Bimota Forum

[steff_dk]

Hi all

As mentioned in the other thread What is the front hub made of? I am currently under way with a new project.

To steer clear of trouble I will just call it a hub-centre steering motorcycle project although it is probably clear from where I am taking my design cues ...

I currently have the hub produced and in hand. 7075 aluminium.
I will keep you guys posted here and I would really appreciate some feedback, suggestions and helpful criticism of the design decisions as we go along.

Here is the hub - probably a wee bit smaller than on the Vdue prototype.


Due to this forum periodically taking a tech-refresh hiatus I have chosen to put some of this content on the blog in my signature as a back-up solution.

Thanks all!
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jsteffensen.blogspot.com

[steff_dk]

The next part of the project is designing an axle.

I have taken a lookat the Bimota one which looks like it is welded together from three tubes.

The ISR model here https://www.isrbrakes.se/products/ns2/ seems to be bolted together which I don't really like although I am sure it is strong enough.

I am trying to see if I can get it milled from one piece of metal.

I am looking at alloy steel or titanium.

I have sent this drawing to the manufacturer for a quote to make it from solid titanium.




The design has to have two faces that interface with the bearings and also have to fit the ends into the front swinging arm in a way that can withstand the braking forces that will try to rotate tha axle itself in the swinging arm.
Hence I have gone with a square shape (27 x 20 mm) to resist the rotation.

The upside of using titanium is that it will be light enough (482 grams) that I will not have to hollow it out which would be labour intensive. Also corrosion is not an issue as it would be with steel.
The downside is obviously the 5x material cost.

The weight of the axle as drawn here in titanium is roughly similar to an alloy steel tube of the same length - 22mm diameter and 5 mm wall thickness for comparison.

If I were to design it in steel I would have to make a design that takes away some material inside the axle as well. I may try to get a quote for that too.

Here is a comparison of the properties:
Titanium
Tensile strength 950 MPa
Yield strength 880 MPa
Modulus of elasticity 113.8 GPa
Hardness 334 HB
Density 4.47 g/cm³

Alloy steel
Tensile strength 1130MPa
Yield strength 715MPa
Modulus of elasticity 210GPa
Hardness 335HRB
Density 7.85 g/cc
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jsteffensen.blogspot.com

[steff_dk]

The actual position of the axle and hub once in the bike is upside down relative to the above picture - meaning at some point this will be flipped 198° in the drawing to give an 18° rake.

Drilling the axle through with a 10 mm hole wasn't excessively expensive so I will go for that. Added some chamfers too because I hear this is what separates us from animals ...

I decided to extend the axle length to 240 mm after drawing it out on paper. Weight of the axle in Titanium (6Al-4V) is now 450 grams (vs 798 grams in steel) and wall thickness at the thinnest place is 5 mm.
So still less weight than a 22 mm steel alloy tube for comaprison.

I knew this was going to be one of the more expensive parts because it has to take a lot of force during braking and bumps while still being as light as possible being part of the unsprung mass.

Can't wait to get hold of the finished part!
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jsteffensen.blogspot.com

[steff_dk]

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jsteffensen.blogspot.com

[atomibart]

Very interesting !

[Anders]

[steff_dk]

I have been holding off from ordering the axle because I wasn’t sure about the length.
I figure the axle length will be dictated by the degree of steering movement and the spacing between the brake discs. I see smaller bikes go with a single brake disc but I will go with two even if I am not sure what the final bike will be.

The freedom of steering seems to be around 21° (42° lock to lock).
Not a whole lot but in reality only a problem if you are moving it around the garage and I don’t want to trade axle dimension off for more movement.



Next problem I have been sort of pushing in front of me is to solve the problem of getting a wheel.
The outer diameter of the bearing it has to sit on is 85 mm.

I see three options:

1) Making my own wheel hub and making a laced wheel – like on the Honda Africa Twin. Think it would be possible to cut the laces to length and make it work. However, this style is best suited for a café racer type bike which was not really what I was going for.



2) Assuming I win the lotto this week is to call up BST and ask if they would build me a custom one-off carbon front wheel if I sent them the wheel hub it should be built on.
I will not even bother as it would surely run in the thousands – and this is after all just a fun little hobby project.

3) Find a donor bike wheel and modify it. Potentially the best and cheapest solution so I began the hunt for a bike with a large hub.

I did not want to go the route that ISR and Stelland Egeland went with the “Harrier” and cut out the hub of a donor bike wheel.
Although it is a crazy cool bike I prefer the hub, axle and outer wheel hub to be one-piece rather than bolted together – and it is a nice design challenge to work with as well.



At first I looked at Aprilia wheels since I am a sucker for Italian bikes (Italian everything to be honest). Couldn’t find anything big enough. Same for Ducati. So started looking at other brands.
In my endless scrolling through eBay I came across some Yamaha R1 brake discs that seemed to have a fairly large inside diameter of the rotors. It turned out that R1 and R6 have a large center part of the wheel although it has this weird almost ball like shape.

Then I found a Yamaha Fazer FZ6 RJ14 wheel...

