350 Ducati suspension help

[Duc1]

Hi, I have recently finished a narrow case Ducati 350 single for classic racing, the motor is great but the handling is far from ideal. The rear has been fitted with new Ikon shocks as recommended by them for this bike. The front 32mm Marzochi forks have been stripped, new seals, re-hardchromed and filled with 150mm of 15wt oil. The head bearings have been replaced with tapered rollers. The two noticeable problems are:-

1 The bike tries to run wide over the hill at Pukekohe and onto the back straight.
2 The bigger problem is braking into the hairpin at Pukekohe it feels like on a modern bike when you grab the front brake too hard and the back goes light and starts waving around. I dont believe the back is lifting as it only has drum brakes which have not fully bedded in yet. The same sensation happens braking into turn 1 at Hampton Downs. Friends on the pit wall can see this.
I have checked swingarm bush's and pin (all new) wheel bearings and frame for true. I am running different tires to what i usually use these are front Dunlop kr 825 (very low profile) and rear KR 124.
Any ideas on what to do next? tnhanks in advance
Terry

[Blackflagged]

Re 2 , How much static sag/ sag have you got on the rear. Topping out can give those symptoms ( Braking for hairpin).

[Duc1]

Okay, without me on the bike the back does not appear to sag, if i sit on the bike it now sags about 18mm. The front forks appear to sag 25mm without me on it. If the rear shocks are topping out could it mean the front is to soft creating excess dive? and as a result the rear pivots in an upward direction?

[Blackflagged]

If you haven`t already.Lift the rear of the bike (take weight off),with out you on it. Should settle, around 3-6mm,when you drop it back to ground.If not adjust preload lock nuts till it does.With some some rear static sag may be useable.The Hard rear/ soft front may be causing the vague feeling out of corners. Are you on/off the power,when your running wide?.You need to cut some tube , fit on top of the springs and get the front sag about same as rear, maybe a little more on the front. After you have done both ends check the bike with you on it. Should use aprox 25 - 30% of total travel both ends. After that you can try dropping front forks though the yokes, a very little at a time on practice day , and see how it turns.Take notes.Theres also damping but.....

"If you want to go further" Crown Kiwi , could setup up and supply fork springs.You may want to fit some fork emulators in the front also,they will help the front end dive.
Go the Ducati`s!

If you get really pissed off with it, i`ll take it off your hands :-)

[Duc1]

1st off thanks for the advise Blackflagged very much appreciated.
I have the rear ikon shocks on there lowest preload setting, only 4 preload positions to choose from, and still cannot achieve the 3- 6mm sag, so i guess the spring is still to hard?.
When the bike runs wide i am on the power ie over the hill at Pukekohe or onto the back straight.
I removed the cap off the top of the forks and there is a 65mm preload tube present. i will make some more tubes and increase this length in say 5mm increments and will check this at the next track day i can get to. do you have a link on the web to find out about these fork emulators?.

The odd thing is i also race a Benelli 250 with the same forks and similar rear shocks and this handles sweet as? Although it has Avon tires and i guess the weight distribution could be quite different.

Once i can get this sorted its going to be a neat bike, so i cant give it to you yet blackflagged..........

[Robert Taylor]

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1st off thanks for the advise Blackflagged very much appreciated.
I have the rear ikon shocks on there lowest preload setting, only 4 preload positions to choose from, and still cannot achieve the 3- 6mm sag, so i guess the spring is still to hard?.
When the bike runs wide i am on the power ie over the hill at Pukekohe or onto the back straight.
I removed the cap off the top of the forks and there is a 65mm preload tube present. i will make some more tubes and increase this length in say 5mm increments and will check this at the next track day i can get to. do you have a link on the web to find out about these fork emulators?.

The odd thing is i also race a Benelli 250 with the same forks and similar rear shocks and this handles sweet as? Although it has Avon tires and i guess the weight distribution could be quite different.

Once i can get this sorted its going to be a neat bike, so i cant give it to you yet blackflagged..........

Note that Ikon shocks are only single acting, they have next to no low speed compression damping and instead rely on a heavily preloaded spring to give a crude form of ride height control. They then require a fairly agressive rebound damping curve that happily can be arranged by a form of fixed orifice damping. That is why such shocks are cheap and their ultimate performance is somewhat limited

You have geometry issues and acceleration squat issues causing you to run wide off the turns. It is also clear that you have excessive forward pitch issues under brakes, in turn causing the rear end to go light under brakes. This issue is more about damping control ( controlling rate of change of position ) and emulators that are suitably pre-specd will make a huge difference. Its very easy to think its all about spring rate but its not, people often overspring or overpreload to crudely compensate for lack of damping control. Dont be tempted to purchase any of the mainland Asian ripoffs of the genuine Race Tech emulators. The Asian ones are a piece of junk.

When I have more time ( next week ) I will be able to elaborate further as required.

[Duc1]

Hello Robert

Ive done a bit of reading on these emulators and taken on board what your saying. As a result i would like to progress towards obtaining some suitable emulators. So what information is required to obtain the correct one's ? looking at the RT website it would appear that I will have to modify the top of the damper rod to fit. The forks are 32mm Marzochi 1974 vintage. The fork spring dimensions are 25.4mm eternal diameter 16.9mm internal diameter, the top of the damper rod being 24.3mm that the spring sits on. i can supply a detailed drawing if needed.

[Brian d'marge]

I had one off those things back in the day , Engine tuned by witty ducati drums with standard shoes ( I am doing this from memory ) and the 19 inch front wheel

it had typical old style Ducati handling ( something around 27 degrees , Cant remember )

on the sweepers , felt planted but in the twistys it wouldn't change direction easily

I remember that the frame was the one to change if you went racing to a cradle also as Robert pointed out the forks are a bit crude
The engine is lovely , I prefeered the 250 as it was smoother than the 3, or 450 and had similar top speeds

Stephen

[Paul in NZ]

I'd also check the tyre size / profile (once you have the suspension sussed). The temptation of going too wide has ruined many an old bike, sometimes low profile tyres don't have a decent radius to allow a smooth transition (only speaking from aggresive road experience here on heavy dungers - I'm not road racer material).

[Blackflagged]

The Emulator, needs to be smaller than the inside of the fork tube.The same as the external size of spring would be good.Have a look at the Race Tech site( That`s what you mean by RT)?, as it sounds like you will need springs also.Yes you will more than likely need to get some adaptor made for between, the Emulator and damper rod.I think Marzocchi`s of that era have some extra valves etc in them stock. Just contact Robert direct.

ps: there`s another thread on Emulators in this section somewhere.