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Grey Flash: Motorcycles that Matter


vibrac

Well Known and Active Website User
VOC Member
What's the story on those tanks? I used to think 'it' was just youthful whim of a young Surtees making up a Vincent special out of a works hack. Then copied by some in imitation. now I am thinking perhaps the works tank to clear the big carbs and the 1950 TT
Tell us David
 

Little Honda

Well Known and Active Website User
VOC Member
I think this bike went to the National Motorcycle Museum years ago after the fire. Originally went to South Africa.

View attachment 26772

Spotted in a promotion for Classic Bike Guide's April 2019 issue.

https://www.classicbikeguide.com/motorcycles-that-matter-vincent-grey-flash/

David
I see a large spare sprocket (52-54t?) on the rear wheel, while the sprocket in use is much smaller.
The 2 in pipe makes only sense above 5000rpm - so what is the use of the large sprocket? What kind
of race would this combination make sense for?
 

timetraveller

Well Known and Active Website User
VOC Member
The point is that it is horizontal which is what both Dave Dunfey and John Emmanuel found was required to obtain the best handling and safety. The works racers knew this but it seems that it never got passed on the road users.
 

Little Honda

Well Known and Active Website User
VOC Member
Well, if the rear wheel will be on the ground and a rider on it, the lower link will point slightly upwards, which
is essential for a pushed fork - opposite to a pulled fork, which should show slightly downwards.
No sidecar with pushed front fork will have it adjusted pointing downwards or even still horizontal with the
rider onboard. If not, the rider will learn his lesson....
 

davidd

Well Known and Active Website User
VOC Member
I think the bulk of the large road racing tanks were made for the 5 Flashes that were entered into the 1950 TT. Kelbin Hardin bought a Lighting that had one mounted on it and I remember that Glyn Johnson recognized it as a Flash tank and tried to hunt it down in the 1970's. Kelbin still has the tank.

After the TT some of the Flashes were loaned to racers like Phil Heath and John Surtees to use, but they had to be returned to the Factory. I think that the shot of Surtees with the large tank is a shot of him on one of these loaners and it is not the bike his Dad bought later.

One of the TT bikes was rebuilt and sent to Cimic for Edgard Soares, who was an exceptional rider. It was ordered with a large tank and it was painted black. This became known as the "Black Flash." It is not known which racers were donors for this bike although the frame and engine numbers are known. The bike did not survive as far as I know.

The big tanks certainly fit the big carb, but they fit the stock tank also. This is a shot of my Flash in 1953 at Daytona Beach.
cycledaytonavincent5.jpg
The rider is Canadian Cliff Venier. Eddie Stidolph, who owned the bike worked for a Harley dealer and was not allowed to ride for himself. Those who have seen the big carb set-up will spot the 1-3/4" big port manifold, which is tiny compared to the 32mm manifold. The 1-7/16" carb is clip fit and the remote float is mounted to the F106/1. You can see a shiny band just under the bell mouth of the carb that is holding the float to the F106/1. So, the fitting of the big carb did not seem to be a problem in the stock tank, although the attached float bowl would not fit. The float bowl, if attached to the carb, has to be adjusted so high that it hits the tank. This is due to the steep angle of the carb.

I ran most of my races with a 52T rear sprocket. I used a 20T CS sprocket and I had a 1.89 primary ratio. I did, however, use 18" wheels. I could have gone to a 19T CS sprocket and run smaller tooth rears. I think the major problem with the Comets is that you have to stay away from any sprocket smaller than 45T. This does not give you a large selection of gears. Ideally, for all racing you would like to carry about 15 rear sprockets. My lowest rear is 42T and my highest is 58T. I use the former for land speed races and the latter at Miller motorsports where the 52T sprocket does not allow me to get out of 2nd gear.
100_2765.jpg
I can run small rear sprockets because I went to an adapter rather than a brake. I use a stock off the shelf 1/4" Suzuki or Honda sprocket made out of 7075 aluminum. This makes it easy to have a large selection.

I gear to hit 100-103 mph on the straight of any track. The single does not have enough power to go faster on the straight, you run out of time. I hit that speed at 7000 rpm. The MK2 is usually losing power over 6000 rpm, so it is not much good up at high rpms. It will do it, just not happily. You need a cam that does 7-8000. It is fine if the power drops, but it can't drop fast like the Mk2. If two Comets have the same gearing and you run one Comet up to 6000 and another up to 7000 on the straight, the 7000 rpm bike will be going 15 mph faster than the 6000 rpm Comet. This is a tremendous advantage between bikes that are similar. If you are passed by someone going 15 MPH faster at the flag you think that guy has tremendous HP, but he may be geared more carefully and using all the rpm's he has.