It was similar to the R1 and R6 in hub size but it actually looked like the hub was more cylindrical than ball shaped. Got in touch with the guy selling a set of front and back and the wheels are in the mail now and should arrive Monday 4 December …



If they are good I will start looking for brake discs etc.
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jsteffensen.blogspot.com

[steff_dk]

So the wheels arrived for my ... project.
Just realized the bike hasn't got a name yet.

Wheels looked to be in good nick and luckily the guy who sold them had packed them very carefully.

I was anxious to see what I was working with so I popped on my overalls and stache and went to work with the angle grinder.



That is Tamburini himself btw - the guy who put the "TA" in Bimota ...

So after cutting a nice little Christmas star in one side I got the hub opened up.



The front hub was much wider along the axle than expected.



The hub diamater however is a generous 125 mm so no problem getting the hub-steering mechanics packed in there somehow.





The material removed with the angle grinder came out to around 500 grams.



The bearings I will add will come to an additional 2 x 127 grams + 2 x 55 grams = 364 grams.

I have had multiple contraptions thought out in my mind how I could mount a fixture on the rim and mill it to size myself but in the end I decided that it was snowballing into it's own little side project.
A thing called "Yak shaving" in the software industry



I saw the warning signs and decided to find the nearest motorcycle shop who kindly directed me to a local machine shop that could help me mill it to size.
I will reduce it to the same width (72 mm) as the inner hub meaning that about 25 mm has to come off each side.



Weight wise I am still fairly certain it will weigh in a smidge less than the original FZ6 front wheel which I am pretty happy about.
The machine shop was backed up with work until first week of January so I am in for a bit of a wait again but I have some thoughts bumbling around in my mind to re-design the axle and front swinging arm axle receivers (again) but that will be the topic of another post.
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jsteffensen.blogspot.com

[Anders]

Very interesting!

How do you plan to do the interface between the cut-away hub and the 85mm bearings? I guess you have to make (turn) some sort of adapter. Will you weld it in place (I see that Tamburini wears a welding mask) or use some other way of keeping the parts together?

I must say that this is a project that really appeals to me. I have always been facinated by "funny front ends" as Tony Foale calls them. A Tesi has been a long time dream, as has the V-Due. I have now finally accepted that both will be just that for me, a dream.
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Anders
bb1, Mille S2, Guzzi / Watsonian, Morini Corsaro 125, EC200, RD350YPVS etc

[atomibart]

Cannot wait to see your next update !

[steff_dk]

[Anders]

[steff_dk]

One of the things I had initially thought I would try instead of just copying Bimota was to make an axle that did not rotate in the front swinging arm such as the Tesi.



This would negate the need for a lever and arm but at the same time it would not make the rake adjustable nor keep the rake constant when the swinging arm moved upwards for road bumps etc.
I figured the effects of the change in rake during riding would be neglible anyway plus it would make the design a bit more sleak without the need for that lever and arm. To stop it from rotating I simply made the axle square and as wide as I could possibly fit it into the inner hub and still preserve enough room for movement.
Another thing I would like to try was to make the front swinging arm parallel with the ground - simply for the cool aesthetic look of it ...

Looking at other designs they all grip the axle in a 90° angle (like my above post from 26 Nov) but after realizing that my steering movement was 21° to each side it slowly came to me that I could just grip the axle directly at that angle since the axle is not rotating.
I opened up the drawing of the axle receivers and brackets and rotated the through hole accordingly.



Next I opened up the axle drawing and cut the ends of in a 21° angle and imported it into the assembly drawing to admire the flush cut-off ends .....





Not very flush huh!?

And this was where my lack of mechanical engineering experience was staring me back in my face ... haha Because when you rotate a square along one axis and then rotate it again along another axis you end up with this funny phenomenon.



So after a few (too long) moments of total brain-f*ck I opened up the axle drawing again and got the ends cut off the right way









Everything should be at the right angles now. The axle receviers grip at 21° but effectively they could have been bent only 20.1° but such is learning. I suppose this project should be called "The Mentor".
The extra 0.9° is the tuition fee. I hope we can keep this secret between us. It cannot be changed because those parts were sent to production 5 days ago and should finish right around Christmas. It will be produced in 6061 aluminium.

The grade 5 Titanium axle has passed review at the CNC shop and that order will be submitted today.

Then, come January, parts should begin to arrive with me again along with the wheel.
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jsteffensen.blogspot.com

[steff_dk]

Brembo Serie Oro floating discs arrived today.



Snapped a picture and put as background in Fusion 360.







This was the first actual stock part I had to draw that was not my own design.

Still waiting for the milling job on the front wheel so I can start taking measurements and get it all fitted together.

Have some Brembo calipers off a 2015 MV Agusta F3 675 in the mail that should arrive any time now ...
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[hindsight]

Amazing work - I'm super-impressed at what you're doing. I've been using fusion360 myself recently, so it's great to see what it's capable of beyond the little things that I've been producing.

Really looking forward to seeing how this progresses!

Mike