1971 Experimental F750 Racer

I was reading through Peter Williams book "Designed to Race" again, and ran across his discussion of a bike he designed (and raced) for Norton in 1971, prior to them getting John Player support and having a real race team. This is a picture of the frame.



The most obvious feature is the lack of isolastics. Also notable is the absence of a full cradle design, and the placing of the swingarm pivots ouboard of the frame. Peter said that it was a decent race bike, but the handling wasn't really right. He had some speculations on why, but never had the opportunity to check them out. Still, he did manage a third in the 1971 IOM race. In 1972, when they built their first JPN F750 bike, he went back to what he called a mini-Commando chassis, complete with isolastics.

There are some more pictures and details in his book, which I highly recommend. It's a great read for any Commando fan.

Ken

lcrken said:

I was reading through Peter Williams book "Designed to Race" again, .........

There are some more pictures and details in his book, which I highly recommend. It's a great read for any Commando fan.

Ken

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+1 it's a great read. Apparently he is writing a second volume.
cheers
wakeup

lcrken said:

I was reading through Peter Williams book "Designed to Race" again, and ran across his discussion of a bike he designed (and raced) for Norton in 1917, prior to them getting John Player support and having a real race team. This is a picture of the frame.

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1917 seems a little early - typo man strikes again. ?

Those top rails pulled together sort of eliminate the twin tube strength advantage.
A few extra horizontal cross-braces would (considerably ?) reduce potential twisting forces, with very little extra weight...

Oops! 1971 it is.

Ken

Besides what Rohan noted, do ya think making the elastic Norton engine cases a stressed member is a good idea? Swing arm definitely more stablized.

The motor is about as stressed in that frame as it is in a Mk3 Seeley - no problem. The major difference is in the way the swing arm is mounted. You could almost box in the mount, I think you would still have a problem with the swing arm bearings giving too much movement, There is a lot of leverage at that point.

That frame is a little H1R ish except for the odd way the perimeter tubes butt onto the gusseted box.
The gearbox case also looks to be used as a frame brace at the lower mount.
Maybe it flexed at the swing arm area being so narrow,the rest looks to be well triangulated,the narrowed top section is only seat and shock top mount.

On a tangent,I wonder if the Commando was ever tested with a bottom rear isolastic for three in total.

Time Warp said:

the narrowed top section is only seat and shock top mount.

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Err, a LOT of force is fed back from the shock top mounts - the whole weight of the bike, plus all the acceleration forces.
Failing to stop (or limit) the twisting forces from that lot could easily be the problem with this frame ?

That simple tubular swingarm looks like it could be gainfully beefed up and braced too, all along its length, not just near the pivot point.
(It is faintly possible its braced inside, with more tubing - but we can't assume that...).

Due to the excessive motions of wallowed out s-a spindles changing chain tension I've seen, I think Norton might of goofed by not doing similar on C'do's instead of worrying about a total non issue amount of chain stretch/contraction by the power plant shifting fro/aft - at most a whole eight of an inch, if even that, by pulling a wheelie to clear logs and slab stone creek ledges, gopher holes and such. I have witness marks from soiled lube smears to nail maximums down. W/o off roading antics 1/16" is all a road racers or drag launch racer could hook up to cause both isolatics to compress before tire spin or flipper over. Only issue I can see is if the chain drive through the 'cushion drive' transmitted engine vibes. Suspension travel has more tensioning effect than isolastics backing each other up in swing arm chain drive plane. With s-a robustly fixed to the frame, that foot pegs and seat and bars on, might give better sense of tire patch grip to have more dangerous fun safer. Correct my thinking on this if I'm missing something. Think with all the brains focused on frames we could come up with a better isolastic set up short of welding em like I asked acetrel not to make a snide joke on his isolastic solution.

Btw Al I got 'puter to decode AU DVD and just started seeing the vintage two smokes taking turns. Somehow too smooth to me, too much time in poor traction leans with resonance building and chrips em off into hay bales. Many of their lines through turns is not how I'd do em on Peel. Some bikes feel good going fast others like new GPs scare the shit out of pilots but allows a bit less time around so they got to put up with it. I tell ya one thing from my very bones I'd rather have the twist taken up by the frame motion rather than forcing it back into distorting tire patches and grip, Boy Howdy No Sir Ree Bob. Maybe that's what Williams didn't like as looks like this one could oscillate in its long axis. No way can I take the Irish race DVD right now or never get shut down my brain static.

hobot said:

Due to the excessive motions of wallowed out s-a spindles changing chain tension I've seen,.

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The Owners Manual gives clear instructions on oiling the swingarm pivot.
Now, I haven't investigated inside too many, but if kept well oiled they appear to stay in good condition.
Greasing them seems to be problem though ?

