Sunday 27 December 2015

Bike Steering




How does a bike steer?

After a ROG's Run on a very pleasant day and leaving fliers at Ryka's where we set off from and lunch at Eastbourne it is time to reflect on what makes the bike go around a corner and what keeps it stable in a straight line after a very slippery ride along country lanes on the way home. The basic component to all this is the spinning wheel. If you have ever held a bicycle wheel by the spindle and spun it around the wheel likes to be upright and it is hard to move it to make it run at an angle but once there it will remain so. It is the gyroscopic effect. The wheel is a gyroscope. On the road this is your forward momentum and if you have seen lots of bike racing many times a rider has been dismounted with a high side and the bike stabilizes itself and continues to run on in a straight line. Usually it is not quite upright so it then has a sideways momentum combined with the shape of the tyre that sends it on a decreasing spiral trajectory until it hits the ground. 



The sideways momentum is created by the shape of the tyre, it being curved, so that the further from the centre line the overall diameter of the wheel reduces. When the diameter gets smaller you have to do more revolutions to cover the same distance so the effect is that speed is reduced. Just by leaning into the turn will set the bike on a curved course because you have moved over to a slightly smaller diameter wheel that has slowed the speed. A slower speed means less gyroscopic effect and helps to tip into the corner. Giving power to increase the speed helps bring the bike upright after the turn by increasing the gyroscopic effect. This is only part of the story. What helps to make the bike turn is around the front end. The best example is seen on the extended forks of a chop. The wheel is raked forward so far you can see and feel exactly what is happening. In a straight line all is fine when the handlebars are turned either to the left or right you will see the wheel being rotated by the forks around the headstock. 


The effect is to move the wheel form the centre line in the opposite direction to way the handlebars are being turned. Pulling the left side will displace the wheel to the right and vice versa. This is also changing the plane in which the wheel is rotating so that turning to the left moves the wheel to the right and tips the headstock to the left. In slow manoeuvring if the tip is too large you end up having to put your foot down or be a heap on the ground. 
You counter balance this with your weight to keep upright because you have little gyroscopic effect from the wheels. Next to look at is how far forward is the front wheel. On a chop its miles away and the steering is a bit slow, bring the wheel closer to the frame and this make the rake less and the steering will become much quicker. Many of the earlier Japanese bikes had quite steep fork angles to help make the bikes turn as the weight distribution was not quite right. They have made impressive improvements over the years but have a look at the older British bikes that seem to have the front wheel a long way from the frame with shallower angled forks and why did they handle so well? 


 The answer is Trail. This is the distance from the projected angle of the headstock to the centre line of the wheel where it touches the ground. The shorter the distance the faster the bike turns. This affects the straight line stability and there is a compromise to get good handling at speed and low speed manoeuvrability. One affects the other. 

Where does counter steering come into this? Look at the chop again, if you have pulled on the left side of the bar the headstock tips to the left, to make it go more to the left you need to change the plain in which the wheel is spinning so do do that you need to push the left bar forward. You can feel this if you lean the bike over to the left and push on the left bar. This puts the wheel more to the edge of the tyre so making the wheel even smaller and turning tighter still


To bring the bike upright you pull back on the left bar. Amazingly opposite!! None of this takes into account the effects of where the engine is placed in the frame or of the position of the rider and where the centre of gravity is. That's for another time.


Tuesday 8 December 2015

Off Road Bike Show 2015




Classic Off Road Bike Show
Kempton Park 2015

After being at the Wey Valley Advanced Motorcycle Club Christmas diner it was hard to get moving first thing but as the show opened at 09:30 I just had to get going. In the last few weeks I had been as far afield as Birmingham and Falmouth. At both places I took the opportunity to leave some fliers. I had heard that the Coventry Transport Museum had revamped their displays and I plan to pay them a visit in the new year. 



This year the Off Road Bike show seemed very busy with lots of interest from visitors about the Museum stand with our exhibits being the Brooklands Sprinter, Doug Heckle's 50cc Moto Minarelli's drag bike, the Cotton Telstar, the Triumph 500T and our very own 500 cc 1970 Barcelona 24 hour racer and to complete the line up we had the 5 stud JAP engined speedway bike. 



The carburettor is missing and Bill has it somewhere. From information gleaned, courtesy of the Speedway club who also had a stand, it was a mono-bloc as there was the manifold left to confirm the fitting. There was much discussion about the frame and the fact that it had suspension fitted. I spoke to Bill about the bike the Monday after the show and he said he had made 5 frames of differing sizes for Jim Tebby. Differing lengths and weight distributions to give better traction for different tracks. You can see where the engine plates have been re-drilled. There was much conjecture about rear suspension being fitted to the bike and that it could be a grass track conversion. More was clarified by Bill as the bike was campaigned in France and won 3 consecutive championships in the 1960's. Bill was not sure which years and so far I have not been able to verify this. Rear suspension was fitted because the French tracks have very rough surfaces and life would have been difficult without it. It was allowed within the French regulations. On the home front Jim Tebby raced for Wimbledon from 1959 to 1972. Many thanks to the Speedway Club for their input.






