Topic: Swing arm pivot stabilization? Ala Big Benly and T-rex

[slikwilli420]

I have run across the Big Benly article where plates attached to a longer pivot bolt and the rear engine mounts which substantially improved handling.

There was also mention of Rex Wolfensen did something similar to his T-rex racer.

What can be done here to improve stability of this area?

I am already planning on another tube above the pivot as bwaller did to his racer. I'm also thinking of a larger pivot bolt like a 20mm. I would use either chromoly tubing our a hollow axle like I am already using. The issue with a bigger built is that I will need to figure out how to jig the frame up so the new bungs are properly aligned.

[754]

Frame is easy to do on a Bridgeport tye mill.
 Make up a 2 dia centering plug that fasten to mill table. Stock hole and new hole diameters on plug..
 Fasten plug to table and clock in, then lower table   add frame and clamp loosely. 
 Then clock in and bore frame  to new size.. then remove frame, flip over  ( now sitting on big dia of centering plug) and repeat
Boring the end new size hole,

 Things may be tight heightwise, May need to cut hole with endmill or something short if  you can't get boring head in. .

[HondaMan]

Make it of waterpipe instead of chrome moly pipe, and it will flex MUCH less...

[kmb69]

Make it of waterpipe instead of chrome moly pipe, and it will flex MUCH less...

You have got to be kidding! 

High grade water pipe has a yield strength of approximately 40,000-90,000 psi and 60,000-103,000 psi tensile.
Typical chrome moly tubing has a yield strength of approximately 46,000-161,000 psi and 81,000-179,000 psi tensile.
Their modulus of elasticity is similar but you're talking closely coupled joints so the difference in physical properties counts in a big way.
I don't see very many NASCAR and NHRA car frames made from water pipe! 

[Captain]

 Here is a clever way of reducing the defection of the swing arm pivot shaft.............. Put a bush or bearing in the "middle" (half way between the swing arm bearings).  This is where the shaft deflects (bends) when unsupported as the load and leverage is being applied externally and at either or both ends and not necessarily by the same amount..........Which results in deflection.

 It is surprising just how much this can improve the rigidity of those with small diameter pivot shafts.

 Captain 

[Jore]

Here is a clever way of reducing the defection of the swing arm pivot shaft.............. Put a bush or bearing in the "middle" (half way between the swing arm bearings).  This is where the shaft deflects (bends) when unsupported as the load and leverage is being applied externally and at either or both ends and not necessarily by the same amount..........Which results in deflection.

 It is surprising just how much this can improve the rigidity of those with small diameter pivot shafts.

 Captain

can you expand on that idea?

[calj737]

Here is a clever way of reducing the defection of the swing arm pivot shaft.............. Put a bush or bearing in the "middle" (half way between the swing arm bearings).  This is where the shaft deflects (bends) when unsupported as the load and leverage is being applied externally and at either or both ends and not necessarily by the same amount..........Which results in deflection.

 It is surprising just how much this can improve the rigidity of those with small diameter pivot shafts.

This is a great technique, but how do you resolve the grease shaft inside which the pivot rod runs? You'd have to eliminate that and switch to needle bearings or roller bearings in order to support the rod midway. Or am I missing something?

[slikwilli420]

The way I picture it is the following from outside to inside:

Pivot tube which is the swingarm itself
Two bronze bushings, one on each end
A third bushing in the middle that shares OD similar (maybe -.001 or so for fit) to the bronze bushings, and ID that matches the bronze bushings
The pivot bolt tube that slides into the bronze bushings
The pivot bolt

If you could press the center bushing in then you would not need to find a way to hold it center.

Now the pivot bolt tube cannot flex in the middle as it is now supported by the third bushing.


I still very much like the idea of a 20mm hollow pivot shaft, rather than the puny 14mm solid version.
My thought there is to make plates that reference the two rear engine bolts and the pivot shaft, bore out the current bungs and replace with 20mm ID bungs, which will be located by the plates bolted to the frame with a 20mm shaft with nuts and collars which will secure everything while it is welded.

[calj737]

The way I picture it is the following from outside to inside:

The pivot bolt tube that slides into the bronze bushings

That's what I would envision too from an assembly, but is there even sufficient room between the tube and pivot tube? And, does the rod literally sit tightly enough within the tube to bear on the bushing?

