Steering head design

Hallo,

after my fathers Maico is finished i have another bike to built. This time for my own. I have a  completely new PVP 252 superkart engine, Öhlins FGR100 fork, Dymag wheels and a self supporting RSW 250 seat. The frame and the swing arm is still missing and so i have to design a steering head for the frame. Swing arm bearing concept is standing already.
My question is about the wall thickness of such a steering head. The alloy is 7020 aluminium, strong and weldable.
The steering bearings are 55mm in diameter and i don't want to use a steel bushing. I will put the outer rings straight into the alloy. A variable steering head angle is not planned.

I want to mill the headstock out of a big plate. It`s planned to have a hollow supporting structure behind the headstock on which the frame "tubes" can be welded on to get a good stiffness. The structure and the steering head are made of one block.
Please don´t laugh at the pictures as I´m not a good free hand drawer. After milling the parts look straight. The project is serious and the capabilities of metal treatment are very good.

Anybody knows the wall thickness of an RSW 250 or got a good tip for me? Anybody here did somethink like this before?
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A picture of my fathers Maico though it´s complete different to the stuff i normaly work with. I built within 1,5 years. Engine is compeltely different to stock andvery strong with the help of Frits Overmars here at Pit-Lane.
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Best regards,

Bernd

Hallo,

can someone help me with the following points:
What is a good offset between the rear sprocket pivot and the swing arm axle for a 250 race bike regarding an anti-dive- effect?
Or got someone the measurings for these 2 points from ground level.
What would you assume as a minimum swingarm lenght (axle to axle) and a maximum wheelbase?
Best regards,
Bernd

I education included 6 failed examinations in the French language so I will stick to English. In my experience, the best source for amateur designers and constructors in "The Racing Motorcycle. A Technical Guide for Constructors". There are 2 volumes. it is written by John Bradley.

In relation to Sprocket positioning, I understand that the swing arm should align with the centre of the sprocket at somewhere between 40% and 50% of the rear suspension travel. There is more to it than that as the "ideal" position is affected by the chain forces and can be altered by the sizes of the sprockets front and rear that are used.

The focus of the design needs to be the tyres. The construction of tyres have been changing rapidly in recent years and the impact seems to be for the motorcycle to need to be softer in the suspension and to be longer in wheelbase. Forward weight bias for the tyres we can buy still seems to be desirable. Most of the frame needs to be stiff.

When you have the luxury of 1000 engineers and technicians working with you, your frame would be massively stiff around the steering head and massively stiff around the swing arm pivot with something like 220 mm in the middle of the frame that is stiff vertically (when upright) but less stiff laterally. Might be fashion; might help with the steering and feel in the turns by allowing a very slight flex in the frame. Without the technical department to back us up, we have to guess.

Hallo,
thanks for recommending these 2 books. I bought them, but the prices reach a ridiculous level. Luckily i got vol1 for 36 bucks on ebay UK yesterday. That balances vol2...
I hope i can use it.
Best regards,
Bernd

Hi
Maybe this  dimensions from GP two stroke racers would help on your project. F and beta angle.[Vous devez être inscrit et connecté pour voir cette image]

Hallo,
thank you for your drawings. They are very usefull!

Another question: I got the opportunity to realise an adjustable steering angle. But what kind of bearings can i use? Someone got a number/description of such a bearing?
Best regards,
Bernd

Hello,

You will find hereafter some information about the steering column with the possibility of change of the caster angle.
We used tapered roller bearings, the same size as the 2006 Yamaha R6:
Bottom: Ø 31 / 55x17
Top: Ø 30 / 55x17

Changing the angle is by swapping two centering pieces which support the bearings.
These pieces are of course machined with an angle equal to the desired variation
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J'ai commencé par la suspension arrière mais le problème était complexe, le bras était trop exigu pour y loger un autre amortisseur que la m…e d'origine.
Le choix s'est porté sur un amortisseur Ohlins TTX36, j'ai donc dessiné un bras modifié et de nouvelles biellettes.
Par la même occasion, je me suis penché sur les différentes actions/réactions qui engendrent les effets "squat et anti-squat" de la moto, au cours de l'accélération sur l'angle.
Et là, la conclusion m'a sauté aux yeux, les positions relatives des axes de bras et de pignon de sortie de boite, n'étaient pas compatibles avec les effets attendus.
Merci à JBB et à Stéphane MEUNIER pour leurs explications, qui m'ont permis d'arriver à ce constat.
Donc, le problème se corsait sévèrement, puisqu'avec ce put… de moteur FPE dont la fixation arrière est montée coïncidente avec l'axe du bras, il n'était pas question de faire varier la position du pignon de manière satisfaisante.
La solution serait de dissocier la fixation moteur de celle du bras, mais cela entraîne des modifications importantes qui ne pourront être faites dans le délai imparti, puisque la moto est engagée en Klass GP au Castelet lors de la Sunday Ride.
On est donc en train de regarder une solution "moins mauvaise" pour la partie arrière et de finaliser le montage de la nouvelle fourche.
J'ai dessiné et fabriqué la nouvelle colonne en ajoutant la possibilité de modifier l'empâtement et éventuellement la chasse, par interposition de cimblots entre colonne et roulements.
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La moto étant à St Michel sur Orge, contact a été pris avec "Panerai" pour l'implantation de la nouvelle pièce.
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Pendant ce temps, le projet continue.
A suivre…

fpayart a écrit:

