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Not WD related... A BSA Plunger fame query.

Hello guys and sorry for the unrelated question, but...

I also ride an M21 plunger, with Ian's big bore.
She really accelerates great and pulls like a sports bike,
Well, compared with my WM20S...
But when I hit about 75-80 Km/h, it begins to be a bit scary...

Every small crack of small depression in the road sends the bike to a small "Belly dance",
and the rear springs lounges me sky high.

The tire pressure is correct, there is no slack to the steering head,
(Closed bearings) the rims are straight and balanced, and there is no "pull" to either side.
At lower speeds I can leave the handlebar and steer with my body easily.

I do have a Suzuki front end, as that what I could find in a decent nick,
(And it is a project bike, from the 60's so I did not care about authenticity)
But the fork has all new oil seals, and the oil type and quantity according to specs of the bike it came from.

It looks like the problem comes from the rear plunger.
All of the plunger parts were cleaned with kerosene and greased before installing,
But it does miss the felt washers on top, as I did not have them when assembling.

Any thought? lower tire pressure?
I am starting to think of changing the frame into rigid...

Thanks and sorry for the unrelated subject,
But could not think of a better place to ask.


Re: Not WD related... A BSA Plunger fame query.

Ah, the dear old plunger... ...The last two I had I converted to rigid by fitting accurately machined aluminium spacers in place of the springs...

You need to take some initial measurements with the weight of the bike on the suspension to determine where the casting that carries the wheel should be positioned (vertically)..

Then strip everything down and accurately measure the components to calculate the exact lengths for the upper and lower spacers...At the same time check the amount of play in the slider bushes and replace them if badly worn...

Advantages are no more 'wobbles' on fast bumpy bends, no lubrication and wear issues once completed and no more stretching of the rear chain on bumpy roads that results from the compromised geometry of the rigid set up...Ian

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Re: Not WD related... A BSA Plunger fame query.

Thanks a lot Ian !

I understand that the idea is to to turn the "Plunger" into "Rigid" frame,
So would what I described, be a normal behavior for the rigid frame?
Is generally, the rigid more stable?

And last question, would relapsing the back chassis into a rigid one,
Also be do the same trick, I guess?
(Though I must say I like your idea better:
Just thinking about taking the engine, gear, clutch, all transfer system and all
Only to be put back on another frame, made me prefer the bikes "Belly dancing" so far...)

Regarding the drive chain tension-
What I did to eliminate that, was to give the chain some slack,
And install a cheap "Universal chain tensioner" (eBay) which eliminated the sound
From the drive chain when driving, and enabled it to tension when the spring compresses.

Thanks again!


Re: Not WD related... A BSA Plunger fame query.

The rigid frame isn't inclined to 'wallow' on faster bumpy bends like the plunger does...

The plunger, you will note, has a very strong wheel spindle...This was to keep the two plungers in line with each other when moving up and down...

However, the two sides do not actually move in unison causing the wheel to tilt slightly momentarily. When this action is repeated on bumpy bends this gives the 'wobbly' feel you describe as 'belly dancing'...

The rigid set up eliminates this, though at the cost of the loss of the suspension movement the plunger set up has (which isn't much)...Ian

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Re: Not WD related... A BSA Plunger fame query.

Hi Noam,

Fitting forks from another bike can open up a whole lot of questions about how well they match the original forks. is the wheel trail and fork offset similar to the original forks? Are the springs and valves correct for the bike and rider? Small changes can have a big influence on how the front wheel tracks and behaves over rough surfaces.

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Re: Not WD related... A BSA Plunger fame query.

Many thanks for your inputs, Ian and Matthew.

If this is the case, I think I would take your advice, Ian,
And replace the springs with spacer for now,
And on the same time have a rigid frame blasted and powder coated,
In order to replace it one day, when I have time for it. which is not in the horizon...Hahaha...

Matthew, I do agree with your comment,
But as the bike, when I got it, was an "Easy rider" copy,
Withe the down pipe of the frame (from under the steering head down to the engine)
Cut and replaced with one about 30" longer,
And the front fork was made of VERY long 2 Harley fork hydraulic pistons
Welded together through a 6" pipe (2 on hydraulic pistons on each side, I mean)
In order to gain a compensation of length needed,
As the steering head was much higher due to the change in the chassis,
And, of course, much higher forks angle, When I changed the front half of the chassis
To a standard one, I had to replace the fork,
And I figured that instead of replacing it with an old and worn BSA front fork
Which is spring-based,
I will benefit from fitting a more modern Suzuki fork which has also oil damping action and a front disk brake.
In my case, however, I do feel that the wobble effect comes from the back.


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