Strange phenomenon

Hi all,
I've been meaning to post this question for some time, but just wanted to make sure i'm not imagining this strange phenomenon. Anyway today I was out for a ride and it was confirmed. When I fill my bike (850 Interstate), it becomes a very much more pleasant bike to ride. Vibrations are drastically reduced, and the motor just seems to run much smoother (maybe it's the same thing). Currently I have a single S.U. fitted but noticed the same when I had the Premiers. At first I thought I was imagining it, but today I rode the Commando for the first time this year ( oh the shame) and had completely forgotten this until I went onto reserve and filled up. It must be something to do with the additional weight, but I would have thought that would have had the opposite effect. Any collective thoughts?

Cheers,

cliffa.

I reckon the heavier load of fuel helps to damp out vibrations. I notice it on my other bikes.

I noticed that after I got old and fat that my bike had fewer vibrations, so I second the damping effect idea.

You are probably getting off on the fumes !

That large tank resonates when close to empty, just that alone will make the bike feel less vibratory when full, the extra weight will add to the effect. I would look at your under tank padding and add some if the current lot has sagged.

Heavier tank dampening out vibes, and resonance as mentioned above, definitely.

I first noticed this on my Harris Bonnie in the 80s, it had a U.K. tank but it still held no where near what an Interstate holds !

Similar experience, I've got the 5G manx alloy tank and definitely notice a difference when she gets to about 1/4 capacity. I've also often wondered about head pressure and its effect on the carbs.

acadian said:

Similar experience, I've got the 5G manx alloy tank and definitely notice a difference when she gets to about 1/4 capacity. I've also often wondered about head pressure and its effect on the carbs.

Click to expand...

I don’t believe head pressure has any effect. The tank fills the float bowl. The carb draws from the float bowl via the Venturi principle. The float bowl creates a ‘neutral point’ in the system effectively disconnecting the carb and tank.

I own a Triumph TR6 car, that I modified for the use of SU carbs. I learned over the years that these carbs are somewhat sensitive to fuel inlet pressure. (long story cut short).
Could it be that the difference in head pressure between a full and almost empty tank plays a role here ?
Note that many cars that were fitted with SU carbs had electric rather than mechanical fuel pumps.
Just a thought.

Fair point SU Peter, I was thinking in terms of Amals and generic bike carbs. I don’t know owt about how SUs work.

Hmmm, to be honest, looking at the diagram below, I can’t see how fuel head can make any difference to an SUs operation...

My PhD is not in fluid dynamics, so I'm just brainstorming here... but I think Eddie is correct: flow rate and pressure are two very different things, in the case of our amal carbed Nortons, gravity is the de-facto "pump" contributing to fuel velocity through the lines, with the size of the petcock, fuel line ID, and the float needle determining the pressure at the carbs. So, as long as the volume of fuel in the tank is greater than the volume of the lines + float chamber... pressure should remain unchanged?????

My understanding is also that flow and pressure are different. But that’s not the primary factor here. The primary factor is that the float bowl effectively de couples the tank from the carb.

I believe the only way that fuel head could effect mixture is if the float chamber became so flooded as to become under pressure from the head of fuel, then fuel could be forced into the carb by the head pressure.

But as float chambers are vented to atmosphere this is most unlikely. As those of us who have ever had a flooded float chamber will know, the fuel pissing out of the float chamber vent tends to make the issue evident.

I think a bigger head pressure would allow more flow, thus filling the float bowl faster. Therefore, it could effect some things, for example, if the float bowl ran dry at WOT with a low head, this might be lessened with a bigger head pressure. But I think this is a tad hypothetical.

Agree - full tank acting as a mass damper. Not enough rubber mounting to consider as “tuned” but nevertheless effective in reducing vibration.

Not unusual for NVH Engineers to hang mass blobs strategically to reduce vibration. On my last program before retiring we increased mass of the accelerator pedal to reduce pedal buzz. However, when NVH guys wanted to weld a blob of steel to the front structural cradle – opps wrong answer – barrier would not like that let alone the additional mass.

I always thought that the pressure created from an elevated tank was due to the height rather than the number of gallons in the tank. The height above the carbs is not enough to generate much pressure from height alone. So the carbs would have to be sensitive indeed.

rvich said:

I always thought that the pressure created from an elevated tank was due to the height rather than the number of gallons in the tank. The height above the carbs is not enough to generate much pressure from height alone. So the carbs would have to be sensitive indeed.

Click to expand...

Yes height is the factor, and a full tank has a higher head than an empty tank.

But I agree the amount of height, and the difference between full and empty, would have a very small effect indeed.

And... as the float chamber introduces a neutral point that de-couples tank and carb, I do not believe it could make any difference to carburation.

Thanks all,
Firstly I'm glad I hadn't imagined it during a fume enhanced euphoria I know the Commando's fitted with S.U. carbs can be hard to kickstart due to reduced fuel head ( and there is a simple fix ), but with an Alton starter I have no such issue. And having an MG-B with twin S.U. 's I am quite familiar with them. I actually noticed exactly the same the first time I rode the bike when it was fitted with twin Premiers. In fact I thought to myself WTF have I bought here, I thought these were supposed to be smooth! Then as soon as I filled the tank it was transformed. At the time I thought the carbs must need balancing as it wouldn't idle reliably either, but now I know that's not the case. I think Kommando' point is a good one, but the tank mounting rubbers seem to be in pretty good shape and I would have thought that if they're shot the tank would be closer to the frame when full and transmit more vibration rather than less ? Does this mean that all Roadsters vibrate more than Interstates ?

One point to note - My bike is slightly unusual in that it has a 3rd ISO under the gearbox, but I don't know if it has rubbers in it or is just used for alignment / stability.

Maybe I could fill the top frame tube with lead shot ?

Cheers,

cliffa.

If the reason for the full versus empty vibration characteristics is purely mass damping then yes, the bike would be sensitive to fuel capacity. However there are numerous sources of vibration on any motorcycle. Sympathetic resonance at the tank can have different sources.

The primary source of vibration from the engine is caused by the vertical shaking forces from those two big pistons. The vibration from the dynamic movement of other internal components pales in comparison. It amazes me how well the isolastic sytem works considering the tools the developers had at that disposal. However, it would be interesting to see the static and dynamic rate curves of the individual mounts as designed and used 50 years ago compared with what we buy today. The inner bushings are smaller in diameter than the outer bushings and at what level of deflection do they make contact with the mount tube and how does the rate build up. The later spring system added to the head steady was designed to keep the rate at idle / low deflections low and provide better isolation. Natural rubber used in PT mounts has a rate tolerance of +/- 10%. Did Norton Villiers monitor rates? Do todays vendors monitor rates or ask for their vendors to “duplicate these samples” How close do the current available mounts duplicate the original? Many sources of variation.

A post several days ago asked about balancing the rear wheel/tire. Another source of vibration that could excite a given frequency that you might not feel through your butt or appendages. Interesting stuff -sometimes I just ignore and enjoy the ride.

Do you have the rear tank strap installed?

We used to go to Brands Hatch a couple of times a year from the West Midlands and used to stop for a break somewhere north of London. There were two of us on Commandos and we both noted that our bikes ran noticeably smoother after the stop. The difference was enough for both of us to notice and comment. We hadn't refuelled, just stopped for about a half hour. We never figured out why but it happened on long trips often.

Ian