... je me suis penché sur les différentes actions/réactions qui engendrent les effets "squat et anti-squat" de la moto, au cours de l'accélération sur l'angle.
Et là, la conclusion m'a sauté aux yeux, les positions relatives des axes de bras et de pignon de sortie de boite, n'étaient pas compatibles avec les effets attendus.
Merci à JBB et à Stéphane MEUNIER pour leurs explications, qui m'ont permis d'arriver à ce constat.
Donc, le problème se corsait sévèrement, puisqu'avec ce put… de moteur FPE dont la fixation arrière est montée coïncidente avec l'axe du bras, il n'était pas question de faire varier la position du pignon de manière satisfaisante.
La solution serait de dissocier la fixation moteur de celle du bras, mais cela entraîne des modifications importantes qui ne pourront être faites dans le délai imparti, puisque la moto est engagée en Klass GP au Castelet lors de la Sunday Ride. On est donc en train de regarder une solution "moins mauvaise" pour la partie arrière et de finaliser le montage de la nouvelle fourche.

Good morning Francis, I remembered having a similar problem, so I dug up an old design that may stirr your brain. Instead of the fixed swing-arm shaft position I bolted an exchangeable saddle to the rear of the engine.[Vous devez être inscrit et connecté pour voir cette image]

Hi Frits,
That's almost what I drew for the future FPE bike. 

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Hallo,

@fpayart: Thank you for your reply. I can supply that to my headstock i constructed. It´s wall thickness in 7020 alloy is 10mm around the bearings and 4mm between. I think there is enought material to put in some excentric bushing to adjust the steering angle +- 0,5°.
What is the material thickness around the bearings on your headstock.

The swingarm pivot should be adjustable, i plan to have the  swingarm mounting point and the engine retainer in one part to save weight and space, as the tandem engines are very long.
On your picture i can`t spot a swingarm pivot. Am i wrong? On your bike, what is the mounting angle of the engine?
Best regards,

Bernd

Hereafter more details:

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That looks really interesting. You want to use a lever system instead of a conventional fork. You don't use other "frame" parts than on the drawing?
Interesting for me would be the angle you mounted the engine in the frame.
You should have had the same problems, to get the long engine into an Aprilia RS frame, than me.
One solution could be to give an angle to the engine to keep the front exhaust away from the tire.

If you got other pictures/drawings, please post them. I like your work very much

Best regards,

Bernd

Yes indeed, it is a motorcycle with an alternative front suspension, strongly inspired by the JBB.
Exact, the chassis is reduced to its simplest expression.

In the Aprilia, we had to strongly tilt the engine to make room for the exhaust.
This image represents the motor in position in the bike, you can measure the angle.

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Here is a solution to separate the motor attachment from the swingarm pivot. With a possibility of altitude adjustment.
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Hallo,

thank you for your very usefull pictures and information. Quite interesting details on your drawings for the future bike.
With which ECU you control the powerjet on your carburettors?
The altitude adjustement is easy to realize your way. I will adapt that though my frame will be more the conventional way.
Best regards,

Bernd

RTD, MOTEC or other

Francis,  As you know, I had already been inspired for my frame design by the sketches you sent to me.  The way the design has developed for me is still in 2D form as i gain knowledge of AutoCad 3D.

I am restricted to using the Rotax 256 engine and the frame is aluminium spars modelled on the MR Kobas.

My solution is to maintain a fixed pivot point for the swing arm and move the engine to achieve 2 things:  the desired position of the sprocket relative to the swing arm; and, a longer swing arm (the pivot point moves forward by 13 mm).  The frame spars are forked (in effect - can't show how without a drawing) at the rear on each side to hold the swing arm pivots. A pair of small steel plates with pins facing into the centre push into the 256 engine's single mounting point. The plates then bolt to the left and the right inner forks of the frame spars.  The engine mounting plates will allow adjustment vertically + or - 2 mm (choose between one pair of bolt positions or the other).  If the "designer" finds in testing that the plates do not allow the "correct" engine position the small plates could be easily manufactured to allow a different engine position in the range of 5 mm below the position calculated as "correct" and > 10 mm above (were the engine will clash with the rear damper).  Achieving rigidity for the swing arm mounting and the engine mounting while allowing the engine to be installed without a large hammer will require a wedged/ground-to-fit plate to take up lateral play between frame and engine.

I have been comparing what seems possible with the 256 engine and Francis' drawings. For the same 1362 mm wheelbase, I have a 527 mm swing arm length (Vs 545 mm). However, that is for 18" wheels for historic racing. If I use 17" and the adjustable head stock mount that moves the steering head 15 mm rearward, the swing arm length would be 539 mm. Maybe the "too long" 256 engine will not be such a disadvantage.

A small sketch, even in 2D, would help to understand your description.
Varying the position of the motor moves the exhausts at the same time. 

Hallo,

there was progress with my headstock. It´s nearly finished.
It´s made for adjusting the head angle with different inserts. The seats and the slits will be made after welding.
The structure behind will be lightened with several windows next week. It will be very stiff and got enought edge lenght to be welded properly.
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Best regards,
Bernd