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PostPosted: Wed Apr 04, 2012 4:58 pm 
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was8v wrote:
Its easy to pick fault....so I'm not going to.

Why concentrate on "retro" designs and make the same mistakes. I would be looking at modern designs where big R&D budgets have optimised the pivot positions and axle paths over the years. To do something different will take a lot of R&D and prototype building, great if you have the capacity.

Remember to think about positioning the brake caliper to avoid "brake jack" , and the practicalities e.g. will the end of the shock catch my legs?


I'm using this design because I like the looks and I want to see if a more modern design will improve the old system.
I don't know what 'brake jack' is so could you elaborate :lol: And I was thinking about it getting in the way of legs yesterday so I'm going to try a few ideas out on the design.

merckx wrote:
the rear end is going to have a lot of pedal induced bobbing if the shock does not have some kind of inertia valve or propedal type valving ala fox..


It's going to have either a preload or lockout, if I can work out the lockout system I'll do that but at the moment I only know about preload.

MikeD wrote:
You're going to want bigger than 28.6mm for an aluminium main triangle. Look around at some production bikes.


I think I've only ever seen downtubes bigger than that. I might make that bigger though, not sure yet :P

Pierre wrote:
Had a bit of a scribble... hope this helps!


Believe it or not but I get it :D And I know how it feels when you can imagine but can't draw. It's been happening alot on this design so far :lol:

I'll work on a new pivot now and try and incorporate this.


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PostPosted: Wed Apr 04, 2012 5:43 pm 
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Glad it helped!

"brake jack" is what happens to some suspension designs when you apply the brakes: extra force is generated with a moment that either works to compress the suspension or extend it. It's not a good thing, and leads to unpredictability when braking.

"pedal bob" is kind of the other thing, when the force applied through the chain causes the suspension to compress or extend.

This is why Linkage is a really good program, I've heard - you can analyse how these forces affect different suspension designs.

I think an ideal suspension design would:

- have an axle path that keeps the wheelbase constant (or possibly even extends it as the suspension compresses, to keep the bike stable)

- have no brake jack

- minimise pedal bob

- behave in the same way whether a rider is pedalling or not, seated or standing (one reason the Klein Mantra is a terrible design - stand up and it more or less locks out!)

- have a lot of shock travel (but not necessarily a lot of rear axle travel), which minimises seal stiction and makes tuning easier.


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PostPosted: Wed Apr 04, 2012 6:48 pm 
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:shock: That's alot of potential issues! :lol:

A little bit of research and this is what I have.

Brake Jack; when the shock compresses, if the wheelbase shortens, the rear wheel would want to move forward but because the brake is engaged, it can't :?: Therefore, if the wheelbase stays the same, brake jack would be eliminated?

Pedal bob; when pedalling, the shock compresses and decompresses. Could be overcome by tuning the suspensions preload or lockout? Platform shocks were developed for this (?)
I don't know what a platform shock is, but could a platform 'fork' be made?


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PostPosted: Wed Apr 04, 2012 8:37 pm 
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'Platform', as far as I know, refers to a function of a shock (front or rear) where it has a gate-effect (Rock Shox have "floodgate" adjustment) so that either low-speed or low-force compressive forces are resisted, but the gate opens to allow high-speed or high-force compressions cause the shock to function normally and compress.

One of the best systems I've seen is Specialized's "brain" rear shock (licensed by Fox?) where a separate damping unit contains a ball bearing in an oil-filled tube - this 'feels' whether the compressive force is low-speed (like in pedalling) or high-speed (like trail shocks) and varies the shock damping accordingly, between more or less complete lockout (which eliminates pedal bob) and fast fluid springiness, when you hit big lumpy things and the rear wheel needs to track over them.

Brake jack is an odd thing that is often the cause of those "stutter bumps" at the bottom of short downhill sections of trail - the rider brakes hard, the rear wheel either lifts (and temporarily loses traction so the brakes lock up) or is pushed down (increasing the wheel traction so digging the treads in a bit more and possibly chewing up the trail). The wheelbase may stay the same, but if the suspension effectively compresses and lifts the back wheel off the ground, you'll still have brake jack.

I'm not a suspension design expert - most of my customers are road cyclists and commuters - but I've serviced plenty of forks and shocks, received training (and marketing hype!) from various manufacturers and suppliers and have a good working knowledge of how suspension systems work.

Keep asking, keep learning, keep refining! Best of luck.


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PostPosted: Wed Apr 04, 2012 8:40 pm 
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...meant to say, brake jack can occur even when the brakes have locked up the wheel (so prevent it rotating) because the bike may still be moving forward so the rear wheel may well experience forces trying to rotate it; as the wheel is locked up, those forces go to rotate the suspension design and "jack" it up or down.


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PostPosted: Wed Apr 04, 2012 9:01 pm 
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There's a fair bit of controversy about brake jack. You'll always get forward weight transfer under deceleration, which is why the front brake is so much more effective than the rear one. Your maximum braking occurs when the rear wheel's just lifting off the ground, ie all the weight's on the front. And when that happens, the back's going to be stuttering over the ground whatever bike you're on...