Back at the show there were some interesting bikes as always with a DMW engined Dot, a couple of Greeves Racers and trials version and a well turned out BSA Bantam. It reminds me that I should get out there and get muddy!





 




Sunday 29 November 2015

Motorcycle Suspension Part 2



Rear Suspension

Part 2.


More words from Pete about suspension. This time at the back. A solid rear end is fine on smooth roads at low speeds as anyone who rides a bicycle will testify to however as the speed rises shocks increase and so does the level of stability. Even ace drag star Bill Bragg fitted suspension because Santa Pod was so bumpy. His first effort in 1960 was with Yellow Peril and with speeds around 145mph and he had rear suspension in 1961 for Scarlet Peril. Just as a side issue to go faster he fitted a supercharger in 1962 with Blue Peril and managed a standing quarter in 11.19 seconds nearly two seconds faster than his previous efforts.










Pete:
There are exceptions but in general it took a long time before designers got round to rear suspension. Many old sages thought that you would loose control if you put springs at the back. However common sense prevailed. It was pretty obvious that if you could keep the rear wheel in contact with the road instead of skipping over the bumps the tyre would grip better and the road holding of the motorbike would actually improve.
Without making a huge change to the rigid frame the larger manufacturers used what was called plunger suspension.







The rigid frame is what it says. Where the axel of the rear wheel is held in lugs fixed to the back end of the motorbike frame.
The Plunger sprung frame has the end lugs replaced by spring units rigidly attached to the rear frame. The component that holds the rear wheel spindle slides on guides and their movement is controlled by the springs contained within the units.
It took some of the shocks and bumps away from the back of the motorbike but it was difficult to damp the spring to maintain control. The other drawback is that the suspension is working in a straight line and this is contrary to the radial path of the rear chain.



As can be seen the chain is stretched at each end of the travel of the suspension. If the chain is run slack to prevent this there is a danger that it might jump the sprocket. Run too tight and both sprocket and chain wear is rapid.
A way of reducing this is to add a link at the plunger to work in a short radius. This was used by Ariel for a short period.
On a motorbike with shaft drive the shaft has to have a sliding component to stop tension in the drive damaging the gearing.
The answer for a chain drive motorbike is to mount the rear wheel on a moving arm with the pivot point as near to the same centre as the gearbox sprocket as possible. A Swinging Arm or Swing Arm.


The arm needs to be as strong as is necessary to provide rigidity between the wheel spindle and the pivot point. The wheel requires springing and damping. This can be done in a variety of ways. Either directly on the arm near the wheel or indirectly via a link.




Sometimes the structure used for the arm is reinforced. The earliest way of doing this was to fabricate a triangular structure. Advantage can be made of the reinforced structure and link to change the suspension rate.


 



Most modern motorbikes will have a reinforced swinging arm and a single spring and damper unit that can be adjusted to suit the rider.










At the museum we see some of the early attempts at making the rear end softer. The earliest is on the 1923 Wooler  with plunger rear suspension. Much later is the 1937 New Imperial with a triangulated pivot and springs under the seat. A friction damper is also fitted  at the linking arms very much like on girder forks. There are a couple of Vincents that have something very similar from 1947. There is plunger rear suspension on the 1953 Norton Model 7. Other interesting bits I have come across and that bis a 1920's Indian with pivot fork and leaf spring rear suspension and the very interesting OEC with short arm plunger suspension  at the Norfolk Motorcycle Museum. And don't forget the Sprung Hub fitted to Triumphs around 1953 and 54.

Saturday 7 November 2015

Motorcycle Suspension Part 1



Motorcycle Suspension.

Part 1 Front


Early on in the development of the motorised bicycle roads were unpaved, stone tracks at best or even muddy paths, much was learned from the development of bicycles and if you look up the history of bicycles all of the suspension systems around today were seen on bicycles by the end of the nineteenth century. Nothing is new, however the technology required to put some of the systems into operation on a motorcycle needed to be in place or invented or even as simple as a patent running out. For example take Leonardo da Vinci, he invented the differential where wheels on the same axle could run at different speeds. Not necessary on a horse drawn cart but four hundred years later necessary on cars. His ideas needed an application and the materials to make it work. I have been to many museums and looked at what manufacturers used to make motorcycles more comfortable. Some things like having tar on the roads that made a smooth surface slowed the progress. Undoubtedly suspension had become necessary as speeds rose. More on the front rather than the rear. Peter has put this together and is available at the Museum to read.

