1971 Experimental F750 Racer

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Dances with Shrapnel said:
You don't need nor get much support for torsional stiffness from the shock mounts on any bike; plan is/was to provide all that through the sloping tubes from the head steady to the swing arm area. .
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Its a mistake though to think that any old flimsy rear shock support treatment will be 'good enough'.

If the rear shocks and top mounts are not sufficiently braced, and are allowed to flex all over the place, this will translate as being less that sure-footed in the front end. ??
Look how substantial swingarms are these days too - if the tubular arms can flex, this does nothing for good handling either...
 
Agreed and no intent that any old flimsy shock support will be good enough, but almost.

One shortfall I see with the subject PW frame is that he relies on gusset plates and bending moment of the tube steel to support the upper shock mounts. A pair of small tubes running diagonal just forward of the upper shock mount to the swing arm gusset area would have been far more superior (and stiffer) and likely lighter; the light tubing was what you got with the Colin Seeley Mk2. Basically two small tubes in tension.

It can look flimsy but really does not have to do too much when you think about it.
 
acotrel said:
I apologise about the snide comment about welding up the isolastics. I have a problem with any bike which does not hold a rigid relationship between the steering head and the swing arm spindle. If I ever buy a commando, I will look at picking up the ends of the swing arm spindle by using a pair of chrome-moly Z plates with reamed holes and shoulder bolts to hold them. That way the engine/gearbox assembly would be forced to rotate around the swing arm spindle when vibrating. The isolastics would still be effective, however only in one direction. I think the isolastics were a good solution to a very difficult problem, however maintaining the firm relationship between the rear tyre contact patch and the handle bars is critical if you want good predictable handling. I suggest Time Warp is correct when he said the swing arm mounts on the frame being discussed are too narrow. Even on featherbed frames cracking occurs where the swing arm mounting plates meet the vertical part of the tubes in front of the oil tank - the loads are not small. As far as the shocker mounts providing feeling - what do the rubber bushes in each one of them do ? Seeley frames are much superior to the featherbed, and I suggest their excellent handling is due to the tubes which run from the swing arm mount up to the steering head, also what is around the swing arm mount is very strong. You will note that the swing arm on this F750 Racer frame looks identical to the Seeley swing arm - I've never felt mine flex. I suggest it would take a lot more power than you would ever get from a commando engine to tie any Seeley frame up in knots. I spoke to one of our top riders who raced a G50 engine Mk3 Seeley, and he said he could feel the front wandering a bit when stressed due to the bolted in ladder. I use a single gusseted piece of chrome-moly push-bike tube in there, and I've never felt the front moving. When I ride my bike it is precise, firm and very predictable, and it inspires a lot of confidence, also it never runs wide in corners.
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so if you buy a commando and you fit chrome moly Z plates reamed to fit the swingarm pivot how will you attach the left hand plate ? as on a commando the swingarm pin almost touches the chaincase behind the clutch and the chaincase is inboard of the frame! so your Z plate would have a huge dogleg in it you would have to move the clutch out or reduce the swingarm or both or maybe lengthen the frame in the engine cradle ? or am i missing something? or you could buy a Triumph T140 AV they pivoted on the swingarm pivot ,or even an MZ250 it seems funny when you say you have a problem with the isolastic setup when you have never ridden one! the rest of the world dont have a problem!the commando frame is one of the best lashups in motorcycling history it amazes me still how well this package works its almost eerie riding a commando as they smooth out as the revs rise .but everything has to right build quality plays a part here ,i have owned upwards of 40 motorcycles of all different types but none have the same feel as a commando on everybodys bucket list should be to take a commando with pea shooters fitted and redline it in every gear and if you dont get a smile on your face check your pulse your probably dead !!!! regards baz
 
Bernhard said:
If only PW went the whole hog and fitted the outside s/arm bearings exactly in line with the centre of the gearbox sprocket – there would be constant chain tension no matter where the rear wheel travelled.
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That would put the swingarm pivots outside the clutch and tranny outer cover. Not impossible, but not advised.
 