Never got on with the Brain until they made it so you could pretty much turn it off :)


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PostPosted: Wed Apr 04, 2012 9:36 pm 
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Ok so brake jack is basically too much friction on the rear causing it to jump?

And a platform shock is kind of like when you have the lockout engaged and it automatically disengages when you hit a bigger bump, but on a less powerful scale?

I'm starting to think I'm taking on too much :lol: Maybe I should just use a normal shock instead of a fork.


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PostPosted: Wed Apr 04, 2012 10:14 pm 
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The Brain shock is exactly as you describe. The most common setup is Fox's ProPedal, which is essentially an additional switchable low-speed compression damping circuit -- inputs from pedalling or rider weight shifts don't activate the suspension, bumps do.

Brake jack is defined as the rear end extending under braking. But bikes that in theory exhibit that behaviour are perfectly ridable if you go easy on the back brake, which is good practice anyway :)


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PostPosted: Wed Apr 04, 2012 10:45 pm 
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I am sorry to appear to be the one to look negatively on this, I love the fact your thinking creatively and exploring possibilities but there are two areas you appear to not really know enough about, bikes and engineering.

I don't wish to stop you from working your design or even building something in the end but please run it by some one qualified to comment on the mechanisms and strength before throwing any time/money/lives at a finished model.

Please note although I am an Engineer (boo hiss),I work in the risk obsessed and overly cautious world of medical devices, bikes are a hobby to which I can claim no professional qualifications on which to base the following.

My main areas of concern:

1- Main pivot, (as per posts on page 1) Design of off centre rear mono stay puts a large moment on the centrally aligned pivot. Pivot as shown would at least wear quickly and flex like a bow and at worst fail.

2-Main pivot , (as per page 3) Similar concerns about the mono stay putting a large moment on what looks to be a very under nourished pivot.

3-Main pivot (page 4), The clamp on pivot will be very difficult to secure enough to withstand the force coming through the stay, you also show it clamped to what is usually an area of extremely thin wall section on the seat tube. (Same goes for the welded version on page 5). This may result in damage to the the seat tube although it is less likely if you choose your tubing carefully. Your claim to have never seen a larger tube than 28.6mm OD does not fill me with confidence that you are in a possession of enough knowledge just yet to make this assessment.

4- Main pivot, "load spreader" (Page 5) more a comment than a concern but the design will do little to spread the load (unless you have the none drive side stay reinstated.doing away with the single stay approach)

5- Chainstays, Page 1 they are like noodles-simply won't work. The page 3 design stiffen things up a good deal, but I am not sure if it is enough. The weight will also be fairly hefty by this point...probably heavier than a (possibly) more suitable monocoque design) If you look at FS designs the ones that have the small diameter tubing chain stays "tend" to rely on a seat stay at a relatively obtuse angle to provide a bit of structure through maintaining a rear triangle to a greater degree. Bikes that have lost the seat stays (such as the Orange 5 etc) use far chunkier monocoque designs that offer far greater stiffness.

All of this is long before you need to start thinking about pedal bob or brake jacking.

Essentially the desire for a mono stay rear fork type shock is inherently difficult to channel force through the frame without it wanting to twist wildly. Your "rear fork crown" is creating a large lever that the compression force will be reacting against and creating a rotational force around the axis of the seat tube.

My advice to you would echo some of the others here.

Research, Look at bikes, new and old, look at how they work look at trends and common features.

Research, Look at fork & shock technology, Look at the types of tubes used in frame building and look at the methods of manufacture used to join them. Understand the properties of the materials

Research, Forces, vectors and torque.

At that point you may well be in a better position to approach this project. Until you have a little more knowledge and understanding you are simply making pretty pictures. (Also 3DSmax is about as useful for this sort of design as a jumbo pack of Crayola.)

Please don't let this put you off, as I said at the start it is fantastic to see creative thought and looking at new, eccentric ways of doing things, even if it is re-inventing the wheel. Please DO NOT take my word for granted, I am not a bike designer and I have never built a frame. But please do regard it as an opinion.


Last edited by JeRkY on Thu Apr 05, 2012 10:55 am, edited 4 times in total.

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PostPosted: Wed Apr 04, 2012 11:01 pm 
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titch__16 wrote:
I don't know what 'brake jack' is so could you elaborate...

...if I can work out the lockout system I'll do that but at the moment I only know about preload...

MikeD wrote:
You're going to want bigger than 28.6mm for an aluminium main triangle. Look around at some production bikes.


I think I've only ever seen downtubes bigger than that. I might make that bigger though, not sure yet


You maybe need to get out and ride some of the designs where this occurs and others to form a view?

Lockout is dependent on what you are using as your shock, are you making these from scratch?

MikeD knows his shit, look at modern aluminium bikes and you will see very few main triangle diameters under 30 plus mm.


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