P11 Taming Vibration

Joined
Oct 7, 2005
Messages
1,170
Knut:

I never got to visit the Plumstead Road facility while at N-V. I heard some odd tales about the condition of their machinery back in '66, but the tales may have referred to machinery moved there from Bracebridge Street when AMC took over Norton. I think the muti-millionaire that resurrected Norton (he already owned Villiers) would have put a load of cash into AMC to get better quality work, but I don't know for sure.

Communication between top management and lower levels was marginal and the top man at Wolverhampton was a former Rolls-Royce Aero Engine manager whose expertise was in the chemistry of kerosene combustion. Very pleasant man, but no familiarity with motor-cycles. I don't remember seeing any technical correspondence between Wolverhampton and Plumstead while I was there.

For a few months at the start of my N-V job, we were living in Kenilworth, about 45 miles from Wolverhampton. I was loaned a rather tired 650SS as a commuter and got very familiar with the vibrations of the parallel twin rigidly fastened to a Featherbed frame!
 

elefantrider

VIP MEMBER
Joined
Apr 6, 2013
Messages
969
Country flag
Last edited:
Joined
Jan 19, 2012
Messages
1,052
Country flag
Which style is correct for P11?

None. As delivered from the factory, the P11A and P11 Ranger didn't have foot peg rubbers for the rider (and neither had the G/N15CS). The pillion footrests had rubbers (NM16249 in the case of P11A/Ranger). Real men cope without rubber!! :)

-Knut
 

elefantrider

VIP MEMBER
Joined
Apr 6, 2013
Messages
969
Country flag
Hey that is great if you have wood feet.
That explains why the rubbers don't fit correctly.
The vibes puts my feet asleep at long distances 60mph or above.
Vibes on handlebars are a lot more tolerable, and actually a lot less than my XR650R.
Any idea what the RPM is at 60 to 65MPH? I don't run a tach on this bike.
(I like to jog/run also, so I value my feet).
 

texasSlick

VIP MEMBER
Joined
Jan 2, 2013
Messages
2,970
Country flag
Any idea what the RPM is at 60 to 65MPH? I don't run a tach on this bike.

My Atlas has overall top gear ratio of 4.53:1. With a 21 tooth engine sprocket, it gets 17.5 mph per 1000 rpm.

With a 20 tooth engine sprocket, it would get 16.7 mph per 1000 rpm.

With a 22 tooth engine sprocket, it would get 18.3 mph per 1000 rpm.

The AMC clutch has 42 teeth.

With this information, you should be able to estimate the mph per 1K rpm for your bike. Rear tire diameter will affect the calculations, but these numbers are typical for a 4:00/18 rear tire.

Slick

FWIW: the vibes on my Atlas are quite tame under 3500 rpm. The vibes start coming on 4000
 
Joined
Jan 19, 2012
Messages
1,052
Country flag
Hey that is great if you have wood feet.
The vibes puts my feet asleep at long distances 60mph or above.

Long distances on a P11? I would say the obvious remedy is to select a different kind of bike. A Commando would be preferable. Do foot peg rubbers really shield vibrations that much? If you are bound to using a "hybrid" (esp. P11 and G/N15CS) for long trips, I suggest you become inventive and create rubber-mounted footrests. Inspiration may come from the 1976 T160 Cardinal and T160 Legend.

-Knut
 

elefantrider

VIP MEMBER
Joined
Apr 6, 2013
Messages
969
Country flag
Depends what type of long distance. I don't think anyone rides their P11 really long distance anymore. I have ridden mine about 100 miles in the last week or so, with a few long 65-70 mph straights. Most of our local club day rides are between 50-150 miles RT. I think foot rubber does help cut down vibrations a little.
With dished 7.5:1 pistons, vibes are not that bad but definitely noticeable. I hear with flat top pistons, vibes increase a lot.
 
Last edited:

xbacksideslider

VIP MEMBER
Joined
Aug 19, 2010
Messages
1,920
Country flag
You might try the lead weights that Suzuki puts in the ends of the handlebars. My ‘86 GSXR and 2002 Bandit 1209 both benefit from those weights.

It Might also be possible to engineer some kind of rubber mounting for the foot pegs. Many Japanese bikes came with rubber bushings that the foot peg forging was mounted upon
 
Joined
Oct 28, 2014
Messages
962
Country flag
My Atlas had Commando pistons and the vibration was ferocious. I could feel the numbness creeping up from my hands to the forearms etc. At 5000 it was unbearable. So I had the crank balanced to 64% installed a Boyer and a single 34 Mikuni. Now it is very pleasant to ride and my main bike. I would do this first rather than a fancy crank - rods - pistons.
 
