JPN Monocoque Specs

If you watch this video, what they DON'T tell you is interesting :

https://www.youtube.com/watch?v=aHVFS8e3cRY

One minute you are comparing the steering geometry my Seeley with modern bikes which have huge tyres on smaller diameter wheels. The next you are telling me my tyres are too big ?

There are 3 bikes in that table with 27 degrees rake, and none has less than 97mm of trail.
For the most part, the trail doesn't dip into the 92 range unless the rake is 25 or less.
I have no idea if this means anything. Just an observation from reading the table...

If there is a formula for calculating what the rake and trail should be to get certain types of handling, I certainly don't know it. I have a friend who has a 350cc Drixton Aermacchi. The rake is almost vertical and the offset is huge - it handles very well. I rode a Manx with 24.5 degree rake and very short offset, it was stable under brakes, yet tightened it line slightly coming out of corners - very confidence inspiring. The weight distribution is also important, because the change in the handling between braking and acceleration depends on minor changes in rake and trail as the steering head rises and falls.
What amazes me is that the McCandless brothers got the handling of the featherbed Manx so right.

acotrel said:

One minute you are comparing the steering geometry my Seeley with modern bikes which have huge tyres on smaller diameter wheels. ......

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No, track back, I have never done that. Maybe others have.

I am telling you what I see in your avatar, and am comparing to the many Seeleys I see at race meetings that clearly their riders don't consider dangerous.

Have you tried anything other than changing the yokes? Have you tried getting it near to how Seeley intended and then just making mods to suit your style or preference.

The principle I am suggesting is to go towards a set up that is seen to work, a huge number of Seeley framed bikes win races! but there set ups are not wildly different.

For example, and the bike model doesn't matter here: I bought an '85 Suzuki GSXR750 slabside and took it straight to a race meeting, it was a bit unnerving, not dangerous, but not right, it exited some corners sideways when the weight came off the front (This thing wheelied pretty easy). These are bikes often called widowmakers so at first I though..oh well, thats how it is. But the more I looked into it the more poor maintenance and poor set up I found. The list was very long, including wheel alignment, yoke height, wear and incorrect parts in the shock mount.

The solution was to modify the set up...meaning....back to standard as much as possible, correcting bad maintenance, poor alignment and poor set up....with modification limited to spring rates better suited to my weight and improved rear damping and a slightly jacked rear end, it was an armchair ride after that, faster and much more fun, and nowhere near dangerous.

And the next improvement based on watching faster riders on the same bikes was going to be reduce the rear tyre size from 150 to 135! But I sold it and got on with my Rickman.

A lot of us worry more about piling modification on top of modification than getting something back to basics and working from there.

Hi Acrotel

Dances is right. I have a Seeley Mk3 roadbike & I fitted the tyre sizes that Dave Watson/Gary Thwaites recommended & ran on their championship race bike. Not for me. If you commit fully it went where you wanted it to go. However the large tyres will make it stand up as you come off the throttle. It did not like bimbling round. I fitted narrower tyres as my mate Rob suggested
( & closer to what they were in the period.) Once fitted the changes were remarkable, you could ride at any speed with neutral handling & it tracked beautifully.
Frame by Graham Hurst, yokes are Dommie, Dommie length forks, Wheels are both 18". Only my Norvil runs a 19" front wheel.
My race bike frame by Graham Pierce, Minnovation yokes, Dick Hunt Dommie length forks. Handles brilliantly, stable, goes exactly where you want it to go. I have had it locked up on the brakes after being pushed of track then picked it up to run across the grass. Never had a moment/reaction from the bike ever. Not sure why your bike behaves as it did but dont know anyone else who has a problem with a Seeley Chassis.

all the best Chris

all the best Chris

The original Mk 3 Seeley was fitted with Metal Profiles forks. I have never owned them. When I got the bike, it was fitted with Ceriani forks and Arces yokes - probably off a 450 Ducati. Because something is standard, it probably means it is neutral - (safe ?). With the TZ yokes my Seeley tightens it's line - that creates confidence. The bike is extremely quick steering - wherever you put you head, that is where it will be in any corner. So I always think a long way ahead. That sort of handling is not for everybody, however I love it. It is interesting when I race it, it can go a lot of places where other bikes cannot. The others are up on the ripple strips while I turn beneath them.
The tyre sizes on my bike are no different to those on a 70s TR500 Suzuki. Obviously it is the whole package which is important, not the individual component parts. The other thing is that if you don't ride your bikes on the limit, you don't become so sensitive to the way it handles.

I don't know if you are aware, but most road bikes are set up to steer slightly on the self-steering side of neutral. So when you brake the bike becomes stable and if you accelerate the bike slightly tightens it's line. If you think about what happens if you get into a high speed bend too fast. If you cannot decide whether to brake or accelerate, braking will often cause you to run wide, accelerating gets you around the bend safely. When I had the incident where the Seeley stoop up and turned the wrong way under brakes, I had pushed it very hard around a bit of track I knew extremely well. After it happened, I was looking for indications that it might happen again. I noticed that as I braked, the bike tended to rise. I have never had the situation where I've had to reverse-steer a bike to get it to tip into a corner - might be a warning sign ?

