Atlas vibration killer ?

docpez

docpez

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Several years ago I had an Atlas with Commando pistons fitted that vibrated so badly that you could not keep your hands on the handlebars.
So it was quickly sold off.
A few weeks ago I felt Atlas ready again and inclined to spend my last UK Pounds before brexit will loom, and bought a nice shiny 1965 Atlas. The owner claimed a smooth ride. When I got it here in Vienna the first ride really proved a smooth engine, nearly as smooth as my model 99. Then, amongst other things, I dedected that the two long 5/16 bolts which go through the hollow main stand location screws were missing. I duly installed those two bolts and tightened them up to fasten the lower frame rails to the gearbox cradle plates. As the factory intended. Test ride, and guess what ? Bad vibrations above 3 grand. I loosened these two bolts and hey-ho, smooth ride again.
I could not dedect any instabilty in the handling department with the bolts loose, so for me thats the cheapest way of reducing Atlas vibrations.
Would be interesting to hear what the community says upon that bodge.
Johann from Vienna
 
Check for less than perfect alignment of the cradle plates and mounting points and mounting holes. Even small errors in alignment can cause stress when ‘nipped up’ by fixing bolts and lead to vibrations. They also lead to fractures if not rectified.
 
I am the original owner of a 1962 (titled 1963) Atlas. It has never had the center stand bolts.

I have always claimed, this Atlas did not vibrate any worse than any other 650 British vertical twin. It is now fitted with 9:1 CR pistons (original were 7.5:1), and I still claim it does not have excessive vibration.

Perhaps this bike's crankshaft was balanced to a different factor, or more carefully than the typical Atlas. I say this because the bike appears to be a special built factory model. It was dispatched from the factory in May 1962, and came with a 6 start oil pump and con rod squirt holes, which were not factory standard until 1966. It was titled as a 1963, because it did not appear at my dealer until April 1963 .... 11 months to cross the pond?? I rather think Berliner had it for evaluation for most of that time.

A recent year's NOC Roadholder issue, had an article on the "missing" center stand bolts. Apparently, missing bolts is fairly common, either because the factory, or owners found out the vibes were less without them.

Thanks for posting.

Slick
 
Interesting to read that those stand bolts were occasionally omitted. I have two non restored original featherbed twins, both with the stand through bolts in place. A 1961 Model 99, which is balanced to 84% and does not vibrate at all. Then a 1970 Mercury, also with 84%, and smooth like a silk stocking. Then a 1965 N15CS with its Atlas engine with Commando pistons (impossible to source the dished low comp Atlas pistons) and this does also not vibrate in that Matchless chassis.
My 1970 Commando vibrates like mad below 2500 and then easies out. But what a horrible roadholding. Bendy is the right nickname for a Commando.....
 
This is most interesting to me as I am building an Atlas based Domiracer. I had the crank of my 650ss dynamically balanced to 70% & it is a lovely thing to ride at all revs, but after hearing of swollen hands & blurred vision with Atlas vibes I spoke to Steve Maney. He told me his Atlas was balanced to 75% & smooth, so I will try that firstly & if not successful will fit J.S. rods & pistons. The stand bolt isn't fitted so it would seem I'm half way there already.

Martyn.
 
Guess the factor depends if the engine is mounted vertical or inclined. Most racers use inclined engine positions.
 
Matchless said:
This is most interesting to me as I am building an Atlas based Domiracer. I had the crank of my 650ss dynamically balanced to 70% & it is a lovely thing to ride at all revs, but after hearing of swollen hands & blurred vision with Atlas vibes I spoke to Steve Maney. He told me his Atlas was balanced to 75% & smooth, so I will try that firstly & if not successful will fit J.S. rods & pistons. The stand bolt isn't fitted so it would seem I'm half way there already.

Martyn.
Click to expand...

Are those wet or dry balance factor numbers, and with stock-style conrods?
 