Having the engine/gearbox and the swingarm moving against each other sounds like a recipe for a disaster.
Nortons plunger framed bikes tended to stretch/stress their chain on bumpy roads, which is good for neither chain nor sprockets.
They don't have a reputation for long life for these components, which were not cheap nor easy to replace - which didn't exactly endear this foble to owners.

The Owners Manual gives clear instructions on oiling the swingarm pivot.
Now, I haven't investigated inside too many, but if kept well oiled they appear to stay in good condition.
Greasing them seems to be problem though ?

Having the engine/gearbox and the swingarm moving against each other sounds like a recipe for a disaster.
Nortons plunger framed bikes tended to stretch/stress their chain on bumpy roads, which is good for neither chain nor sprockets.
They don't have a reputation for long life for these components, which were not cheap nor easy to replace - which didn't exactly endear this foble to owners.

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Ugh, spindle only need oxidation protection and if area not modifed for grease then it just clogs oil in flow path but not out flow so spindle rusts then wears by corrosion removal way way way more than friction. Don't know about the plunger deal except it don't apply to C'do's chain or spindle life.

I've used down/dirty emery/dowel method then fitted grease zerks and flow grooves. I think ya' all worry warts on this as friction ain't ever going to be an issue with the loads so even dry as a bone the soft bush would just burnish[polish against the super hard spindle about forever, unless of course there was not enough oil or grease on spindle to keep super hard surface pitting and flacking into rust dust. I dare ya to even leave a mark with a file on spindle, so how can sintered brooze? The lube ain't doing anything but chemical oxidation protection to matter. I've had two dry as bones all trapped by swollen rust , yet shock action and ride was fine with routine hard tests. Now don't get me wrong on this friction issue, as oil or grease makes less friction to remove and install. Moral of my tale, if spindle drools oil out below half way, the sintered pours do not leak out enough to protect the hard finish so its a non issue if they are smeared over or not. Now if used in a higher speed device the heat would sweat out some oil to flow and lube. Btw before you operate on the insides might check the bushes are actually round as so soft handling can distort them, which requires more material removed to pass the shaft. Correct my thinking if i'm missing something.

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hobot said:

. Don't know about the plunger deal except it don't apply to C'do's chain or spindle life.

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It will apply if the engine/gearbox can joggle about, but the swingarm not !!

I would imagine that the reason the frame tubes are 'tucked in' and the reason the swinging arm pivots are outboard of the frame, is to allow the riders legs to tuck in tighter, remember PW was (is) obsessed with aerodynamics.
I love how he says it 'didn't handle' yet he still finished 3rd at the highest level of the sport!
Must get the book.

Pretty nice looking design. 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. I suspect this frame design was heavily influenced by the seminal Colin Seeley design. You see the concept today in just about every new sport/race bike design where two spars go from the head steady down to the swing arm spindle, everything else on the motorcycle is almost jewelry or Christmas ornaments. When you think about it, how do mono shocks work.

I am speculating here but the inboard swing arm spindle support and narrowing of the back bone were for weight reduction and maintaining a narrower profile (aerodynamic or whatever). Furthermore, with supporting the swing arm spindle inboard as in the picture versus outboard like a Seeley Mk2 I just don't see any sweeping grand improvements from a stiffness standpoint. Swingarm bushing centers would be about the same distance and I doubt the spindle will see any significant flexing so a single shear inboard support as shown in the picture will perform pretty much the same as a double shear outboard support.

Keeping it in context, as Fast Eddie stated, this bike did finish third.

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 :?: :shock:

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 :?: :shock:

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If this is the case with the subject frame it could have been a deliberate move on the part of PW if he were looking to develop an anti squat condition - this is a road racer.

In the book, PW mentions that the primary reason for the swingarm pivot design was the keep the footrests as close in as possible to minimize frontal area.

Regarding the handling, he says the problem was front end feel. He speculates that the engine was mounted too far forward and too low, and that the suspension was "wrong". He says that the bike worked well at Mallory Park, but that he didn't have enough confidence in the steering to ride harder at the IOM.

Ken

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.

lcrken said:

I was reading through Peter Williams book "Designed to Race" again, and ran across his discussion of a bike he designed (and raced) for Norton in 1971, prior to them getting John Player support and having a real race team..........There are some more pictures and details in his book, which I highly recommend. It's a great read for any Commando fan. Ken

Click to expand...

I missed out on getting a copy. Now available on Amazon......£199.00! :cry: Didn't realise it would be such a limited print.

lcrken said:

In the book, PW mentions that the primary reason for the swingarm pivot design was the keep the footrests as close in as possible to minimize frontal area.

Regarding the handling, he says the problem was front end feel. He speculates that the engine was mounted too far forward and too low, and that the suspension was "wrong". He says that the bike worked well at Mallory Park, but that he didn't have enough confidence in the steering to ride harder at the IOM.

Ken

Click to expand...

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.