Getting back to the JPN Monocoque Specs..............

Thanks for posting this Ken. When you posted this I noticed the peak horsepower was listed at 7,200 rpm and this caught my attention. The commonly understood redline for an 89 mm stroke Norton twin is 7,000 rpm. It's not that 7,000 rpm is an absolute but more of an administrative control or useful limit (limit of usefulness) for a street engine. I had this preconceived notion in the back of my head that all peak HP lagged redline but I see no reason why it would have to be that way. As a side note, the 76 bhp at the crank to 67 RWHP as cited in the article infers about 13% loss.

To the point, Ken, would you or anybody have a dyno chart of this or a near comparable Norton engine build you would be willing to share on this forum. I would be interested in seeing both the HP and torque curves, including beyond 7,200 rpm to see the nature of the torque drop off.

lcrken said:

I just ran across an article from the magazine "Moto Tech" back in 2008 about the John Player Norton Monocoque and Peter Williams, and realized it had more detailed specifications than I had seen in other articles on the subject. In light of some recent discussions on the forum, I thought the fact that it had a 27° steering head angle, 98 mm trail, and 18" wheels, 3.00" rear and 2.25" front, with a 48/52% front/rear weight distribution to be quite interesting.

Also, it had a nice summary of the engine and transmission development. I had never heard before that the engines (short stroke 750s) in the monocoques had one-piece crankshafts. Also a good explanation of how they finally overcame the transmission breakage problems.

Ken

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Sorry, John. I don't have any dyno charts for a similar engine. I have some 920 and some short stroke 750 charts, but nothing for a standard 750. I had some at one time from runs on my 750 Sportsman bike at AMI in Daytona, but I lost those a long time ago. All I recall from them was that the horsepower peaked almost exactly at 7000 rpm, and then was essentially flat to 7500 ( wiggly line flat, that is). I didn't want to run it past that on the dyno. I used a 7200 rpm red line in those days, but normally shifted at 7000. I do recall that the peak HP was 63 at the rear wheel. I ran three race bikes on the dyno that day, an SRX-6 that made 60 rwhp, a single cam Rotax 600 that made 62 rwhp, and the 750 Norton at 63. I was surprised at the time that they were so close. A good 89 mm stroke 750 race engine today, with modern aftermarket parts, can easily make another 10 HP over what I had. And you can run them regularly above 7000 rpm without worrying about broken cases and cranks!

Ken

I try to never rev my 850 over 7000 RPM, I'm worried I might leave a piston and the top of an aluminium rod behind. In any case, as I have gone on, I have raised the overall gearing and it still has not started to stall in any gear. It will go straight up through the rev limit and very quickly. Might be due to the methanol fuel that I use. I noticed in what you have posted, that 33mm carbs are used. I wonder if the inlet ports were opened out to that size ? I use 34mm carbs, however the inlet ports are tapered back to standard within 25mm of the flange - NOT parallel.

wakeup said:

Well I've found two photos of the Croxford Lamp Stand in the making, apparently I have to jump through some more hoops to get them on here. Problem is I'm away for a few days and the boss is telling me I have to get packed. Sorry about that.
cheers
wakeup

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Posting photos on behalf of wakeup
Andy

Racing motorcycles are like table-tennis bats. When you bust them, you just throw them away. I wonder where it ended up living ?

acotrel said:

I noticed in what you have posted, that 33mm carbs are used. I wonder if the inlet ports were opened out to that size ? I use 34mm carbs, however the inlet ports are tapered back to standard within 25mm of the flange - NOT parallel.

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Probably not. In the factory race heads I have seen, the intake ports at the manifold surface are 32 mm, and the manifolds are internally tapered to match the carbs at one end and the head at the other, i.e. 33, 34, or 36 mm at the carb, depending on carb used, tapering to 32 mm at the head. The 33 mm carbs at the time were just standard 932 Amals bored out to 33 mm. The factory tuners didn't generally go crazy on opening up the intake port diameters. Even the heavily modified big valve head that I have from one of the factory flat track bikes still has the intake port at 32 mm, even though it was used with 36 mm carbs (1036 Amals). All the tapering was done in the intake manifold.

Ken

acotrel said:

I wonder where it ended up living ?

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Mention of a 'lampstand' might give some sort of clue ?

I don't believe I've ever seen a pic of it in that format though.
Anyone ?

Isn't there mention too that it has recently been transformed back into a bike. ?
Not surprising, given the rarity of them...

If you have the money, you can settle for a replica.
http://www.peterwilliamsmotorcycles.com/jpn-replica.html#panel22

Dances with Shrapnel said:

If you have the money, you can settle for a replica.

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If only 4 originals were ever made, we might all have to 'settle' for replicas.
For considerably less shekels than an original too.... !

Rohan said:

acotrel said:

I wonder where it ended up living ?

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Mention of a 'lampstand' might give some sort of clue ?

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Has Dave Coxford done the same to his on the garden of his house I wonder :?:

Bernhard said:

Has Dave Coxford done the same to his on the garden of his house I wonder :?:

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I had visions of it in the study/library.... ?

Again, its has (recently) been repaired and rebuilt back to a JPN monocoque racer ?