[Technique] JBB for 80cc Bike

Hello there,

now we try to built our next chassis with an JBB front suspension. I have smoe pictures of the first drawings and a question. We will chance the drawing explizit for lower weight because at the moment it will be to much of it.

The Question is about the offset. I really know why on a standart fork have to be a offset, not only for the geometrie of the overrun...also for the fork dynamics.

But i dont understand why every JBB Suspension have a offset in center of the Wheel...the center is no center

With all the things we can do with the geomtrie of this Suspension we can built it with the Ball in the middle an its easyer to built. Mayba there is smomebody who have an Answer.

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Grieützie mitanand...Jannsen

Interesting project!

I think that you have found the right place to get some answers..

Here are the newest drawings...not perfekt but much lighter then before.

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JanSchäffer a écrit:

But i dont understand why every JBB Suspension have a offset in center of the Wheel...the center is no center

I also wondered about this, and JBB tells me it is because he tried with no offset and it didn't work as well. It is hard to argue with that!

You must remember that the angle of the steering axis also contributes to the dynamics of the steering: with a more horizontal steering axis, the weight of the bike makes the wheel want to fall to the side. However the trail will try to pull it straight again when the bike is rolling at reasonable speed (and so the two make a nice perturbation-correction system that can potentially oscillate, but that is another story).

To be able to adjust independently the trail and the steering axis, you must have offset. With the pivot in the centre, you must have a steering angle of 16.7° to have trail of 90mm, for eg.

On the Vyrus page on facebook, the guy from Vyrus told me the Vyrus has adjustable offset, but he said other things that were plain silly, so I don't believe him

OK, thanks for the answer. It did not tell me how to do but there are any things i have to think of.

My friend who make the drawings had today a lot of time so he did any more work on it. At the moment i am happy and i think the first hub will look nearly like the following picture.

Maybe there is sombody who can tell me more about experiences with the JBB system.

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Why don't you have a look there :

JBB tech

I think your ball bearings will be hard to put between the blue and the purple part.
And then, how will you remove the wheel ?

It is not going to be an endurance racer, so quickly changing the wheel is not that important.
Maybe you could divide the purple part through the middle. Then you would not need any spring rings to lock the bearings in place, not even the center sphere. I do not like the sharp-edged grooves that those spring rings require. And I feel unhappy about the narrow collar outside of the spring ring that holds the sphere in place (it is a lot narrower than in the first design).

Yes...it is "narrower" but that was my will.

Hey Ed...i have took a lot of looks at this side, but there are a big problem...the language!

Maybe it was you who answered my e mail for the same question and this was your answer:

This is the same with a standard telsecopic fork. You will see an offset between the steering axle and the wheel axle.
It is necessary to put this offste to get the right values for trail and caster angle. The geometry is designed with all parameters.

Maybe you remember.

But this answer was not enough for me to built it like this. With the question in this board i hope to become answers thats more comprehensive.

Jannsen

Frits Overmars a écrit:

It is not going to be an endurance racer, so quickly changing the wheel is not that important.
Maybe you could divide the purple part through the middle.

Ed, un truc qui tourne dans ma tête depuis un moment... vous avez pensé à faire une JBB symétrique avec un disque de chaque côté ? Evidemment cela compliquerait les chose pour le changement de roue, et il faudrait une porte-moyeu de deux pièces qui se boulonnent, mais on ne serait plus obligé d'utiliser de frein automoteur qui a l'air assez lourde...

But not at all relevant for an 80cc, so carry on :)

GrahamB a écrit:

Ed, un truc qui tourne dans ma tête depuis un moment... vous avez pensé à faire une JBB symétrique avec un disque de chaque côté ? Evidemment cela compliquerait les chose pour le changement de roue, et il faudrait une porte-moyeu de deux pièces qui se boulonnent, mais on ne serait plus obligé d'utiliser de frein automoteur qui a l'air assez lourde...

Concernant le system JBB symetrique, je m'étais penché au peu sur le sujet et j'avais pondu [Vous devez être inscrit et connecté pour voir ce lien] à l'époque. ça vaut ce que ça vaut, mais ça remplit la fonction :)

JanSchäffer a écrit:

Yes...it is "narrower" but that was my will.

Maybe you remember.

Jannsen

I remember your e mail. We will try to prgress in english.
You do not want any offset in your hub. I don't think it's a great idea.
Anyway, you should say first which trail and caster angle evolution you want for your bike.
Then it will be possible to draw or calculate and see if your geometry is good and possible to built.

Even if your motorcycle is not a endurance racer I still thinking it's necessary to remove the wheel easily sometimes...

Graham,
I would like to see a symetrical JBB mechanism too.
You will see a nice try from SDVS on pit-lane (I have to find the link).
But sometimes you have to make compromises. And I think that Jean-Bertrand made it very well, more than 30 years ago.
If you want to put 2 discs it's very difficult to manage room enough for the steering angle.
This parameter is easier to achieve with a central single disc.

Here is the Metiss mechanism :
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There is another mechanism, built for the "Atomo 2002". It's lighter then this one. The wheel is held by 4 bolts.
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sdvs a écrit:

et j'avais pondu [Vous devez être inscrit et connecté pour voir ce lien] à l'époque.

Not bad for a "premier jet" !

JanSchäffer a écrit:

...I dont understand why every JBB Suspension have a offset in center of the wheel...

