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Primary Belt drive by Bob Newby


davidd

Well Known and Active Website User
VOC Member
As I remember, the Newby clutch is 68 teeth for both the single and the twin. The single clutch\belt is 30mm wide and the twin is 40mm. The engine sprocket that I use is 36T, which produces the 1.89 primary ratio. The stock primary drive ratio is 1.6, which is quite a difference. With the 36T on a twin, I would use a 22T CS sprocket and an adaptor so I could run rear sprockets in the mid 40's.

The 38T = 1.789

The 40T = 1.7

The 42T = 1.619

A 42T sprocket is pretty big. I do not know if belts are available from Bob for all these. I am sure he has them for the sprockets he sells.

I would opt for the 36T, which we know fits, and change the gearing at the back end to compensate. Of all the items that would have to be modified, it is easiest and the least obvious to change the rear sprocket.

David
 

vincenttwin

Well Known and Active Website User
VOC Member
David
The twin belt I have got is 30mm ,I think he suggests the 40mm for racing or for the big bore fast ones.
cheers
 

davidd

Well Known and Active Website User
VOC Member
David
The twin belt I have got is 30mm ,I think he suggests the 40mm for racing or for the big bore fast ones.
cheers
Peter,

Bob did not have the parts to make this many variations when I last spoke with him. Although it is great for the builder, I can now see why he cannot say whether or not a specific set-up works. The 30mm width will help considerably in getting everything under the stock cover.

David
 

Glenliman

Well Known and Active Website User
VOC Member
David,I wonder if all of the belts Bob uses have the same pitch? The setup I have here is 40mm with a 71 tooth clutch drum and 43 tooth engine pulley. This gives a ratio of about 1.65 to one. Bob was aiming for 1.6 to one and had a setup nearly finished in that ratio when he realized it wouldn't fit. I've forgotten what part wouldn't fit, but it was likely the engine pulley which would have been larger than the one I currently have.
Looking at the diameter of the clutch drum gave me concern that it would not fit, so I tried it and the pulley on an engine Robert Watson was building for his son. The engine pulley fit with good clearance, however the clutch drum did not. Some material will need to be removed from the crankcase for it to fit on.
As far as the primary cover goes, Bob and I discussed that a bit but decided that it was not important for this engine as I can always build a primary cover to suit.
I may have put the fear of God in him re clutch requirements, but neither one of us wanted to end up with a clutch that was not up to the job. That is why he used seven thinner plates rater than five standard plates and I suspect it is also why the diameter of the clutch is so large, which goes against his other goal to get the primary ratio close to stock.

He had some concern about the large engine putting too much torque thru the trans and one way to reduce this is to bring that primary reduction down. I believevwe talked about Roy Robertsons 1272 egli which has been raced so much and apparently has eaten several transmissions (early Triumph 5 speeds?). Roy has one of Bobs primary drives, must be in the old higher ratio he wanted to avoid.
So he managed to get the ratio quite close to stock Vincent , has made a clutch that is probably good for about 200 hp and it looks as though it will all fit in there with a bit of case trimming.
I may or may not use the stock type primary cover, but if I do, it won't be a bolt on fit, there will be some renovating required.

Glen
 

davidd

Well Known and Active Website User
VOC Member
Glen,

That is interesting and I had no idea that Bob made a larger diameter clutch. When I first started ordering clutches from Bob there were only two sizes of 30mm belts available and only a thick and a thin clutch. So, I would agree that he has enlarged his offerings significantly. I have always thought that the 68T 30mm clutch was good for about 70 hp. If you are building a Comet special and you use a plate rather than the inner primary cover, there is no restriction on the size of the components. However, on the twin the inner primary is part of the case. Also, on a Comet the conering clearance is pretty good, but on the twin the clutch cover becomes a problem unless you rais up the engine in an Egli frame. You could also do the Reg Bolton mod and jack up the rear suspension to get a little clearance on a stock machine.

I have made a few outer primary covers for Comets, most of them seen on my racer. I have done them in fiberglass for two reasons: first, my sheet metal tools are in storage and second, I liked the ability to make multiple parts for a racer, which seems to go through multiple parts. The down side is they have to painted. The upside is that they are easy to modify, light and expendable.

100_2618.jpg

David
 
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Glenliman

Well Known and Active Website User
VOC Member
The cover looks great David.
Re cornering clearance, perhaps it is a lucky thing that I had to shift the engine 5/8" to the right in order to get the chain to clear the 180 rear tire. I would like to say I had great foresight on this matter, but my nose is already a good length :D!
 

grahamsharrock

Well Known and Active Website User
Non-VOC Member
When I took all my engine bits to Terry Prince he said the 40mm belt (which I have) wont fit under the standard twin primary cover and needs to be spaced out. Also needs some machining to the inside. Just more money I suppose :cool:
 

davidd

Well Known and Active Website User
VOC Member
I do not think this answers the question about fitting, but it does answer the question "Is anybody using it?" David Tompkins has been using one on his Norvin. Dave reports it has worked flawlessly so far on the trips he has made to various outings and rallies.
Dave Tompkins 8.jpg

David
 
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Spqreddie

Well Known and Active Website User
VOC Member
Thanks David,
do you know how many teeths has this front pully in order to know the ratio?
The generator sproket doesnt seems to fully engage, it looks like the theeths might escape and wear the belt, but might just be the photograph perspective.
Ciao,
Edmondo
 

davidd

Well Known and Active Website User
VOC Member
Edmundo,

It is a 40mm wide belt with a 36T CS sprocket and a 68T clutch. David says the gap at the generator may be a little big and he will close it down with the eccentric adjustment, but he has not had any issues with it.

David
 

Spqreddie

Well Known and Active Website User
VOC Member
Thanks, so it gives you a 1,88 ratio.
To go near the stock 1,6 rato, you would need a 44 or sproket but as you wrote in a previous post it will be difficult to fit it under a cover, therefore soultion would be as suggested to get a small front sprocket and change the gearing at the back end to compensate.
Any how i think that i would still opt for 44 sproket if it fits without need of grinding the cases to not get to far away from the orginal set up. of course a custom cover will be needed.
Thanks,
Edmondo
 

johnmead

Well Known and Active Website User
Non-VOC Member

mm Newby
Here is a picture of a 40mm Newby primary cover being developed by a member of the VOC Dutch section. He has been working on it for a couple of years and is near completion. I have a standing order for one.

John Mead
 

bmetcalf

Well Known and Active Website User
VOC Member
In your reply, click on the icon to the right of the one with the globe with a red X.
 

johnmead

Well Known and Active Website User
Non-VOC Member
Beltdrive Primary Cover.jpg

Here is a picture of the 44mm Newby belt drive cover.

Thank you Bruce, it worked. See you at the next section meeting at seans.

John Mead
 

minivin

Well Known and Active Website User
VOC Member
Has anyone had any more experience with fitting one of these twin belt set ups, into a standard primary? I am still questioning whether I am willing to run a standard triplex chain, what with the engine I am building hopefully being a bit spiced-up. However, I don't fancy having spaced-out covers destroying the looks of the engine. Has a way been found to hide all this under the standard covers?
 

dagriise@online.no

Well Known and Active Website User
Non-VOC Member
The covers are one thing, but I discarded the belt, when i found out one had to shorten the mainshaft to fit the clutch..

Regards Dag
 

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