Steering problem

Fulvia-hf

Good to see this forum back! It was sorely missed.

My 1989 Ducati 851 monoposto Lucchineli tribute build is getting to the final stage; the fine tuning of all modifications. The engine is superb. Cilinderheadtuning performed by Toine vd Biggelaar and the Eprom adapted to suit got an increase of 15Hp to 102 Bhp.
The only problem I need some guidance for is the steering. I fitted 3.75x17 front and 6.25'x17 Marvic penta magnesium wheels. As the 851 has the narrow rear swingarm I was limited in tyre size. Following the advice of a formal 'Battle of the Twins' race series rider I fitted a 180/60ZR17 to the rear rim and a stock size 120/70Zr17 to the front. The curvature of the rear tire seemed ok even though you could see the sidewalls were vertical. The steering of the bike is odd. It seems the front is falling into the corner and you try to catch it by counter steering. Then the front tucks in again ending in a turn existing of small loops. I have no other way to describe it.
To compensate for the taller rear tire I reduced the amount the front forks protrude the top yoke. I ended up with just 10mm. I replaced the steering head bearings to make sure they were not causing this strange behavior but they make no difference.
I measured the width of the fitted 180 tyre and measured 194mm. This meant the tyre is stretched a lot over the wide rim. This seemed not well so i decided to fit a 190/55ZR17 tyre (same as before diablo rosso II) This tyre measures fitted 190mm so seems ok for the rim size.
The strange steering is now a lot less strange but the rear does still not inspire confidence.

Can somebody help me with some advise? Is there way to tune the rear suspension using the adjustable linkage?

marvin

I had a problem with my '92 851 falling suddenly when leaned over. (180/55/17 rear, 120/70/17 front)

I made 27mm offset triples like the Corse bikes (30mm is OEM) and the problem went away.

Fulvia-hf

Just came back from a 100Km ride. It seems the wider 190 tyre is a definite improvement. The odd steering is not gone but much much less than it was before.
What is the stock protrusion of the (M1R) fork legs through the top yoke?

I guess by having fitted the larger section rear tyre (190/55 compared to 180/55) and the raising of the front I raised the CG somewhat. It turns in without much effort but it feels a bit as if the diameter of the rear wheel is huge when you flick it through some twisty bends.

Cheers,
william

JSR29

Taken in isolation your description of the handling characteristics describe a setup with too little trail. However as you've changed many things away from stock and provided no info on rear geometry it's difficult to make any specific recommendations.
I'll give it a try anyway😁
Since the ship of the extra wide rims and big tires has sailed the best thing you can do is start from stock geometry settings both front and rear. Fork height, rocker setting, rod end length. Set the sag appropriately. Then reduce the rear ride height (ride height, not sag)5mm to compensate for the big rear tire. At that point you'll be starting from stock geometrywise. As Marvin points out the bikes benefit from less offset (more trail) particularly if they are Corse frames with steeper rake or the back end jacked up which has the same effect. Bearing in mind that the front cylinder head can have a say in how much you can reduce the offset.

JSR29

Also bear in mind that a 180/60 tire, regardless of the designated size is actually quite a bit bigger (height and width) than most 190/55 so it's not a surprise the bike feels better on the 190

Fulvia-hf

Thanks a lot for these usefull tips.

But your remark on the 180/60 surprised me. Is this tyre fysically bigger as the noted size?
Why would this be?

How can I reduce the rear ride height? The balljoints on the hoop are screwed in completely already.
The protrusion of the front fork tubes is now 10mm were I believe (seen on period brochures) over 20mm is stock. This would restore trial a bit AFAIK.

Cheers,
william

JSR29

Yes. That's the thing with the 180/60. You measured it yourself 194. I'm advocating that you return the bike to completely stock geometry front and rear and work from there. And since the front end, with the exception of the slightly wider rim, is stock sized, leave that alone and concentrate on changing what you can on the end of the bike that is most changed from stock.
I say this based on my experience with my racebike with stock sized rims, stock cast rocker, stock triples, 120/70 and 180/55 tires. Handles well enough for me to win a few AHRMA races over the last few years. Now, this is a later bike with Showa forks but I think the actual stock geometry is the same. Even if it's not, I'd still recommend starting at stock and adjusting to compensate for the changes in the rear at the rear.

econti

I have the same sort of issue except I have standard size tyres.
Rear ride height is not wound the whole way low but fairly close, front forks are maybe 10mm from the triples.
I'm not too well versed in bike geometry (more race cars), what would be recommended? Drop the back further? Or drop the forks to be level with the triples?

marvin

econti wroteI have the same sort of issue except I have standard size tyres. Rear ride height is not wound the whole way low but fairly close, front forks are maybe 10mm from the triples. I'm not too well versed in bike geometry (more race cars), what would be recommended? Drop the back further? Or drop the forks to be level with the triples?

I don't know enough about steering geometry to give you any help. But I do know that my friend Rick has a 996 track bike. Like almost everyone else he jacked the rear up as far as he could and dropped the forks in the triples to get it to turn in more quickly. At the track he let a well known AMA racer take it out. When he got back the guy said,,,,,"yes, it's nice but lower the rear a bunch. If you ever get into trouble you will never be able to save it." FWIW

griff851

Perhaps it easier to think about it like this. If the steering head moves towards vertical the front end wants to become a unicycle. A chopper on the other hand being the opposite is like an ocean liner due to the large angle. If you raise the rear, the steering head angle is changing from a chopper to a unicycle.
The front end is similar, raise the forks is moving towards unicycle but this time it’s shortening the wheel base, thus changing the steering head angle as well.
The trick is setting up for what you want.
Chicken strips can help. The ideal set up is zero on the back and a some on the front. In theory the back will wash out before the front giving you a fighting chance.5-10mm is pretty good for most on the road. Racing, well 5-10mm is a waste of tire. So you tune a little tighter.
But then comes the road or track. Very few tight corners go long. Forks level and rear low. This gives the longest wheel base and the largest angle between the steering head and the bitumen and hence more stability at high speed.
Tight corners need quicker steering as does direction changing when chasing bike 42. But there is a point where the bike will feel like it wants to tip in too easily. You’ll have quick steering, but as Marvin mentioned if anything goes wrong, your going down.

Each rider is different in the way they like the bike to feel. But looking at chicken strips will help you set up for you. I personally think with lift extenders on the rear and forks 20mm I’d pile in. I have 10 at the front and about 12-15mm of thread at the rear, on tight slow mountain corners it just feels a little tippy.