FWIW I have a 650ss with a dynamically balanced crank and an Atlas that does not. Where I to do it again I would dynamically balance the Atlas. The 650ss is surprisingly smooth for a 360 degree twin at all rpm up to about 5k. The Atlas has a thick base gasket to lower the compression and a single carb. The Atlas feels "balanced out" between about 3k rpm and 4.2 so is decent at 65 mph. Over that things start to get buzzy. At the end of the day any 360 degree twin without a balance shaft is an inherently unbalanced motor. But the idea is that we like these bikes because they are archaic so it's not really logical to complain about vibes if they are within what was considered acceptable back when.
 
docpez said:
My 1970 Commando vibrates like mad below 2500 and then easies out. But what a horrible roadholding. Bendy is the right nickname for a Commando.....
Click to expand...

You could have several factors making your Commando feel so bad. While they bend, in good shape they were never considered horrible.
 
The balancing factor will have virtually no impact on the amplitudes observed at the handlebar. Various balance factors will shift the rpm band where amplitudes are significant up or down. Requirements will be different for a racing bike than a roadster. The statement "Atlas was balanced to 75% & smooth" is rubbish!

-Knut
 
I Restored My 650SS 10 years ago and have just finished an Atlas both of which are late '66/early'67 models, and both rebuilt back to std specks with GPM pistons, the Atlas has the dished type, neither crank was rebalanced, so far the Atlas does not feel any worse or offensive with vibration.
 
Speaking of Steve Maney.(From above)


Steve Maneys' cool but it appears (from scopeing his pages) that a few major engine components are discontinued. For example cases.
Maybe permanetly?. Dunno
 
If most of what you feel is in the bars, you could try filling the bars with silicon. It helps a little on some bikes. Sometimes very little, and a waste of time.

I don't know squat about the model of Norton you own. All the bolts are tightened up on my solid mounted Atlas 750. I feel high frequency vibration in the foot pegs at various times. Nothing that bothers me in the bars. However, I rode off road 560 and 604 Rotax singles for a few years and may have a different tolerance level for vibration. Plus I'm not particularly sensitive. Ask my wife.
 
mdt-son said:
The balancing factor will have virtually no impact on the amplitudes observed at the handlebar. Various balance factors will shift the rpm band where amplitudes are significant up or down. Requirements will be different for a racing bike than a roadster. The statement "Atlas was balanced to 75% & smooth" is rubbish!

-Knut
Click to expand...

This statement is not rubbish, just poorly explained by myself. When he says smooth he means relative to the majority of road going Atlases used within the normal rev range. In his opinion it is as good as it is possible to make one using standard parts, but will not turn an Atlas into a CBX.

Martyn.
 
All of these old bikes vibrate to some degree. I divide them into two camps- 1.bikes that can be ridden at 70mph all day without really noticing the vibration. My MK3 is good for this as are the Vincent twins.
2. Bikes that are ok for an evening ride at speeds mostly under 60 mph with the odd blurry foray up to 70-75 while passing.
A lot of old British parallel twins are in category two. The owners might claim them smooth, but try putting the thing on a highway at speed for 400 miles per day for six days straight.
The truth will come thru, as will the vibes!

Glen
 
I'll add that my 650 ss is very close too moving from the second group into the first.
A number of changes have improved it. One of the biggest reductions in vibration came from replacing the stock clutch with a lightweight Newby clutch.
Other improvements came from trueing and balancing the wheels and obtaining the correct rubbers and bolts for the fuel tank and seat.
A new seat squab would help.
I can feel vibration coming thru the seat at 4 k revs and up.
When I slide back to the noncompacted pillion part, those vibrations disappear.
Will try it tomorrow with the centrestand bolts slackened off & report back.

Glen
 
Last edited:
Given how remarkably smooth my 920 is with the JS kit I’d love to try a 650 or 750 featherbed Norton with the JS kit. He does extra lightweight kit for these that should work brilliantly in my mind.
 

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