Wednesday, 24 December 2014

Why are dirt bike riders supposed to be tall?

Recently I lowered the seat height on my Honda XR400R.  It was needlessly high, adding suspension movement on top of the tyre diameter still left a lot of excess space. Shortening the sub-frame down tubes would allow the bolted on sub-frame to rotate downwards  from the top mounting lowering the rear of the seat by a considerable amount, although not so much at the front area of the seat. Dropping the rear makes it much easier to get on and off the bike and I later removed about 30mm from the thick foam to lower the seat overall. For a 1.75m high rider (me) subsequent riding confirmed the benefit of the lower seat.

Original subframe showing where it was shortened.

Lowered subframe.

If only things were that simple. The lowered subframe meant that the air filter moved closer to the carb, but that wasn't a problem, the flexible coupling easily accepted that. The problem was that the airbox now fouled the shock. For obvious reasons I didn't want to move the shock so I had to cut away part of the airbox which leaves an entrance for water ingress when riding across rivers.  I think that I'll have to make a new box in CF.
Another thing to fix was that the relationship between the subframe and muffler and its mountings was changed. I had to modify the muffler pipe to align everything again, not a big problem.
I found a short piece of steel tube in the airbox, it turned out that a cross tube on the top of the subframe had fractured due to a lack of proper bracing by Honda, and had fallen into the air inlet.  In other words the subframe was not sufficiently rigid in a lateral direction. Continuous flexing had lead to the fracture. To improve on this rather than just replace the broken tube I welded in a diagonal bracing tube as well as a new cross tube.

Welding in the bracing tubes.

Cross tube and diagonal brace.

Monday, 6 October 2014

Fixing the cheap and nasty (or "Doing it right")

Production bikes are generally built down to a price rather than up to engineering excellence.  This means that there are plenty of areas where improvements can be made. Usually people only think in terms of those big modifications which invariably cost a lot of money, such as new suspension, wheels etc.  However there are many small detail features that are equally important but which only cost a little time.  Sometimes these small detail fixes are necessary in order to provide a sound base on which to add the fancy bits.  Of course there is rarely any Bling benefits to be had from the small oft hidden changes.  We are all familiar with the term "blueprinting" when it comes to engines, but "blueprinting" the chassis is equally important.
The pix. illustrate just what I am referring to.  The Suzuki SV650 internal swingarm axle spacers cum chain adjusters are terrible bits of sheet metal poorly tack welded to a spacer. The external spacers are of too small a diameter and when the assembly is tightened, pressure is only applied to a very local area of the flat swingarm sides instead of the whole vertical face.  In combination with new larger diameter external washers/spacers the pictured chain adjusters provide a more rigid connection between axle and swingarm.

On the left is the original chain adjuster, the witness marks above and below the axle hole clearly show the limited area of contact with the swingarm.  The new block is shown to the right.

Front and rear of the replacement blocks.

Fitted block.  Note how the upward force from the axle is passed to the top piece of the swingarm section, also how the sides of the swingarm make full contact with the block.  Larger axle spacers and washers are needed as well to ensure a more rigid connection when the axle is tightened.

Sunday, 5 October 2014

Cologne show 2014

I have just got back from a couple of days in Cologne.  Probably the two most noteworthy exhibits were the super-charged Kawasaki with 300bhp claimed and the Ducati Scrambler range.
The Kawa uses a mechanically driven impeller type supercharger.  The impeller is of small diameter and so it must be spun at very high RPM. There appears to be a planetary gear system on the drive shaft which is there to gear up the drive to get the required speed.  There is definitely a trend away from the dual beam aluminium chassis toward the oft called trellis tube frames and the new Kawa is no exception.
Personally it was the Ducati Scrambler range which appealed to me.  It looks like a real fun bike and quite small by modern standards.  A welcome change from the overweight monstrosities which seem to be in the midst of a breeding cycle.
I took about 500 photos but here are just a few for now.

The new Kawasaki with integrated supercharger.

Cutaway of supercharger.

Entrance and exit doors and where it goes BANG.
Upper sprocket on supercharger shaft.

Upper sprocket on supercharger shaft.
Lower sprocket for chain to supercharger shaft.
More superchargers at the show. 

Early BMW racer with integrated blower.

Early BMW racer with integrated blower.

Early BMW racer with integrated blower.
Even earlier BMW racer with integrated blower.

Even earlier BMW racer with integrated blower.  Note longtitudinal drive shaft.

 Ducati scrambler.

I wouldn't mind one of these.

Retro is the future
These are certainly nicer looking than the majority of over styled modern bikes.

Kawasaki got in on the retro phase before most others.  Even got TT100 tread on the tyres.

It is amazing what the absense of "do nothing" plastic panels does.

A bit of Deja Vue.  CB900 given a bigger engine but it looks much the same.

What else should we expect from Triumph, but "THRUXTON" haven't the new people heard of Velocette?
What's in a name?

Identity crisis?  I'd have said scitsophrenic but I can't spell it.

Fans of Hawkeye perhaps.  Young viewers should Google.

Dual identity, is this the new HD?  If they called it the Monster then it would be the Monster Mash.   Young viewers should Google.

Kawasaki concept. 

Giant thermos for coffee?

Black hole.

Shades of the Chrysler Tomahawk.

Fuel cell Burger van.

Fuel cell Burger van.

BMW's find the spark plug puzzle.

Odds and sods of interest

If you thought that there was little left to develop with the humble spoked wheel then consider this. There is no penetration into the interior and so tubeless tyres are no problem, The straight spokes should give a long life and are easy to insert for wheel building.  Expect to see them on a new MV.

This Horex uses the new wheels.

Multi-cylinder Horex is an example of smart packaging.

Sawn through head from the Horex showing clever packaging.

We are used to modern brake discs being mounted on separate carriers, here is an example of doing the same with sprockets.

Look mum, no hands.  Nobody would let me ride any of the bike exhibits but I got to try a knee steer Segway rip-off in the exit hall.  The real Segway was there with their electric bicycle, a far cry from the advanced electric motorcycle planned 6 or 7 years back.
Extreme limits

If there is an absolute maximum to the scale of ugliness then this has to be close to it. At the show it was a hard fought battle with many bikes vying for the title of worst styling.  This picture hardly does it justice though, it looks far worse in the flesh.

Thursday, 25 September 2014

Port flow improvements

More examples of previous engine work.  This post is concerned with modifications to improve port flow.  Starting from a head off the street models which had an updraft inlet port of about 10deg. in comparison to the similar factory race heads with a small down draft angle.  I wanted to create a better inlet port shape without any welding on the head.  Any significant head welding requires the removal of valve guides and seats, re-heat-treating and re-machining the seat and guide holes.  That is if you want to do the job properly.

Cross section of a stock road head showing updraft inlet port.

The red area shows where I bore a circular recess to allow the fitting of a solid block from which to fashion a new port.

Making the insert block.

Machining the new port with the block fitted to the head.

Final block shape after machining the new port.

Comparison of flow between the standard port and that modified with the block.  This was achieved without any other detailed port work.  Further improvements were achieved by the removal of small amounts of material around the valve guide area.

There are more pix. in the Aermacchi engine folder on my picasa page and there is a PDF report with a lot more detail available for download from

I managed to get a race head casting from Dick Linton which was cast without ports or combustion chamber recesses.  I intend to machine this to use a narrower valve angle to allow for a more compact combustion chamber and steeper inlet port. It will also have significant squish areas which are absent from the original 1950/60s design.

Showing the basic casting.
More details are available for download in a report at