# Rear suspension linkage



## eng (Apr 9, 2014)

Hi,
first let me introduce myself:
36 years old, mechanical engineer, 14years + experience in engineering, almost none in bike frame building.
But I started working on a full suspension frame in a 3D CAD software and need advice from you guys that actually seen and build frames.

My main problem now is that I don't now where to start in regards to bearings for rear linkage, how they are assembled in the DT, what kind of bearings do I need, how is the shaft manufactured, dimensions and tolerances for the bearings housing and shaft, etc.

Basically I know nothing about the rear suspension linkage. any *documentation*, *pictures* or *drawings* will help me make an idea about how I need to start the linkage design, since the main frame (TT, HT, DT, ST, SS, CS) are all already in 3D. I have chosen the position for the rear dumper, chosen the rear dumper but now I need the linkage designed.

In the afternoon I will come with pictures if anyone is interested where I am stuck.

Thank you all.


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## dacosta (Jul 23, 2010)

I found it easier to buy mountain bike specific bearing and hardware, and incorporate these parts into my own design. Rather than buying industrial hardware or designing everything from scratch. If there is a particular style of suspension that you like, see if you can purchase the hardware from your local bike shop.

Since my project was a titanium soft tail I purchased hardware from a Cannondale Scalpel. Although my implementation was different the parts worked wonderfully.

Shameless plug to my own project:
http://forums.mtbr.com/frame-building/foolys-major-glory-822434.html

Fooly


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## adarn (Aug 11, 2009)

eng said:


> Hi,
> first let me introduce myself:
> 14years + experience in engineering...
> 
> My main problem now is that I don't now where to start in regards to bearings for rear linkage, how they are assembled in the DT, what kind of bearings do I need, how is the shaft manufactured, dimensions and tolerances for the bearings housing and shaft, etc


That's all the engineering part! sounds like you should have no problem there. Are you asking about where to buy these things? Are you planning on making your own?
There isn't really anywhere to just go pick out your parts for a full suspension bike. You could scavenge stuff from old bikes if you really didn't want to start from scratch, butt that is pretty limiting.

Decide how you want the rear wheel to move, see what kind of mechanism you need to make that happen, and then design your pivots from there yeah?

I'd like to see pictures of what you are working on. I am just about to jump into my second FS build and it is fun to see what others are doing.


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## eng (Apr 9, 2014)

In the end it will look just like yours,
here is what I have so far:







I need the guiding link and what's inside it?
Bearings, axle, etc?


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## eng (Apr 9, 2014)

I need to know what's under this screws:







Like: in the link part there are 2 bearings.
In the frame there is another bearing.
The axle goes from end to end, or it has shoulders, etc...
This kind of stuff...


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## eng (Apr 9, 2014)

Or even better:
what's under here?


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## dacosta (Jul 23, 2010)

Here is the diagram of the Cannondale linkage that I used. And a few other pictures.

I tried to press fit the bearings into the linkage but I found that the bearing action was hindered. So I reamed out the bearing holes to .75" to fit the bearings and had to use the loctite to secure the outer races. 

The linkage is NOT held together by locktite but rather locked into place with the shoulder on the linkage and the bolt heads.

I'll try and see if I can get some renderings of my own design, and post it later.

Fooly


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## Testmule (Jul 27, 2013)

Have a look at iGus bushings and bearings if you want a high-tech quality product. They have all the engineering data right on their website - pretty interesting stuff, and they cover a huge range of applications. They are a proprietary self lubricating material that I will go out on a limb and call plastic, but they are more than that; something like Delrin.

I have used their bushings in projects that would normally call for heavy ball bearings.
A friend of mine installed iGus bushings in a specialized moving skid that is used to move 15 ton machines where the original needle bearings were being crushed. He expected them to just squish out and 2 years later they are still going strong.

I'm into building rigid fat bikes right now, but if I was to go with full suspension I would use iGus products for pivot points.


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## pvd (Jan 4, 2006)

Eng-
The shock should NEVER be used as a part of the structure. It must remain free of any loads other than compression or rebound forces needed to control the wheels movement. Any side or bending loads going through the system will destroy the shock and cause poor performance.


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## Drew Diller (Jan 4, 2010)

pvd said:


> Eng-
> The shock should NEVER be used as a part of the structure. It must remain free of any loads other than compression or rebound forces needed to control the wheels movement. Any side or bending loads going through the system will destroy the shock and cause poor performance.


Devil's advocate kinda question...

