Shock design ?

[kylejamers]

The frame has identical axle paths (63 degrees). It uses tires that are 30" (approx.).

My only real preference is that the rear suspension be linear (or mostly linear).

I want to know how you guys would incorporate a shock into this design.


Kyle

[adarn]

What is the white tube connecting the BB to the pivot?

[kylejamers]

It is a tube, it supports the pivot shell. Pretend it doesn't exist (if you like).

[dr.welby]

We'll assume you have a really, really good reason for a pivot that high, so I'd google:

Paramount Buell SASS Bicycle
GT RTS

[kylejamers]

The SASS frame is funky.

I am not set on the pivot height. Just like the idea of mimicking the fork's travel, as a design exercise.

[Eric Malcolm]

Ok, trying to get my head around your original drawing and this one. You have not really expanded on your explaination well.

Assume the wheels are 29" to = 30" Dia? And 405mm theoritical ChainStay?
That pivot to BB tube is confusing. You would be better to curve the lower end of the S/T to achieve the support the BB requires. * EDIT: If this meant to be an attached BB to pivot via those arms, the drawing is confusing me by showing it connected to the DownTube. We cannot see your suspension action drawn this way.

Assume that this is a top pivot system that will need a beefy T/T to carry the load as it is moved forward from the better position of the T/T and S/T junction.

And you would like to know where to fit the shock? Because this would function in an Elevated Chainstay, single Chainstay/Seatstay combo = Extra beefy pivot to axle set-up.

Do I understand this correctly now?

If you want to build this up, more study of Zerode will be needed, its very 'out there' but not impossible. But as Walt points out, this leans to the experienced adventurous builder than the first attempt and hope I get it right type.

Anyway. I suggest that you render a second drawing correcting what we see, as the sale is not working well for me.

Eric

[kylejamers]

Didn't think any of this info was necessary, but here:

It is a 29er Jackshaft bike. The axle paths are identical (63 degrees).

The pivot shell will pierce the TT.

It will use a floating brake too (which has been left out of the draft)

The frame has been designed this way to allow the shortest chainstay length possible, a Full length Seattube, and 7" of travel. There are no interference issues with the chain, brake, tire, seat, etc. (although there are two streamlined tubes that run from the BB shell to the pivot shell in my design, that I've left out of this draft - for visualization)

So far, I am reasonably satisfied with the design, and would be glad to leave it unchanged. However, developing a quality rear suspension design may make that impossible - we'll see.

That's why I am here! (and made two threads, and confused everyone)

[adarn]

OK, I see what you're saying.

I just think that for designing a FS, if you have particular traits in mind, you should put those things first.

So in this case, put your shock where ever you'll have the most linear ratio. I would start with that, find the points you need and build your frame to match as close as possible. It looks like you could find a place on the DT that would allow this.

[Eric Malcolm]

Thanks for presenting a clearer picture.

Now, there is a serious problem for your drive train. How do you propose to deal with a fixed BB and a rear axle path moving away from the BB? The chain length is stretching and shortening on every movement of the suspension.

Eric

[dr.welby]

And if you're really going to have a single swingarm, then maybe this might give you some ideas. I also recommend that you put SketchUp away and just work with paper sketches. Your design is way out there in concept territory and trying to make it look real is just going to work against you. You've bothered to show a relieved head tube, but we have no idea where your jackshaft is, and whether it's really a jackshaft or an idler or an IG hub.

[shirk]

Pivot on the top tube??? You need to read about things like chain growth. Unless this is going to be a run bike with no drivetrain you're going to have serious issues.

If you want to build a full suspension bike copy a Santa Cruz Heckler. It's a proven design and the least complicated to make.

[kylejamers]

Thank you for the responses guys.

No worries about chaingrowth, as this bike is jackshaft drive - meaning that a chain runs from the BB to the Pivot, then runs from the pivot (on a different cog, obviously) to the rear wheel.

dr. welby - The pivot is the 'jackshaft', even though the shaft won't spin since I intend to use a pair of cogs mounted on a bearing, while the shaft pivots on IGUS bushings. The bike will use a traditional derailleur on the rear wheel.

I would love to get one of those 'pull' shocks, but the only decent one is the (Fox Suspension) DYAD that Cannondale uses. It is also very expensive, and not intended for this sort of thing.

Generally speaking, there really isn't much room behind the ST (between the tire and the ST). Definitely not enough room for the shock, or any sort of linkage. Which leaves only the front triangle.

[xy9ine]

have a look at other jackshaft precedents. do you have room to stuff a shock & linkage behind the seat tube a la the brooklyn race link?



oops. just read your post above. hmm...

[kylejamers]

I've seen those designs (even spoke with Doc).

This bike uses a 30" tire and has 15.9" CS length, unweighted.

There is no way for a shock in that position unless the seatmast gets changed majorly, which I would like to avoid. If the seattube was floating like a Risse or something, then I could probably build something like that.

Like I said though, looking for a 400+mm seattube, at an acceptable angle.

[xy9ine]

i suppose a simple solution would be a lever extension of the swingarm forward of the pivot actuating a vertically oriented shock. could add a linkage somewhat like the current scott gambler if you wanted to get tricky.

[dr.welby]

Balfa BB-7

"The pivot point was originally placed 7" (about 178mm) above the bottom bracket (now you know why it's called a "BB7"). In later models this distance was increased up to 9" (228mm).