I am curious about the TT Flashes being red lined at 9000 rpm's. I don't think Mk2's will do well up there. Also, at 7000 rpm the piston speed is 4133'/min. in my engine. The Nascar guys run 4800'/min to prevent the piston from self destructing. 9000 rpm would produce 5315'/min. I suspect that with the technology and oils available in 1950 this would have meant certain destruction.

I build the pipe so that the inside diameter will pass the cubic feet/sec somewhere between Bell's target of 250 and Denish's target of 280 mean port velocity at 6500. That would be a 1-3/4" pipe with .049" wall thickness. These are just starting targets.

Norman is correct. It was a large light bulb that went off when I saw how the Factory set up their racers for the best handling.

David
 

vibrac

Well Known and Active Website User
VOC Member
Thanks David
How that all fits under a standard tank beats me I had to put a hole in the top of our tank
26775
 

Little Honda

Well Known and Active Website User
VOC Member
Some years ago, I had a friend´s Comet for some time. It was an original "C"-Comet. When it topped at 75mph, I asked a friend, to follow in his car and tell me his speedo readings afterwards. He confirmed my
readings, stating, that his Moto Guzzi Falcone did the same speed with a sidecar on!
So, I remembered, that this speed was also achieved by my first motorcycle, a 1952 NSU Max, 250cc, 18hp.
And I decided, to find out why.
Next step was, to fit a bicycle speedo, carefully adjusted. The Comet´s engine was in good trim, everything
well tuned, nice plug face, etc. So I started on a sunny afternoon on a flat, free motorway nearby and revved it
in 1st, until it did not accelerate, anymore, remembering the speed, the same in second and third. In fourth,
it topped at 75 with me sitting up. I stopped and noted the figures on a sheet of paper and went back home.
With the figures from Paul Richardsons book, I calculated the engine speeds in each gear and found out, that,
with works sprockets, it stayed 1000rpm below max. in fourth gear, which is app. 17 - 18 hp at the rear wheel.
So I fitted a 52 teeth sprocket, which was 4 teeth over standard.
Same day, same road, same conditions, and it achieved 90 mph, exactly works figures!
I do not torture my bikes, but they must deliver works standards!
 

timetraveller

Well Known and Active Website User
VOC Member
David's experience with Mk 2 cams mimics my own. When I used to sprint my twin I tried revving to 6,000 rpm and changing gear twice on a quarter mile or revving to 7,000 rpm and changing gear once. It was repeatedly faster over the quarter mile with the extra gear change. Lacking access to a rolling road in those days my conclusion was that one is far over the peak of the power band at 7,000 rpm with a Mk 2 cam.
 

vibrac

Well Known and Active Website User
VOC Member
Some years ago, I had a friend´s Comet for some time. It was an original "C"-Comet. When it topped at 75mph, I asked a friend, to follow in his car and tell me his speedo readings afterwards. He confirmed my
readings, stating, that his Moto Guzzi Falcone did the same speed with a sidecar on!
So, I remembered, that this speed was also achieved by my first motorcycle, a 1952 NSU Max, 250cc, 18hp.
And I decided, to find out why.
Next step was, to fit a bicycle speedo, carefully adjusted. The Comet´s engine was in good trim, everything
well tuned, nice plug face, etc. So I started on a sunny afternoon on a flat, free motorway nearby and revved it
in 1st, until it did not accelerate, anymore, remembering the speed, the same in second and third. In fourth,
it topped at 75 with me sitting up. I stopped and noted the figures on a sheet of paper and went back home.
With the figures from Paul Richardsons book, I calculated the engine speeds in each gear and found out, that,
with works sprockets, it stayed 1000rpm below max. in fourth gear, which is app. 17 - 18 hp at the rear wheel.
So I fitted a 52 teeth sprocket, which was 4 teeth over standard.
Same day, same road, same conditions, and it achieved 90 mph, exactly works figures!
I do not torture my bikes, but they must deliver works standards!
So you geared it down and it went faster.......
 

davidd

Well Known and Active Website User
VOC Member
Tim,

I am not sure how well the 1-7/16 works as a down draft. It clearly had problems that the Factory had to fix due to the angle. With the replica I ran I did not have a problem, but I did not run it because I did not have enough time to do a lot of dyno runs to dial it in. I chose to run the 32 mm instead.

I think the big carb will fit under the tank, but it does "dead end" the bell mouth into the tank more than I liked. Because I could, I made a special tank cover to allow air to the bell mouth.
100_0271.jpg
Coburn Benson ran the Flash with the big port head and an 1-1/2 GP in all of his races. The GP does not do downdraft well (except for the GP2) so he made a manifold to hold it.
IMG_2118.JPG
This is probably the best way to run a vintage carb. Kurt Fisher ran 113.851 mph to qualify for the Daytona 100 miler in 1968. Coburn figured 38 MPG at full throttle and ran the race without a pitstop. Coburn got up early and grabbed one of the original 20 sheets that Daytona copied.
Daytona Qualifying 1968 1.jpg

David
 

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