In the beginning motorcycles were developed from the ‘Safety Bicycle’ and had no suspension. The addition of the engine made the machine heavier and faster and so the shock from road bumps became more violent. This had several effects. Broken frames were common but worse, the rider could loose control and even be thrown off.
So the early motorcycle engineers tackled the front of the bike first. Mainly because that’s the end where you control from, but also because road shocks were felt through the handlebars. The constant jarring was really uncomfortable.

After a whole rash of experiments from different manufacturers the ‘GIRDER’ fork became almost universal. 



The ‘Girder Fork’ that holds the front wheel is suspended from the steering head by two parallel links. A spring between the links provides the suspension. The links swing in an arc and this motion is transferred to the wheel.
As the links need to be short for stiffness, it makes a sharp choppy action.
Non parallel links or links of different lengths can change the way the wheel behaves.






Motorcycles later used Telescopic forks but the early “Tele’s” were flexible so engineers tried different types of linked suspension to reduce the flexing.



By using stronger metals the flexing in the Telescopic Fork has been reduced. They can also be made to give a long travel so that Motocrossers can take big jumps without jarring the rider.
Now because they are visually more acceptable Telescopic Forks are used on nearly all large Motorcycle. There is another variation of the leading link fork and that is the Earles fork. On this the link is extended to pivot behind the wheel. This can be seen on the Around the World Triumph outfit of Richard and Mopsa English. The links are now able to form a fork that keeps the wheel in line with the motorbike. While in theory the wheel is under better control the unsprung weight on this suspension means the damping system has to work harder. The extra weight behind the steering axis tends to add a pendulum effect and can cause a wheel wobble.
More of Peter's views when we look at rear suspension.

Some interesting examples of early designs one is the plunger front suspension on our 1923 Wooler that I feel may have been more like riding a pogo stick as the sideways rigidity is only from the front spindle.



An even more intriguing set up is on this OEC that has a type of hub-centre steering along with suspension sliders within the upright tubes on the front fork assembly. This must also have been interesting to ride relying on the gyroscopic effect of the rotating wheel to maintain stability but still lacking sideways rigidity.


 On telescopic forks greater rigidity is obtained by using larger diameter tubes, compare the sizes of the EMC of 1947 to the B31 a few later across the isle from it and then look at any big Harley for comparison.
One question to ask is how does a motorcycle steer? We'll come to that after looking at suspension.

Wednesday 21 October 2015

Norfolk Motorcycle Museum



Norfolk Motorcycle Museum 2015

On another visit to the Broads for a little family R&R on the Buell. This is where I catch up on some reading along with eating and drinking at one of the local pubs I keep an eye out for interesting places to visit while I am there and of course a little blast around some very nice country lanes. Last year it was the Caistor Motor Museum and this year I found the Norfolk Motorcycle Museum that is at North Walsham. 



There search on my phone said South Walsham but with the correct post code. After a stop at the post office and a friendly local we tracked it down. It is next to the North Walsham Railway station and a breeze for those using public transport. It is in an old railway workshop and packed with bikes British and European. It is run by another motorcycle enthusiast who is doing his best to preserve our biking heritage.The man is George Harmer and with his son Steven are bent on expanding the collection of bikes repairing if possible or restoring a precious relic. One of the rebuilds is of a BSA Sunbeam with fan cooled twin cylinder engine.




He has a 1928 Raliegh that needs a greta deal of work doing to it. We have a 1927 model in the museum and I have sent a number of pictures of that bike that may be of use during the restoration. There are still barn finds around and he has kept this 1934 BSA as it is for a cameo set. 



We had some discussions around suspension systems and one different way was the front set up used on this OEC. It has a type of hub-centre steering with a spring slider within the front tube and a short swing arm on the rear. There are many machines that are similar to the ones at the LMM but they do have a good selection of foreign machines with Gilera, Motobecane, Suzuki and Yamaha. I was particularly interested in the NSU Max being offered one in the 70's. I declined as I had no idea of how and where to get spares. The case with almost everything then.



There is even a Puch split single two stroke. A number of British and foreign scooters. There is even a motorised skateboard that was featured in one of the Bond Movies. 





There are more interesting bikes in this museum with a Rudge Racer that was ridden by Sam Allerdice. Other riders at the time was Murray Walkers' dad Graham. 































Walking around the museum there are some more novel bikes like the diesel Enfield, the Triumph powered Dot and even a dual control scooter. There was a good selection of small Velocettes with all of the LE variants lined up as well as the British Scooter Brigade.


In the adjacent building is another store of bikes and filled with bits to rebuild almost anything. This building is not able to be used as a museum as it has a problem with the gas processing plan just across the road. Apparently it is in the line of blast should the gas plant explode, however the museum and railway station behind it are safe and so is the housing estate along side it!!!!!!! There are enough bikes there to fill every inch of space they have. It is a most interesting place to visit and the next time I am in that area I'll have to pay them another visit.