While most frame manufacturers place the pivot point according to the chain, Balfa placed the chain according to the optimal pivot location. This is done by a pulley routing the chain close to the main pivot to avoid chain tension problems. The rear triangle is then fully active and independent in its travel, giving freedom of movement unequaled to this day. This unusual chain line permits the application of the true high pivot and allows for a better rear end with ample travel and no feedback.

A high pivot point also makes it possible to achieve a rear axle path nearly parallel to the movement of front wheel."

[kylejamers]

I will set up designs with a simple actuation - like those you guys just mentioned.

[Jayem]

A high pivot point also makes it possible to achieve a rear axle path nearly parallel to the movement of front wheel."

Also makes it so your center of gravity will move while the suspension compresses (in a G-out). I don't know about you, but I'd rather have a constant relative wheel position relative to the CG, that way I don't have to change my position a bunch for maximum performance. Not sure why people think the same path as the front is such a good idea. Yes, a little initial rearward helps, but look at dirtbikes or any modern successful DH bike (where bump absorption is super important), no one is doing anything as crazy as this.

[kylejamers]

Also makes it so your center of gravity will move while the suspension compresses (in a G-out). I don't know about you, but I'd rather have a constant relative wheel position relative to the CG, that way I don't have to change my position a bunch for maximum performance. Not sure why people think the same path as the front is such a good idea. Yes, a little initial rearward helps, but look at dirtbikes or any modern successful DH bike (where bump absorption is super important), no one is doing anything as crazy as this.

Dirtbikes have terrible axlewrap, riders adjust pitch with the throttle constantly - hardly a fair comparison.

Also, this design allows the bike to have adequate seat to tire clearance, and maintain great straightline speed (which is sort of the point, with the 29" wheels and everything).

If you think the pivot's location should be changed, then where do you think it should be moved to?

[dr.welby]

Dirtbikes have terrible axlewrap, riders adjust pitch with the throttle constantly - hardly a fair comparison.

But he also says "any modern successful DH bike". With limited human power you can't Enzo Ferrari* your way out of a bad suspension design.

maintain great straightline speed

How do you know? Your design goal of having the wheels paths parallel is really arbitrary, because the front axle path is related to steering geometry. There's no real point in copying it.

You could also argue that a rearward axle path is actually slower, because on every bump the rear wheel slows down, and then has to be accelerated back to its original position in both translation and rotation. TANSTAAFL.

I'd also guess the pivot is too high and the bike will have too much thrust antisquat, but I can't tell without seeing a complete design.

And as already said, you don't see highly rearward axle paths in *any* performance offroad vehicles, whether they are DH bicycles or Baha trucks. The only application I know of are those are softtail muscle bikes that are trying to look like hardtails by hiding the shock under the engine.

If you think the pivot's location should be changed, then where do you think it should be moved to?

I think you should go buy Tony Foale's book.

* He supposedly once said something like "Aerodynamics are for people who don't know how to build engines".

[Walt]

Man, if this ever gets built, I want to ride it - by far the weirdest CAD proposal on the board in a while.

-Walt

[shiggy]

This bike uses a 30" tire and has 15.9" CS length, unweighted.

And about 20.8" at full compression.

Not a design I would ever consider, for so many reasons.

[kylejamers]

How do you know? Your design goal of having the wheels paths parallel is really arbitrary, because the front axle path is related to steering geometry. There's no real point in copying it.

You could also argue that a rearward axle path is actually slower, because on every bump the rear wheel slows down, and then has to be accelerated back to its original position in both translation and rotation. TANSTAAFL.

I'd also guess the pivot is too high and the bike will have too much thrust antisquat, but I can't tell without seeing a complete design.

And as already said, you don't see highly rearward axle paths in *any* performance offroad vehicles, whether they are DH bicycles or Baha trucks. The only application I know of are those are softtail muscle bikes that are trying to look like hardtails by hiding the shock under the engine.

Let's assume that I know nothing, have read nothing, didn't go to school for any of this, and don't get paid to do design like this.

Where do you put the pivot, dr. welby?

I see this as a hobby, and don't take it too seriously. Love bikes, but view their design as constant compromise. (Already read the book, and surprisingly enough, contacted Foale about the rear suspension)

[Walt]

Kyle, you can't come on with a super weird idea, ask for advice, and then just get defensive about every question. YOU have to tell us why you want to do what you're doing, and it's also helpful to really do a good job showing that you've done your homework by writing clearly and at length about your ideas. So really, we ARE assuming you know nothing about this. Prove you do, and also prove that you're not just wasting everyone's time and are actually going to build something.

Sketchup designs are a dime a dozen and you've got zero cred here (or really anywhere on teh interwebs) until you build something or at least do a good job explaining your ideas. Bike design is something that everyone should approach with some humility - because really, really smart people have been designing them for 150 years. If your idea doesn't exist in the market, there's probably a good reason so the burden of proof is on YOU to explain why what you're doing is going to be superior to something conventional like, say, a Heckler.

Maybe you get paid to design bikes or some sort of suspended wheeled things all day. Great. If that's the case, why are you asking us how to put a shock onto your wild sketchup drawing?

-Walt

[kylejamers]

That doesn't make sense.

It is my place to show the action of the bike, and ask that the users of this forum to reply about how to actuate the shock (which I have done).

It isn't my place to convert all the users to my ideas, impress them, teach them, or any of that. I don't want to do that, and it seems abundantly clear that would be unwelcome.

Again, this design is for fun, and from a program that I had never previously used. I apologize if you all expected more from a new poster.

It is a bit confusing why the response to this has been very critical, but entirely unhelpful, when that should be the whole motivation to operate a 'framebuilding' forum - to help people with framebuilding.