I would think a centered bushing would demand the rod bear on the bushing directly to offer real support.

[slikwilli420]

Essentially just press a third bronze bushing without flange into the middle. The pivot bolt tube would now ride on all three instead of just two outside ones.

[calj737]

Essentially just press a third bronze bushing without flange into the middle. The pivot bolt tube would now ride on all three instead of just two outside ones.

Yeah, but thats the tube not the rod. And is there is enough difference in the OD of the shaft and the ID of pivot tube to allow a bushing? And then the rod to shaft OD/ID... What's that clearance?

[kmb69]

Essentially just press a third bronze bushing without flange into the middle. The pivot bolt tube would now ride on all three instead of just two outside ones.

That should work.

Frame is easy to do on a Bridgeport tye mill.
 Make up a 2 dia centering plug that fasten to mill table. Stock hole and new hole diameters on plug..
 Fasten plug to table and clock in, then lower table   add frame and clamp loosely. 
 Then clock in and bore frame  to new size.. then remove frame, flip over  ( now sitting on big dia of centering plug) and repeat
Boring the end new size hole,

 Things may be tight heightwise, May need to cut hole with endmill or something short if  you can't get boring head in. .

The guys up at JesBuilt that do frame modifications, converting to mono shocks, replacing steering heads, and such, tell me the necks on the Honda frames are usually not perpendicular to the swingarm pivot. You need to be able to measure this and orient the setup accordingly when milling the 20mm holes. I'm doing one right now on a DOHC frame and found .014" clearance between the frame and pivot bolt which results in nearly 0.25" play at the end of the swingarm. Frank's method will work when you flip the frame for the second side to line up X-Y on the first 20mm hole. I would still check the neck perpendicularity when flipping the frame. Takes some care to get it perfect but it should keep the wheels inline.

[kmb69]

Essentially just press a third bronze bushing without flange into the middle. The pivot bolt tube would now ride on all three instead of just two outside ones.

Yeah, but thats the tube not the rod. And is there is enough difference in the OD of the shaft and the ID of pivot tube to allow a bushing? And then the rod to shaft OD/ID... What's that clearance?

Yep, might require line boring the swingarm through.

[slikwilli420]

Plenty of room. The tube that is part of the swing arm is the same ID across the whole thing. The bronze bushings press in from either side and stop on the flange. If you took a third and cut the flange off you could press it to the middle. The pivot tube already rides on these bushings with the bolt inside. The third bushing would keep the bolt and tube assembly from flexing at mid point. The clearances for the outside bushings can be used on the third one as well.

[slikwilli420]

Essentially just press a third bronze bushing without flange into the middle. The pivot bolt tube would now ride on all three instead of just two outside ones.

That should work.

Frame is easy to do on a Bridgeport tye mill.
 Make up a 2 dia centering plug that fasten to mill table. Stock hole and new hole diameters on plug..
 Fasten plug to table and clock in, then lower table   add frame and clamp loosely. 
 Then clock in and bore frame  to new size.. then remove frame, flip over  ( now sitting on big dia of centering plug) and repeat
Boring the end new size hole,

 Things may be tight heightwise, May need to cut hole with endmill or something short if  you can't get boring head in. .

The guys up at JesBuilt that do frame modifications, converting to mono shocks, replacing steering heads, and such, tell me the necks on the Honda frames are usually not perpendicular to the swingarm pivot. You need to be able to measure this and orient the setup accordingly when milling the 20mm holes. I'm doing one right now on a DOHC frame and found .014" clearance between the frame and pivot bolt which results in nearly 0.25" play at the end of the swingarm. Frank's method will work when you flip the frame for the second side to line up X-Y on the first 20mm hole. I would still check the neck perpendicularity when flipping the frame. Takes some care to get it perfect but it should keep the wheels inline.

Keith, if I made plates to reference the old pivot holes and the rear engine holes then I would at least be able to keep the same geometry that is already in place on my frame though right?

How does a humble weekend warrior check the perpendicular nature of the pivot to head stock?

