Frame Geometry

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johnu
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Frame Geometry

Post by johnu »

Ok so trying to lay the tr5mx frame geometry out in autocad and not having much success. Can anyone tell me if the bottom frame rail is supposed to be parallel to the ground when set up properly or should the cylinder base be parallel to the ground? I have the bike on the work stand right now with the cylinder base parallel to the ground and it's giving me a crazy 30* of rake!!!
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Re: Frame Geometry

Post by minetymenace »

johnu, this doesn't answer your question, but have you seen this?

I do hope that once you have drawn the frame in CAD you will share the files here for the benefit of all... <038
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Re: Frame Geometry

Post by johnu »

minetymenace wrote: Thu Mar 26, 2020 9:13 am johnu, this doesn't answer your question, but have you seen this?

I do hope that once you have drawn the frame in CAD you will share the files here for the benefit of all... <038
I do have that drawing yes but there are a couple of crucial dimensions missing. That drawing shows the bottom frame rails as being parallel to the ground which would then give me 27* of head angle on my bike. I am in the process of taking dimensions from my bike to layout in autocad. I will set it up so I have 27* of rake then take dimensions. I also have a partial drawing from frank the welder on here which has an acurate layout of the engine mounting holes. FYI my bike is a 74 tr5mx which I assume is the same as the b50 drawing <201 And yes no problem sharing drawings.
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Re: Frame Geometry

Post by madsundaysurvivor »

I don't know if I understood your query well, but you must keep in mind the fork suspension snicking, with the springs loaded, this changes the rake.
you can find a lot of information about this in the books of tony foale books
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Re: Frame Geometry

Post by Shawn_Mc »

madsundaysurvivor wrote: Thu Mar 26, 2020 6:51 pm I don't know if I understood your query well, but you must keep in mind the fork suspension snicking, with the springs loaded, this changes the rake.
you can find a lot of information about this in the books of tony foale books
But that isnt the way things get setup on the drawing. Normal suspension sag is a real life setting where drawings typically get made with suspension at full top out of stroke.

Have you ever used the Tony Foal suspension Geometry Software? It's pretty cool.

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Re: Frame Geometry

Post by johnu »

Shawn_Mc wrote: Thu Mar 26, 2020 7:22 pm
madsundaysurvivor wrote: Thu Mar 26, 2020 6:51 pm I don't know if I understood your query well, but you must keep in mind the fork suspension snicking, with the springs loaded, this changes the rake.
you can find a lot of information about this in the books of tony foale books
But that isnt the way things get setup on the drawing. Normal suspension sag is a real life setting where drawings typically get made with suspension at full top out of stroke.

Have you ever used the Tony Foal suspension Geometry Software? It's pretty cool.

https://motochassis.com/freeware/
I know eventually sag will come into effect but as you say there has to be a starting point. My starting point will be with the suspension topped out like you mentioned. I use John Bradleys "The Racing Morocycle" vol 1&2 for reference. I will look at that Tony Foale software, sounds interesting :thumb Right now I am just trying to get everything drawn up as the frame and swingarm were built then see where I'm and go from there.
I appreciate you guys interest and comments.
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Re: Frame Geometry

Post by madsundaysurvivor »

I agree with you, but I just wanted to say that if you draw the lower tube parallel to the ground, with the fork at the beginning of the stroke, in practice the chassis will not be parallel and the angle will be less than the theoretical
Sorry is dificult explain me in english
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Re: Frame Geometry

Post by johnu »

madsundaysurvivor wrote: Thu Mar 26, 2020 8:01 pm I agree with you, but I just wanted to say that if you draw the lower tube parallel to the ground, with the fork at the beginning of the stroke, in practice the chassis will not be parallel and the angle will be less than the theoretical
Sorry is dificult explain me in english
No I understand, thanks!
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Re: Frame Geometry

Post by Canberra »

On the BSA frame drawing the datum is through the center of the swinging arm pivot and is parallel to the lower top tube. In no way I would consider that is parallel to the ground. I assume all B50 frames are same. In that case that datum would never relate to the ground as the MX has 21 inch front wheel and the SS 18 inch. I believe the MX and SS forks are identical unless the triple tees vary to alter rake.

Maybe someone can confirm that last point.

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Re: Frame Geometry

Post by johnu »

Canberra wrote: Fri Apr 03, 2020 6:22 am On the BSA frame drawing the datum is through the center of the swinging arm pivot and is parallel to the lower top tube. In no way I would consider that is parallel to the ground. I assume all B50 frames are same. In that case that datum would never relate to the ground as the MX has 21 inch front wheel and the SS 18 inch. I believe the MX and SS forks are identical unless the triple tees vary to alter rake.

Maybe someone can confirm that last point.

John
Hi John thanks for your reply.
The horizontal datum line as far as I can tell runs parallel with both the bottom frame tube and the lower top tube. The bike would have to sit as drawn for it to have the 27* head angle. I understand what you are saying about wheel sizes affecting how the frame will sit although a 21" front and 18" rear with tyres on are almost the same diameter. What I am trying to do is build my bike up to sit with as close to 27* head angle as possible. There is no way these bikes were ever sitting at that head angle from the factory as far as I can tell. As far as your last point I can't see how different triples would make a difference to rake <201

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Re: Frame Geometry

Post by FranktheWelder »

I can describe the drawing I sent to johnu. I took my B44 engine cases and located the bolt holes with a CNC mill. When gathering the points, I made the barrel mounting surface horizontal. I collected the three motor mount bolt positions and a very approximate envelope. I then created a new drawing from the BSA B44 drawing showing motor mount bolts, BDC of head tubes, swing arm pivot, shock mounts and a horizontal line representing the approximate available height needed to remove the rocker box using bolts. Using the BSA drawing as a platform I brought in the engine bolt locations as tried to fit it in the BSA drawing and it didn't fit. I was very thorough and confident so I located a single bolt hole and rotated the engine drawing 2.5 degrees and it fit perfectly. I think it was off Y.005" and X .013".

Once that was done, I drew the lower cradle/upper tube horizontal and designed a three piece head tube that uses tapered bearings that fit my Spanish forks.

minetymenace wrote: Thu Mar 26, 2020 9:13 am

I do hope that once you have drawn the frame in CAD you will share the files here for the benefit of all... <038
[/quote

The drawing johnu has may have mistakes. It's really construction notes and all drawn in Autocad 2000 and in 2D. I am fine with sharing as long as people understand it hasn't been proven. Sorry about botched edit.
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