GrahamB a écrit:

I also wondered about this, and JBB tells me it is because he tried with no offset and it didn't work as well. It is hard to argue with that!
You must remember that the angle of the steering axis also contributes to the dynamics of the steering: with a more horizontal steering axis, the weight of the bike makes the wheel want to fall to the side. However the trail will try to pull it straight again when the bike is rolling at reasonable speed (and so the two make a nice perturbation-correction system that can potentially oscillate, but that is another story).

Ed a écrit:

You do not want any offset in your hub. I don't think it's a great idea..

As far as I can see, Jan Schäffer is not opposed to an offset; he just would like to understand why it is necessary.
JBB's reaction is of course thankfully accepted, but it does not give any insight in the matter. Graham's explanation makes sense to me, but I really would like to hear from JBB or Ed if that is the fundamental reason for using an offset in the JBB front end.

Voilà, c'est parfait ! L'auteur SVDS nous rappelle le lien vers sa [Vous devez être inscrit et connecté pour voir ce lien].

About this famous offset. I said, as Graham, that it's necessary to get the right angle and the right trail (and the right evolution along the suspension travel).
But it would be good to observe the contact between the tire and the ground, in a spatial model.
I never tried to make the drawing but I suppose it's different if there is an offset or not.

I remember (as JBB does) that the first TESI was very unstable (the journalists wrote that).
I went to ASSEN and I saw the Vyrus and Rodorigo Ascanio. His motorcycle works well on the track.
I did not see any offset on the Vyrus. So ?

I think you have to choose your geometry and draw all parts before giving a conclusion. Then you build and you test the bike !
I like theory but I'm not sure to be able to understand everything at the begining of the design.

@JanSchäffer: you can construct an offset of maybe 20 mm in your purple part and choose the bolt pattern so that the whole purple part can be fixed in two positions: with the offset effective, and rotated 90° around the hub's axis so that the offset 'disappears'. You would have to adjust the length and position of the upper carrier arm to preserve the desired trail, but you would not have to fabricate two different hubs.

A drawing to illustrate what Graham said :
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Then you have to imagine or draw (3 dimensions) what happend when the steering is working...
Like this :


Do you see the difference ?
But you also see that offset / trail / caster angle are dependant parameters. So if you choose a trail (90 mm) and an offset (0, 15 or 30 mm) you will get an angle and only one.

I'm always impressed by Ed's ability to whip up a video

I'm not sure it's just the way the tyre touches the road, since in principle that can always be determined from tangency and is a function only of the orientation of the axle. What I suspect matters is how the contact patch moves relative to the steering axis, and the CoM of the bike moves relative to the ground, as the steering moves.

Another consequence of offset is to change the dependency of trail on tyre diameter. This is importany since the more you lean, the smaller the tyre becomes. If you had zero rake and negative offset, the trail would not change, whereas with non-zero rake lean angle reduces the trail.

I find it quite hard to really understand what is going on: for one thing, the differential equations that describe the motions around the equilibrium point are fundamentally non-linear... and simply visualising the whole system is quite a challenge.

Many thanks Graham. I would like to be able to describe with a video all the interesting things you are explaining.
you are right about the size of the tyre when it leans. It is the same with the rear tyre in another topic about "Marquez power".

Ed a écrit:

JanSchäffer a écrit:

Yes...it is "narrower" but that was my will.

Maybe you remember.

Jannsen

You do not want any offset in your hub. I don't think it's a great idea.
Anyway, you should say first which trail and caster angle evolution you want for your bike.
Then it will be possible to draw or calculate and see if your geometry is good and possible to built.
.

Attention:
quelque soit le type de train avant d'une moto, les lois géométriques pour l'épure de direction sont les mêmes.
Ce qui veut dire qu'il n'est, à mon sens, pas judicieux de s'écarter des valeurs de référence. En effet même si une moto légère sera moins affectée, un déport nul obligera à beaucoup diminuer l'angle de chasse, ce qui rendra la moto très sensible aux changements d'assiette et donc pointue dans son comportement (instable).
Un déport nul certes permet d'utiliser des roulements de roue plus petit mais même seulement une réduction du déport affecte la conduite.

Pour terminer une anecdote: Dans les années 80 Tomkinson me rendit visite et je lui présentait ma 250, son regard se portant sur le moyeu de roue avant, il me dit "vous n'avez pas fait le même érreur que moi vous avez mis du déport"

Hey...here is a lot of traffic. Thank you very much for all the answers, its great to write and read here.

I can see the difference in the video and i think i understand what i can feel of it at the handlebar. There are really a lot more parameters that are important to built a system like this. I hope i can learn here more of it.

the overrun should be about 95mm in cero position and i want that it have to go back to 65mm without wheelbase changing. I dont know "the best" parameters for it, but my experiances of the past say that this could be good for the tracks i want to ride. The weight of the bike is about 65 kilogram and i built the whole frame by myself, so i have a lot of place for playing. The wheelbase will be about 1,20m.

In the past we have driven some street bikes an race tracks, that was ok. But last winter i built my first complete Bike with its one frame and it works so much better than the other ones in my class. I dont think its perfect but now i really understand how important the chassis is for laptimes...years ago it was all about horse power.

Best regards, Lucky Jannsen

Hi Jannsen
What is your front wheel travel between these values of overrun ?
100 mm ? 120 mm ?
I give you a small sketch of a JBB...
This is just an example. You can change many things.
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JBB = Open Source...

Three years ago, I built a Di Fazio hub (Offset = 0). The wheel was 12".

Drawing and Photo :

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Only one bearing (two rows). hub : two parts bolted.
Yes, Ed, it wasn't easy to take off the wheel.