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Centre of flywheels and lateral centre of weight for a twin engine.

Monkeypants

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VOC Member
I'm planning to use a wide rear tire on an Egli-Type build and have been looking at ways to get adequate chain clearance.

The standard Egli, if there is such a thing, approximates the original Vincent engine mounting very closely.

The original Vincents have the engine mounted so that the crankcase joint, which I believe is also the lateral centre of the crankpin, about 9MM to the left of the centreline of the frame/wheels.

I suspect it would be ideal to have this joint right in line with the wheels so that the three really large heavy gyroscopes are all in alignment (2 wheels and engine crank).

For high speed handling it seems this arrangement would be optimal. Shifting the engine 9 MM to the right also gives another 9 MM of chain clearance which allows for 18 MM more tire width, good for my situation.

One other consideration is the lateral centre of weight of the engine. It would be ideal if the engine could be mounted such that the finished bike had its lateral balance line in the same postion as the wheel /frame centre line.

This doesn't necessarily mean there would be exactly the same amount of weight each side of centre as the balance point is affected by the position of the weight. In other words, there is a lack of total symmetry so simply having equal amounts of weight each side of centre is not the same as being balanced on centre.

A good starting point for this would be finding the lateral balance line of the engine and seeing how that relates to the centre of flywheels or if it is feasible to make the lateral balance line be the same as the frame/wheel centreline.

I'm posting this because I suspect others have already given consideration to these factors when building a Vincent Special.



I'll be happy if I can get this close. Being a very practical person, I'm quite willing to wear a second sock on one foot in order to achieve perfect balance:)
 

davidd

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VOC Member
I think that most of the specials that use wider tires use jack shafts. However, most of these have been racers or employed "sawed off" engines, thus no need for self contained starters. I do not know what you are shooting for so I can only try to give you totally inappropriate advice. Modern compounds have pretty much obviated the need for wider tires on vintage bikes. You would be hard pressed to lose grip on the narrowest treaded racing tire with any amount of horse power that a street twin could supply. I have checked the cornering clearances on a standard Vincent, but I have not done so on an Egli. They are not very good for racing although the Egli is far superior. It is something you should keep in mind when moving the engine to the side and setting the ride height.

David
 

timetraveller

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VOC Member
Have you considered the exhaust pipe clearance on right handers? Even a standard Vin with a determined rider can ground the pipes. You might have to rethink the exhaust layout as well as balance etc.
 

Monkeypants

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VOC Member
timetraveller, hadn't thought much about the exhaust yet but thanks for the reply as ground clearance could be an issue with the shifted engine.
I'm planning to use an exhaust similar to that on Jos Van den Ouden's Egli. The pipes twist under the engine an rise up on the left side. The way the pipes are tucked in should allow for very extreme lean angles on either side.

David, I don't know what the HP of this engine will be. I don't think any exactly like this have been built just yet, not with this particular combustion chamber design.

The top end is Terry Prince's latest version of his 92MM bore squish head setup. The crank is 102MM, which gives 1350CC, compression ratio is 10.5 to one , carbs are 40 MM, cams are Terry's lightning cams. I think Peter Volkers has these cams in one or more of his bikes. They keep producing more HP as the RPMS go up past 5500 whereas the Vincent Lightning cams drop off in power above 5500 rpm.
Terry's 1200cc RTV Vincent's were 85HP when those were built about 20? years ago. Since then his head design has had almost as many changes and power upgrades as Terry has had trips to Bonneville.

I think the fat tire will be needed, if not at least it will look the part!
 

Howard

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VOC Member
Just a talking point, because I can't claim to have thought it through.

When you blip the throttle the gyroscopic action causes the front end to lift (at least I seem to remember it lifts) so if you offset the engine, the offset weight may be offset ( :eek:) ) by the gyroscopic action????

Opening the throttle coming out of a left hander may tip it back in.

As I said, just a thought - any clever people out there?

H
 

Monkeypants

Well Known and Active Forum Website User
VOC Member
Here is one other question I came up with after realizing that the 3 big gyros are out of alignment by 9 mm on a standard Vincent.

Does this in some way contribute to the tank slapper problem?
 
Last edited:

timetraveller

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VOC Member
I doubt that it is as simple as that. The only time that I had a tank slapper (while racing at Cadwell at about 100 mph) there was no way that the gyroscopic effect of the front wheel did me any good at all. Instead it went from lock to lock, faster than one could see clearly, with the kinetic energy of bike plus rider; say about 600 – 650 lbs at 100 mph being fed into the whole of the front end. The curved black skid marks on the track were about 3’ long with paired marks curved left and right at about 10 foot intervals. It is a long time ago but if I am remembering those figures correctly then, letting 100 mph be about 140 feet /second, means that the total cycle of left, right left again was occurring about 14 times per second. So much energy is fed into the front end as the forks hit the stops with the whole force of the bike behind the impact that the front wheel gyroscope doesn’t stand a chance. On a lesser bike the stops would have been fractured. It can be instructive to play with a gyroscope and see how it actually twists when a torque is applied to the axel. It does not do what one would naively expect. My scars are still visible in a bright light!
 

Howard

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VOC Member
I think you'll find the reaction from the flywheels is in the wrong plane to cause a tankslapper (lots of chat about tankslapper causes on the forums).

At worst it will probably cause the bike to rock a la BMW Boxer, but since it's a turning moment, you'd need a large force at 9 mm radius to make a significant difference.

Why not make the frame/engine fittings wide enough so that you can move the engine off centre to suit various tyre widths including standard. A batch of spacers of varying thicknesses should allow quick adjustment, and if the offset causes problems you can simply go back to a standard wheel/tyre.

H
 

Tom Gaynor

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VOC Member
That's torque reaction, acting in the opposite sense to motor rotation. The old joke about racing BMW solos was that they were fine unless off the deck over a jump: then the flywheels stopped and the bike revolved. It is very noticeable on a Sunbeam S7: whack it open out of a corner and the bike revolves clockwise from the riders point of view, picking itself up out of left-handers, laying itself further down out of rights. Because the Vin motor revolves "forwards" - anti-clock from the drive side - then either the front will rise or the back sink when power is applied.
 

Pete Appleton

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VOC Member
VOC Forum Website Administrator
Offset flywheels

What about the outside flywheel Guzzis and Nortons? Quoting from the National Motorcycle museum website about their 1954 350cc Norton racing bike:-

"To minimize crankcase size and to reduce oil drag, the internal flywheels were replaced by a single external flywheel mounted behind the engine sprocket"

Presumably Nortons didn't think that it would be a problem to have an offset flywheel.

I hope that they are right as my new off road comet has the engine offset by an inch. We will see!

Peter
 
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