AN Acquires Thruxton Club Racer

[acotrel]

Why would you build a short stroke motor, if the objective was not to spin it faster to get more horsepower at the expense of mid-range ? The limiting factors then become the valve train and the number of gears in the gearbox.

 

[Kvinnhering]

Why would you build a short stroke motor, if the objective was not to spin it faster to get more horsepower at the expense of mid-range ? The limiting factors then become the valve train and the number of gears in the gearbox.

Lost off mid-range on short stroke. See the graf above?
In my opinion, there's a lot of torque there. Do you not agree ???

 

[Dances with Shrapnel]

Why would you build a short stroke motor, if the objective was not to spin it faster to get more horsepower at the expense of mid-range ? The limiting factors then become the valve train and the number of gears in the gearbox.

Your premise is flawed and we have gone over this with you maybe dozens of times already. So one more time: It is not at the expense of mid-range.

Furthermore, I am seriously considering going back to a five speed from a six speed on my 75mm ultra short stroke Norton 750 as it is really counter productive since my mid range is broader and higher torque. And the valves are not the limiting factor on this build as there are other Norton twin builds that spin much faster.

 

[MichaelB]

The only picture I recall seeing of a short stroke Commando is the red roadster in the Norton sales brochure, but it doesn't say "Short Stroke" on the side panel. I think it is the one that they put together for display at a motorcycle show (I used to know the name of show, but I've forgotten it in my old age).

FWIW, the short stroke 750 was, in theory, offered as an option on both the regular Commando and the JPN replica, but I've never heard anyone claim to have actually ordered one with the option.

View attachment 3442

Ken

I have that same brochure and have often wondered if they just put 750 tank and covers from a MK V 750 on Mk 1 850. How would you know?

Edit, Thinking about this. The 1973 Mark 1 850 had silver barrels. So, it's either real short stroke or a Black barrrel 850.

 

[acotrel]

Isn't the 'shortER stroke 750 norton' about the same stroke as a 650 Triumph, which has plenty of torque, even when fitted with race cams ? Perhaps a standard Norton 750 has a LONG stroke motor - very long ! All I know is the long stroke 850 is a real shit when you only have the standard gearbox to go racing with. With a close box, it is gem. I believe late BSA A10s and A7s had 74mm stroke - that is what I would almost call short - like the late 80s Thunderbird 650.

 

[ZFD]

The Vienna bike has meanwhile turned out to be the "prototype" that Norman White raced. Several details are different to all known "production" Thruxton Club Racers.

Back to shortstrokes: The brochure offering a short-stroke engine on normal raoad Commandos was probably an idea at the time they printed the brochure but never actually executed. When Wolverhampton had their sell-out in 1975- which is when Norton was bankrupt, the Mk3s were built under the receiver till 1977- Rhind-Tudd of WASP was landed with most of the short-stroke engines gathering moss. There was no use for them after Norton factory racing had ceased. He took the useless short-stroke cranks, conrods and pistons out and used the major parts as spares for 850-based engines.

My Signal Orange Roadster- the bike I normally ride to work on- sports one of those short stroke engines that came to me via an Andover Norton trade customer who belongs to the off-road scene.

Mike Jackson, whom I know very well, is not, I repeat not, a reliable source of historical Norton information. Sorry to say this, but if you want factual information there are other sources that are more relable.

Joe/Andover Norton

 

[Dances with Shrapnel]

............He took the useless short-stroke cranks, conrods and pistons out and used the major parts as spares for 850-based engines.............
Joe/Andover Norton

Thanks for the history. Can you elaborate a bit more on what became of the "useless" short-stroke cranks, conrods and pistons? Futhermore, my understanding was that the short-stroke heads (RH7) had full hemisphere combustion chambers; were they also removed and....?

 

[acotrel]

Was Norman White successful when he raced the prototype Thruxton Racer ? How much development went into the bike ? The theories of building a fast motor need to be put into practice and proven, before they mean anything. And even then the handling of the machine has to cope with the different power characteristics of the improved engine.

 

[Chris]

According to Normans own website he did race one. How often someone would have to ask him. I only ever raced at the same meetings when he returned to classic racing with his wonderful pannier tank bike.
I have to say, boy was he quick.

Chris

 

[ZFD]

Thanks for the history. Can you elaborate a bit more on what became of the "useless" short-stroke cranks, conrods and pistons? Futhermore, my understanding was that the short-stroke heads (RH7) had full hemisphere combustion chambers; were they also removed and....?
The "useless" parts circulated in the then Norton world for several years. I remember Mick Hemmings offering the steel conrods in the early 1980s. The heads without squish band- hated by my tuner Rudi- were probably used on the WASP engines.
The crankshafts still circulate and can be found on the net (ebay etc) if they are not gathering moss in the garage of hopeful ex-youths who once intended to build the Norton engine to end all Norton engines with it but have since found they never will but cling on to their youthful dreams- and the crankshafts... I heard of several such cases.

According to Normans own website he did race one. How often someone would have to ask him. I only ever raced at the same meetings when he returned to classic racing with his wonderful pannier tank bike.
Yes, I saw Norman in his workshop when in Andover a couple of weeks ago and we discussed the bike. He raced it once but with a long-stroke (conventional) engine in it. The bitter, untold truth is there was practically no development of the short-stroke engine. Peter Williams told me it came mid-season and they had no time for it after testing it back to back against their conventional engine and finding no more horsepower. It was shelved and he admits to never riding one. The whole exercise was overtaken by the "Cosworth" hence the short-stroke was history and never developed.