Dances with Shrapnel said:
It can look flimsy but really does not have to do too much when you think about it.
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If the rear shock mounts AND the swingram can flex, then its unlikely that great handling will result.
As you say, another tube giving triangulation support could have been better.
And I'd like to have seen both sides around the upper shock mounts much more connected - so the shock mounts cannot dance independently, so to speak.
Modern (and cheap) cameras could easily be set up to view any flex while travelling, make a great diagnostic tool these days ?
Confirm if it was a problem at all...
 
Dances with Shrapnel said:
Interesting PW attributed the lower confidence to the presumably forward and lower weight of the engine. On the other hand, maybe he did not give it enough of a chance and moved on to other things. I am not a frame builder but I understand the recent (last few decades) move toward more weight to the front. Mr. Herb Becker has done this on my 500 Seeley and at least one of his Commandos that I raced with no apparent bad results.

Thanks for sharing Ken.

Keep at it with this thread and this will become your reading of bedtime stories to us Nortoneers from the book of PW. :)
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It doesn't sound like he raced the bike all that much, so you might be right about not giving it a serious chance. Once Norton got John Player money and started building serious F750 bikes it sounds like the experimental bike was left behind. The 1972 JPN 750 had what PW called the minimized version of the Commando frame, complete with isolastics, but smaller in size. It still had the spine frame, but the reinforcing tube was above it instead of below as in the Commando. This is a picture from his book showing pretty good detail of the frame. He does mention that the isolastic mounting system "worked very well."

1971 Experimental F750 Racer


This is a shot of the complete bike as ridden by Mick Grant. I think this picture is from a talk PW gave down under a few years ago, but I'm not sure.

1971 Experimental F750 Racer


Ken
 
lcrken said:
This is a shot of the complete bike as ridden by Mick Grant.
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Thanks for the pics Ken, and exerts from the book.
I for one hadn't noted that that JPN frame was a 'mini Commando'.
Explains why roadbike bits would be tough to make into a replica...

That blue bike is quite a rework from the bared pic one above it.
Both zorsts are now sticking out the same same, and longer megaphones.
Rear hub has gone from manx type to a separate brake away from the sprocket.
Front disk brake has switched sides.
Carb bellmouths are less downdraft, and now pointing more at oil tank (on this side anyway).

What sort of tach is that shown in the b&w pic ?
 
Rohan said:
lcrken said:
This is a shot of the complete bike as ridden by Mick Grant.
Click to expand...

Thanks for the pics Ken, and exerts from the book.
I for one hadn't noted that that JPN frame was a 'mini Commando'.
Explains why roadbike bits would be tough to make into a replica...

That blue bike is quite a rework from the bared pic one above it.
Both zorsts are now sticking out the same same, and longer megaphones.
Rear hub has gone from manx type to a separate brake away from the sprocket.
Front disk brake has switched sides.
Carb bellmouths are less downdraft, and now pointing more at oil tank (on this side anyway).

What sort of tach is that shown in the b&w pic ?
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The tach is a Krober I think.
 
Rohan, this shot of another works bike shows the same exhaust as the picture from PW's book, as well as the steeper carb angle and Manx rear brake. Norton built at least three of the 1972 bikes, one each for PW and Phil Read for Daytona 1972, and a later one intended for Tony Rutter, but he only rode it once. Mick Grant replaced Tony as the third member of the JPN team, and was later replaced by Dave Croxford. Period pictures taken during the year show the bikes with slightly different configurations, including the exhaust sytems, There are pictures of PW racing the bike with both exhaust configurations.

1971 Experimental F750 Racer


The tach is Krober, as Fast Eddie surmised. Norton used Krobers on all three F750 designs, the mini-Commando, monocoque, and space frame.

Ken
 
hobot said:
Could you throw some light on that engine 'helmet' fitted?
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Steve, that was a fiberglass shell that PW designed to duct cooling air to the head and cylinder. He had used something similar on the Arter Matchless G50 he raced so successfully. It didn't work out to well on the Norton, and was eventually discarded.

Ken
 
Ok my brain static overlooked the text Rohan pointed out. I know air craft and also Sir Eddie put some air flow duct plates on so considering it too. I think if Willaims had sprayed water under the 'helmet' it might of worked. Heat in engine and tires will be main limiting factor in Peel. Have thought to spray water or CO2 on tire sidewalls if laser measuring tops some threshold. Peel may be able to squat lower than this racer, if that matters on bee line spurts.
 