So all conventional front suspension forks are an abomination?

I have an incredibly simple leading link design in my head that depends on a thru-axle. Uses a typical rear shock. It's like... a fork, with a leading link, that's *it*.

No one would want it because it looks gross, but I'd ride it.


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## dacosta (Jul 23, 2010)

Here are some CAD rendering of my design for the suspension linkage. As you can see the linkage grabs the outer race while the frame grabs the inner race.

If I had to do it again I would make the chain stays a double shear joint instead of a single shear joint. At the time I couldn't figure out how to make the part with the limited machine shop I had access too.

Fooly


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## eng (Apr 9, 2014)

Are you sure I want friction between part 1 (fixed in frame) and 2 (pivoting) in the spot indicated with the arrow?








I cannot use any kit, it needs to be unique design.
Thank you all for contributing.


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## eng (Apr 9, 2014)

Or the same friction between parts 1 and 2 in this picture.
I don't know what's the use of the bearing if we have friction here.


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## dacosta (Jul 23, 2010)

"If You Steal From One Author, It’s Plagiarism; If You Steal From Many, It’s Research"
Unknown 

Without the linkage shown, the inner race is bolted to the frame. This allows the outer race to spin freely. The shoulder washer is slightly smaller then the shoulder in the linkage. Its not obvious but there is a .020" clearance (.010" radial) between the washer and the linkage.

The lower red bolts are intended to bottom out against each other. the Teflon washer is not load-bearing and is only to take up the slack and play of the bolt. The lower pivot does turn freely. While not ideal, it was what I came up with at the time. Again if I could do it again I would make this bearing a double shear joint instead of a single shear joint.

You don't have to actually use the kit but having the kit in your hands to play with my be more informative than looking at my renderings.

Fooly


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## ktm520 (Apr 21, 2004)

Drew Diller said:


> So all conventional front suspension forks are an abomination?


Your not comparing apples to apples. The structure of the conventional fork is designed to handle side loads. A rear shock is not, compression/tension only.

Eng, I have to agree with adarn. You're the engineer in this situation. That's the beauty of designing your own bike. Axle sizes, bearings, press fits, that's all easy stuff. I mostly ride full suspension bikes and suspension tuning is one of my huge passions in this hobby, but I doubt I will ever build anything but hardtails.

What software package are you using?

You'll be fine with single shear at the stay.


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## cheesy (Apr 15, 2009)

If you go with bike specific bearings I found that this site was useful as it has all of the dimensions and lists a lot of the actual applications, no load ratings though, cant have everything.

Shockcraft Suspension Bearings


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## eng (Apr 9, 2014)

cheesy said:


> If you go with bike specific bearings I found that this site was useful as it has all of the dimensions and lists a lot of the actual applications, no load ratings though, cant have everything.
> 
> Shockcraft Suspension Bearings


That's my main question: can I load the bearings sideways? (can they take the side to side load?) Or they are pure radial bearings?

Edited: according o skf they can:
Double row angular contact ball bearings


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## ktm520 (Apr 21, 2004)

They will take sideload, although its a fraction of the radial rating. 99% of the fs frames out there use radial balls. Not the greatest solution but consider the alternatives.


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## eng (Apr 9, 2014)

ktm520 said:


> They will take sideload, although its a fraction of the radial rating. 99% of the fs frames out there use radial balls. Not the greatest solution but consider the alternatives.


Well, if 99% use radial bearings Double row angular contact ball bearings should work just fine.
Over the weekend I'll come up with something.


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## ktm520 (Apr 21, 2004)

I'd just stick with single row radial's, or single row angulars, if you want to get fancy. Their cheap and work well enough. The pivots don't really see much pure side loading. The problem with any type of radial ball is that they are meant to spin more than 360 degrees. On a suspension pivot that only reciprocates a few degrees, they live a short life. Like I said though, good enough.


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## cheesy (Apr 15, 2009)

I would stick with single row deep groove balls, if you look at the site I linked, some have"FC" in the name, these have more balls than a standard deep groove ball and are more suited to the small angle rotations.


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## eng (Apr 9, 2014)

Well, since 99% of all rear suspensions use double row angular contact ball bearings, I'll stick with that.


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## ktm520 (Apr 21, 2004)

eng said:


> Well, since 99% of all rear suspensions use double row angular contact ball bearings, I'll stick with that.


What??


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## eng (Apr 9, 2014)

ktm520 said:


> What??


Have a look at post #18


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