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Attachment, 3 point or 4 point?
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DianneB
Posted 11/18/2014 7:43 PM (#81179)
Subject: Attachment, 3 point or 4 point?


Member

Posts: 18
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Location: Manitoba, Canada
Back in the 1960s I had a homemade sidecar on a Honda 305. I used a 3-point attachment with two high points and one low. It seemed to work just fine.

I see a lot of the new sidecars use a 4-point attachment and I don't see any advantage (aside from not requiring as substantial a frame on the sidecar). Am I missing something in the design?

[My plan is to make (mill) my own attach fittings from structural steel and use Aircraft-grade bolts and clevises for the attachment.]
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Peter Pan
Posted 11/18/2014 9:48 PM (#81182 - in reply to #81179)
Subject: Re: Attachment, 3 point or 4 point?



Expert

Posts: 1914
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Location: San Isidro de Heredia, Costa Rica
I had a 3 point attachment on the MZ while on Jawa and Ural 4 points...way safer, way less stress, way less things to worry about.
The MZ I had to rebend into shape every 3 month. Look at it this way: You distribute the forces over more spots more evenly into the bikes frame.
And when 3 point, then better 2 low ones (on the same height) and one top one.
There are now even 5 point attachments in Europe used (Jewell-Sauer-EML...).
Sven

Edited by Peter Pan 11/18/2014 9:49 PM
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jaydmc
Posted 11/19/2014 10:54 AM (#81200 - in reply to #81179)
Subject: Re: Attachment, 3 point or 4 point?


Expert

Posts: 1513
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Either way will work fine as long as the underlying design work is done correctly. 4 point is more common then 3 point, there are also hybrid mounts like we use on some scooters where we have 4 points on the sidecar but only 3 points on the bike. Harley only used 3 point.
Unless there is a real need to only have 3 points as is the case with some scooters (Silverwing) then I see no reason why you would want to go to 3 points. Remember your mounts not only need to hold the alignment statically but also dynamically and should be strong enough that should you crash, nothing moves during the crash that could impair your ability to recover from what ever is happening.
I like the old adage, "when in doubt, build it stout out of things you know about" Also why the air craft grade bolts? Grade 8 SAE or even grade 5 are fine as long as you design for what you are using. In some applications air craft grade may not be as good. This is where the "things you know about" comes into play.
Jay G
DMC sidecars
www.dmcsidecars.com
866-638-1793
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DianneB
Posted 11/19/2014 11:32 AM (#81201 - in reply to #81179)
Subject: Re: Attachment, 3 point or 4 point?


Member

Posts: 18
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Location: Manitoba, Canada
Thanks Jay!

I lean toward aircraft-grade and structural components because that's what I "know about" and I know the QC for aircraft parts is one of the best. I trust the specs for those two.

I'll have to have a close look at the frame of my FXD and see how many SOLID attachment points I can find on the frame.

With the 3 points on the Honda 305, the only disadvantage of having two points up high was having to step between them to mount the bike, something I got used to, but a small price to pay for the rigidity of 3 attachment points widely spaced.

It looks like many of the modern mounts try to keep the attachments low and use a cantilever arrangement to avoid having to "step over" the mount.
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jaydmc
Posted 11/19/2014 11:54 AM (#81202 - in reply to #81179)
Subject: Re: Attachment, 3 point or 4 point?


Expert

Posts: 1513
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Most modern mounts use 2 lower mounts rather then the way you did with only a single. This makes it easy to set your toe in. I suspect that with the set up you had you may have found at higher speeds that your toe setting was not as stable as you would have liked assuming that with a 305 you were ever doing high speeds. With two lower mounts spread as far apart as practical your toe setting is very stable meaning it will be the same statically as dynamically. When building lower mounts not only should they be as far apart as practical they should also be about the same distance off of the ground. The reason for this is when you set your alignment you first set wheel lead where you want it, then you set toe where you want it to be, then you set lean out. Then take it out and test ride it, If you find that it has a bit of a pull one way or the other you then fine tune the lean out. If the lower mounts are not the same distance from the ground when you change lean out you also change your toe setting. If the lower rear is higher then the lower front then the more you lean it out the more you toe it out. So if you are working on fixing a pull to the right then the more you lean it left, the more it toe's out and the more it pulls to the right.
The FX mounts we make are year dependent as Harley has had a few different frame types. Some we pick up the lower engine mounting bolts for the lower front and on the lower rear we pick up two bolts on either side of the frame. The upper front mount is usually one of our frame clamps and the upper rear bolt is usually a custom eye bolt that replaces the upper shock mounting bolt. Of course we sell mounting kits, frame kits as well as the hardware that goes between the frame and the bike side mounts. We also sell bits and pieces such a clevis's and eye bolts for the DIY builders.
Jay G
DMC sidecars
www.dmcsidecars.com
866-638-1793
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DianneB
Posted 11/19/2014 1:33 PM (#81205 - in reply to #81202)
Subject: Re: Attachment, 3 point or 4 point?


Member

Posts: 18
0
Location: Manitoba, Canada
jaydmc - I suspect that with the set up you had you may have found at higher speeds that your toe setting was not as stable as you would have liked assuming that with a 305 you were ever doing high speeds.


Thanks for the information Jay.

We set the camber and castor of the sidecar of the wheel at the spring attachment point and it worked very well but I can understand why, in a production environment, manufacturers would want to "do the trim" at the motorcycle attachment points. There was some difference in 'trim' between empty and loaded but not enough to cause any problems or adverse handling. As a matter of fact, my boyfriend at the time very much enjoyed riding in the sidecar and manoeuvring on twisty dirt roads. He was pretty good at throwing his weight where it was needed and we probably would have tied sidecar racing if there was any nearby.

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WinginCamera
Posted 11/23/2014 8:03 PM (#81285 - in reply to #81179)
Subject: Re: Attachment, 3 point or 4 point?



Veteran

Posts: 205
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Location: Spanaway, Wa.
Back in the 80's I had a Vetter Terraplane attached to a Kawasaki 1300 that used a three point mount system. This was my second rig and I didn't think about structural strength back then, but it ran well, kept it's alignment and tracked well. I pulled a tent trailer behind it, using the inside car rail to attach the steel tubing for the hitch mount. My first rig was a Ural hack attached to a Moto Guzzi 850 which was four point attachment. Both were easy to ride and easy to remove or re-attach to the bike.
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