MDPE Swing Arm Bushes?

[Elitepete]

Hi all, has anyone used or considered using MDPE [Polybushes or similar] for swing arm bushes? If so were they any good? Considering swing arm vertical movement will only be about 10 - 15 degrees it is not that much different to how much a car wishbone would move, and I would have thought they wouldn't allow any horizontal movement to the swing arm.

[graeme]

My old 1980 SS black and gold, had swing arm bushes manufactured from a stuff called Lubron .
I did 40k kms on that bike and never had a handling issue or detected any movement.
Never greased
No doubt there is better material now.

Graeme

[Jon Pegler]

Nylon swinging arm bushes were used as standard on most Spanish narrowcase Ducati singles.
They appear to have survived intact in all the Spanish singles that I have worked on, with no obvious lubrication.
I have fitted grease nipples to some Spanish swinging arms, but I am not sure that they are really necessary.
They seem to get a bit brittle with age, so tend to break up somewhat on removal.
They also need removing if a swinging arm is to be powder coated or stove enamelled, as the heat used in curing will melt them.
Replacements are readily available in Spain, cheaper than the Italian bronze alternatives.

Jon

[Duccout]

Hi Jon,

I asked the question of lubrication when I dismantled the swinging arm of my Strada; I always fit grease nipples to Ducati swinging arms, but was not sure that it was necessary for plastic bushes. In the end I just coated the pin in white grease and didn't bother with grease nipples.


AFAIK, the parallel twin type frame bushes are not available, but Delrin, which is an industrial plastic used for many of these applications, is easily machined and is probably not much softer than phosphorous bronze, with the added advantage that it cannot seize like bronze. I don't know what size the O/D should be to give an interference fit without closing up too much and being too small for the pin to fit, but it would be worth trying in place of the bronze bushes.

There are advertisements on eBay by machine shops who will turn up bushes in Delrin to your requirements, so if you know the O/D that you require, this may be worth trying.

[Jon Pegler]

Colin,

I have replaced Forza swinging arm bushes in the past, and they are the same size as Stradas.
I chose to fit sintered bronze bushes because they are fairly cheap and easily available.
It did involve machining them to size and reaming them to 28mm bore once pressed into the swinging arm.
A grease nipple was fitted centrally on the underside of the swinging arm as the clearance on the Forza/Strada frame is even tighter than on the Italian widecase frames.

As you say, it would be easy enough to machine up some Delrin to size, although there is not a great saving in cost as the bronze bushes are cheap.

Jon

[Duccout]

I have wondered if Darmah swinging arm bushes would fit Forzas and Stradas? I believe the bore is the same (28 mm), but I don't know whether the O/D is the same, or the length, although that wouldn't be a problem.

I'll have to ask Jesus Guzman if he has any plans to have a batch of bushes made.

[Jordan]

Bronze is good, but I'm not too keen on using sintered in this application.
Its porosity makes it much weaker than solid metal.

The bushes should be reamed after installing to the swing arm, but if done by hand (most likely!) do it with an expandable reamer and stop before it seems a nice easy fit. Rather, leave it a bit tight with the spindle in the bushes. They will soon bed into size in use.
Reason is that it's almost impossible to achieve a truly round hole by hand reaming.
If you go for a "nice" fit, it will soon be "worn out".

[Duccout]

The problem with reaming is why I would do anything to avoid replacing bronze swinging arm bearings, always trying to get away with a new pin and copious amounts of grease. Years ago, before I knew this, I replaced the bushes in my 750 swinging arm, but then could not find anybody to line ream them. Eventually, Steve Wynne's machinist did them for me, poorly, and they had more play than the ones that I removed. I believe that Ducati used a line-honing machine. If I have to replace bushes again I will make up a needle roller conversion.