Joined
Nov 10, 2012
Messages
2,951
Country flag
Correct. Vibration is chiefly a result of acceleration of masses, in this case pistons and rods. However, the crankshaft flexes under load and will contribute to vibration as well.



A 270 degree crankshafts is state of the art engineering, but should be complemented by other measures, such as lighter pistons. I'd really like to try out the balancer devised by Doug Hele. Too bad AMC didn't have the time and money to develop the bike fully.

Agreed, the Norton has a lovely sound, but if I can buy myself an enjoyable ride, I am prepared to give away the sound experience. A badly balanced (err, stock) P11 is not an enjoyable ride, IMHO. Maybe they were better when new.

-Knut
P11 has much less vibrations than a A65!.
 
Joined
Oct 15, 2011
Messages
222
Country flag
My A65 Lightning had relatively low vibration I thought this was due to the roughly square
Bore x stroke of 74 x 75mm rather than the longer stroke Norton 750cc of 73 x 89mm ?
 
Joined
Apr 20, 2011
Messages
4,893
Country flag
P11 has much less vibrations than a A65!.

Only if you stick to the 7:5 to 1 pistons. as soon as you put the higher compression Commando pistons in make an appointment with your dentist for new fillings!
 
Joined
Nov 10, 2012
Messages
2,951
Country flag
Only if you stick to the 7:5 to 1 pistons. as soon as you put the higher compression Commando pistons in make an appointment with your dentist for new fillings!
I have the flat commando pistons +0,20 and i dont feel more vibration than Commando
 
Joined
Jan 21, 2015
Messages
123
Country flag
My P11A has had flat top Commando pistons since the engine was rebuilt in the 80’s, will rattle the fillings out of your teeth at highway speeds. I’m using NOS 7.5:1 pistons in the Ranger I’m doing now, I’ll see how much difference it makes once it’s all together. Stay tuned!
 
Joined
Dec 23, 2010
Messages
695
Country flag
My A65 Lightning had relatively low vibration I thought this was due to the roughly square
Bore x stroke of 74 x 75mm rather than the longer stroke Norton 750cc of 73 x 89mm ?

I once bought a 1969 A65 Lightning which had the original BSA pistons & the crankcases had never been apart. It vibrated so badly at anything over 4500rpm that it was painful. I pulled it apart & sent the crank to SRM for dynamic balancing. They balanced it to 68% & it was like a different bike. This to me shows how important getting the correct balance factor is.
 
Joined
Oct 23, 2012
Messages
1,001
Country flag
Do foot peg rubbers really shield vibrations that much? If you are bound to using a "hybrid" (esp. P11 and G/N15CS) for long trips, I suggest you become inventive and create rubber-mounted footrests. Inspiration may come from the 1976 T160 Cardinal and T160 Legend.

-Knut

late T160s had the rubber mounted footrests, Les Williams Legends didn't, they were all fitted with their own design of rear sets.
 

p400

VIP MEMBER
Joined
Jul 14, 2014
Messages
1,154
Country flag
The P11 tested by Hot Rod Magazine, July 1967, had foot peg rubbers fitted.
I do not see foot rubber fitted for other period magazine tests.
I recommend foot rubbers on P11.


HRMpg66TopView.jpg
 
Joined
Mar 7, 2020
Messages
1,021
Country flag
I owned an early high pipe model for 10 years and never found it over bearing in standard tune, but coming from a BSA single that might account for it!
Wouldn't call it a motorway cruiser, but it was never designed for the pavement. Once up to 70-80mph those bars would put the wind in your chest and the front end would go light. Just the bikes way of saying: 'I wasn't built for this'
 

p400

VIP MEMBER
Joined
Jul 14, 2014
Messages
1,154
Country flag
I put thousands of back road miles on my P11 after a conversion to JS Motorsports rods, pistons, thick lo comp headgsket.
As a daily ride it was very satisfying, avg 150 mile lunch run.
My legs gave out and I have moved on to a Mk3 Interstate....with just slightly less vibes.
20151015_P11InTheClouds.jpg
 
Joined
Apr 22, 2020
Messages
200
Country flag
This response is going to be way off topic before I get done.

I use those football shaped grips you were using. They are a minimum of 30 years old. I like the way they feel more than any grip I have ever used on any bike I've owned. My P11 only vibrates sitting still idling waiting for the light to change. Lightened and balanced rotating assembly helped some. I doubt I ever rode it without the rubber on the foot pegs. That would probably be annoying unless I was climbing the widow maker.

Off Topic:
Where are the attachment points for a chain guard on a P11? I've never had a chain guard. May end up making a short one, but wonder how the original chain guard was attached. I did a search for pics, and couldn't find any closeups on the interweb.
 
Top