[kmb69]

.....
Keith, if I made plates to reference the old pivot holes and the rear engine holes then I would at least be able to keep the same geometry that is already in place on my frame though right?

How does a humble weekend warrior check the perpendicular nature of the pivot to head stock?

Yep, relatively easy to line up on existing holes and maintain current geometry.

The way I'm doing it is with a straight, precision shaft mounted in the neck with bearings for centering and extends out each end for an indicator. I line up the swingarm pivot holes then check where the neck is and adjust the attitude of the frame to the mill spindle as necessary. There is plenty of material going from 16mm to 20mm to make adjustments. I am reaming the 16's to 20's with an endmill that cuts maybe 0.001 under and then running an on-size 20mm reamer through. My target diameter is 0.001" clearance to my pivot bolt. Frank's fixture idea is pretty good for lining up X-Y on the second side but any clearance between the frame and fixture can let the frame tilt a little so I recheck the neck.

[754]

2 things,  the neck is a different deal  , they are usually out and can be straightened  back.
 If you want different rake  now is the time.
 I would put the swingarm shaft parallel to rear motor mount bolts,
Slick, not all can do this at home,  sometimes  you got to send out.
 I line boring setup tacked to frame would do it. Tiny boring bar though. Might be able to home build that.

 I would not worry about final hole being 1 thou under,  build sleeves to fit?

[slikwilli420]

Frank, I like the idea of doing it on a mill, but mine is a small mill so I don't have that kind of room.

I would think plates located to existing holes would work just fine. The bushings you weld in would be exact to a 20mm shaft and may only need some lapping after welding to get back to perfectly round.

Does anyone have more information on the Big Benly and T-Rex bikes? I have the pics of the Big Benly from the article which is helpful, but it doesnt talk about that mod at all. I want to know how much of a difference these mods really made?

[HondaMan]

Plenty of room. The tube that is part of the swing arm is the same ID across the whole thing. The bronze bushings press in from either side and stop on the flange. If you took a third and cut the flange off you could press it to the middle. The pivot tube already rides on these bushings with the bolt inside. The third bushing would keep the bolt and tube assembly from flexing at mid point. The clearances for the outside bushings can be used on the third one as well.

Are you talking about the stock swingarm here? It is not the same ID all the way through...

My 2 cents' worth of info: here's the particulars on the OEM arrangement -
The clearance of the bushings is less than 0.0010" over the ends of the collar where they ride. I use 0.0004"-0.0008" (depending on straightness of the swingarm's tube) when making bronze ones. You can fit a flangeless bushing of up to 1.800" length inside the arm and still fit the seals and end caps.
The overall length of the outside of the [new] collar must be 0.001"-0.016" more than the length of the outside of the bushings and their seals, as tightening the bolt usually shortens it a bit. Too short, and it all binds tight when tightened.
The press-fit of the bronze bushings (as I make them) is 0.0004"-0.0006" into the arm, more will usually force you to hone the ID afterward from bushing crush during insertion.
The collar is narrower in the region between the bearings: it usually must be for assembly reasons. At the very least, the inside edges of the bushings must be beveled on both OD (to insert them) and ID (to help insert the collar, later). The collar OD in the bushing areas is either 0.8440", 0.8442", or 0.8444", depending on which collar you have.
The big bolt is 0.550" OD where it meets the narrower bearing zones of the collar: the collar is also 0.5515"-0.5520" ID at both ends, and in the center, to ride on the bolt in 3 positions, to reduce flex. The collars have a bearing surface in their centers to ride on the bolts at this location, too.

Installing a larger OD collar can be done by simply making one with the same ID size (and center support zone, like the OEM version), using the same bolt to stabilize it. Some have thus adapted Kawasaki and Suzuki swingarms to the 750 and 550 frames when looking for longer arms. These (that I have seen) have 1" ID sites where the (originally needle) bearings fit: thus the OD of the needle bearings was 1" size and a collar was made to fit the ID of these bearings, with a hole that fit the OEM bolt in the 3 support sites.

Most of the mods I have seen had more to do with longer swingarms than for stiffer arms: the early-style arm is rock-solid against flex (only the phenolic bushings in the OEM setup cause flex). The post-1975 style arm (with round tubes instead of box sections, welded together) are not as stiff.