on the throttle chasing an ST4. Should have let a few squinches out of the tyres before going out. Around the corners, though, the bike was a match for virtually anything else out on the track in my session.
The work on the swinging arm can be done by Spondon, at a cost of £100-200, and the remaining work (other than welding pieces in the centrestand) can all be accomplished by your average “home mechanic” using nothing more than hacksaws, drills and a bit if ingenuity. In practice, a 3” chain adjuster slot will be more than adequate. The first (Mk1) adjuster was much longer than needed, and I was thinking of shortening it anyway when it cracked at the weld, so I got Spondon to fix it with a shorter Mk2 chain adjuster block, which looks much neater.
File comment: The later, shorter, neater item.
newbits (1k).jpg [ 96.29 KiB | Viewed 1087 times ]
File comment: The mk 2 version on the bike.
sw-arm mk2-med.jpg [ 149.34 KiB | Viewed 1087 times ]
To conclude, there is a way of keeping your Paso on the road without going to the lengths of changing all the running gear to accommodate 17” wheels, and it can be done at a reasonable cost too. Getting hold of a second hand swinging arm to modify would be the ideal way to do it, so that if you ever wanted to (I don’t think so!) you could convert back to standard easily. The bike does not look significantly different to its standard incarnation, but it will handle better, and you will be able to find replacement tyres from a number of manufacturers in a commonly available size.
Should you need any further information, or help, just ask. firstname.lastname@example.org
More photos of the project can be seen here - http://s350.photobucket.com/user/spaggy ... so%20Tyres
Spaggy. March 2006.
PS – Replacement original size tyres are now available from Golden Tyres and Shinko. This might well be a good bet for the rear but I’d still stick with a 130/70 on the front as the handling is far better for it.