Back in the fifties I saw a bike with a duct running from the front direct to the side of the carb bellmouth to get cool air to the intake. I think aerodynamics would have defeated it, however it might have worked at low speeds. I know that when I use methanol my bike is much better, probably due to the high latent heat of vaporisation which chills the intake, and it's slow burn rate. The 850 commando engine seems to love it. With petrol, it is all so much more difficult. I think a well designed ducted airbox could be a good thing on a petrol fuelled commando.
 
For my money Gus Kuhns were great. When I lived near Bath (South Westish UK) I could get to Gus Kuhns in London in about 75 minutes. To get to my favorite Norton dealer in Southampton took about 90 minutes. Both dealers supported Production and open 750 racing and were pretty damn good. The GK Commandos with Mick Andrews (?) were always good value and shared the proddie racing wins with the works.
A bit peripheral to the experimental 1971 racer, but anyway....when I started at Andover the Commando frame came up in conversation. The comment was made that it was torsionally far stiffer than a featherbed frame. During the development of the Commando there were a series of tests to measure torsional stiffness, it seems that the Commando frame was (is) three times stiffer than a featherbed, and weighed far less. I'm a bit vague on the numbers but featherbed = 27lb and Commando =19 lb rings a bell. Maybe someone with a bare frame sitting around could verify those numbers. For what its worth those weights are English pounds.
cheers
wakeup
 
wakeup said:
I'm a bit vague on the numbers but featherbed = 27lb and Commando =19 lb rings a bell. Maybe someone with a bare frame sitting around could verify those numbers. For what its worth those weights are English pounds.
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When you add in all the swingarm bits, and yokes to each, the differences are less.
Within a few lbs I'd say.
And all closer to 40-ish lbs ??

We'd also wonder how they tested 'torsional stiffness'. ??
That could be anything from driving a lorry over each of them, to suspending a piano off it someplace.
Or doing a fast lap of the IoM, and comparing times...
 
Rohan said:
wakeup said:
I'm a bit vague on the numbers but featherbed = 27lb and Commando =19 lb rings a bell. Maybe someone with a bare frame sitting around could verify those numbers. For what its worth those weights are English pounds.
Click to expand...

When you add in all the swingarm bits, and yokes to each, the differences are less.
Within a few lbs I'd say.
And all closer to 40-ish lbs ??

We'd also wonder how they tested 'torsional stiffness'. ??
That could be anything from driving a lorry over each of them, to suspending a piano off it someplace.
Or doing a fast lap of the IoM, and comparing times...
Click to expand...

The differences will be less in percentage terms of the whole, but the difference will always be there

The torsional stiffness is easy, stick a well fitting steel bar through the steering head tube and clamp it securely to something....a surface table mayhap. Stick another steel bar through the swinging arm pivot point with a fair bit sticking out one side, hang weights on end of sticky out bit, and measure deflection. I'm not sure if the featherbed showed three times the deflection for the same load, or the same deflection for a third of the load. This test is fairly easy to do for a moderately well equipped workshop.
Assuming that the designer has decided that increasing torsional stiffness is A Good Thing, doing this kind of test is a lot easier, more accurate, cheaper, more objective than building a complete motor cycle and riding it around a circuit. Let alone driving a lorry over it. We won't even mention pianos.

Who was it said that An Engineer is someone who can make something for a penny, when anyone else could make the same thing for a quid?

It just goes to show how much work goes on in the background, that the wider population, interested or not, never hears about

cheers
wakeup
 
wakeup said:
The torsional stiffness is easy, stick a well fitting steel bar through the steering head tube and clamp it securely to something....a surface table mayhap. Stick another steel bar through the swinging arm pivot point with a fair bit sticking out one side, hang weights on end of sticky out bit, and measure deflection. I'm not sure if the featherbed showed three times the deflection for the same load, or the same deflection for a third of the load. This test is fairly easy to do for a moderately well equipped workshop.
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Do you KNOW that this is what they did, or are we just guessing here ??

Me, I'm still wondering how testing sideways torsional stiffness like that actually relates to anything in the real world !
And why doesn't the Commando go around the IoM 3 times faster then ?
 
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