 

[Chris]

Thanks ZDF

Will have to ask Norman about the frame/handling, as it looks a useful item. How does the Thruxton measure up against a standard framed Commando? Now at least we know Norman rode one.

all the best Chris

 

[ZFD]

The frame is bog-standard Commando with some modifications nothing to do with its geometry. But the engine sits quite a bit higher in the frame thus giving more ground clearance. Some of this is offset by the 18" wheels. Mike Braid- who has been very helpful, having what he calls 1 3/4 Thruxton Clubs himself- tells me he has ridden his with the 18" wheels and on a set of 19" wheels and finds the handling and the motorcycle generally with the 19"ers better.
I can't say, my bike is currently in bits. The gearbox didn't want to shift as-bought, the engine has supposedly not run since 1980 so needs to be inspected.

 

[acotrel]

I have always believed that the squish band made the Norton Commando head superior to the Triumph 650/750 head. If the bike handles better with 19 inch wheels, that is probably due to a more suitable rake and trail - steering geometry. Handling can be either quick or slow. If I was using a more responsive short stroke top-end motor, I'd probably use quick steering. The opposite occurs with bevel Ducatis which are usually slow steering and take the high line in corners. This means you can ride them faster on bumpy tracks.

 

[acotrel]

The only time I've reduced wheel size from 19 inch to 18 inch was on a featherbed frame. The steering became much slower and heavier. However that is not to say that the type of change would not be very different if a different rake and yoke offset were involved. It can be a dangerous area to play with.

 

[Dances with Shrapnel]

The heads without squish band- hated by my tuner Rudi- were probably used on the WASP engines.

Sounds like Rudi has some passion. In discussions with Norm White several years back he gave me the nickel tour of how to make power where he seemed adamant about a full hemisphere combustion chamber and piston dome to match, presumably like the factory Norton short strokes. I've heard some impressive power numbers from the Norton short strokes but now thinking about it a bit, these may be of the Ron Wood era, the motors may have deviated significantly from the factory configuration.

The real high performance Norton motors I have seen and heard of employ squish and the serious ones seem to use a kidney or bathtub shaped combustion chamber. In discussions with others there are claims of some loss of flow as one moves from a full hemisphere to a kidney or bathtub. No surprise as naturally aspirated spark ignition IC engines are just one big bundle of compromises.

 

[ZFD]

My Signal Orange Roadster shortstroke runs with squish, standard rods and a shortened barrel (Another Rudi no-no is steel conrods- "too much weight"). So we may shortly have yet another RH7 head gathering moss in a corner... knowing my Rudi. What he currently waits for is the new Andover Norton forged pistons for 850s with piston crown basically to his design that we expect sometime in the winter. Should he want to stick to the original pistons I still have several pairs Shortstroke NOS in STD and +.020" that he could play with...

 

[acotrel]

If the cylinder head is full hemisphere, it is still possible to get squish by using higher pistons and machining the edge of the crown, so the squish is up in the chamber rather than down at gasket level. It can be done on Triumph 650s by using BSA Gold Star 350 pistons. To my mind removing the squish band from a Commando head is a backward step. I think it is the main reason that Commando 750s are superior to Triumph 750s. A lot of the information about the short stroke 750 Commando engine reminds me of what you have when you fit a bore kit to a Triumph 650, except you don't have the advantage of the separate cams.

 

[Dances with Shrapnel]

If the cylinder head is full hemisphere, it is still possible to get squish by using higher pistons and machining the edge of the crown, so the squish is up in the chamber rather than down at gasket level. It can be done on Triumph 650s by using BSA Gold Star 350 pistons. To my mind removing the squish band from a Commando head is a backward step. I think it is the main reason that Commando 750s are superior to Triumph 750s......................

Getting appropriate squish on a true hemisphere is not so easy (precise) as one has to factor in the lateral slop movement of the piston within the bore. Have you ever rocked a piston left and right in a bore along the axis of the wrist pin? Now think about the piston dome and combustion chamber wall geometry; what happens to your squish while your piston can rock and slap forward & aft as well as side to side and you have a nearer to vertical clearance between the piston dome and combustion chamber. One side near closes up and/or hits and the other side becomes sub marginal or ineffective squish. I seem to recall the Ducati mono cylinders have a nice compromise approach with a gradual (nearer to flat) dome piston and sloped squish to match on the sides; at least it looks pretty. This brings up the question on how the HDXR750 managed squish. Maybe I am missing something here.

As for a Commando 750 being superior to a Triumph 750, though squish may certainly be a factor in this I believe it is the demonstrated superior intake port efficient flow characteristics of a Commando that make the big difference which naturally translates to greater volumetric efficiency. The "why" of this better flow is a topic of another thread. Furthermore, when Norton splayed the exhaust ports, the intake ports were rotated about the axis of their respective valve stems in order for the two ports to be parallel. This new orientation of the intake ports to the center-line of the cylinders imparts considerable swirl to the intake charge. I have not run a flow bench swirl test of this but I am stating what appears to be obvious. When one looks at the true orientation of the port to cylinder center-line, the intake flow comes in at a considerable tangent to the cylinder rather than straight in like on a Triumph.

Efficient flow and turbulence are good and are why a Commando generally can out perform a Triumph of that era.

 

[acotrel]

Commando ports are slightly steeper than Triumph ports, but the biggest factor which seems to hold Triumphs back is the dead space on the side of the piston crown away from the plug. That side of the crown always has much more coke on it, in motors which have high-crown pistons. Talking about squish bands and piston slap, have a look where the squish band is in a two-valve Jawa speedway motor. The slop doesn't seem to stop them from still being effective.

 

[Fast Eddie]

I built a Triumph motor with a Norton crank. 840cc with a bathtub type squish. 72bhp on the same that my current Trident Hunter Commando produces 64bhp. I developed it a little further still after this so it was probably closer to 75rwhp in the end.

Then the crank broke !

Conclusion? F**k knows...!

But it seems to say that Triumph motors do have power potential, and that’s squish bands do deliver.