8N Front End "Kinematics"

S10Vette

Member
My front axle was "ovaled out" and cracked at the pivot pin, so replacing axle, pin, spacers, and bushing with new.

I noticed that the new bushing has quite a bit of clearance to the new pin (+.020"). That has been discussed in other threads, so won't repeat all that.

What I am confused about is the actual kinematics of the front end with this design. If you have the ball end of the radius rods disconnected, and with the pins in place in the axle at the other end and rotate the axle about the pivot, each ball end wants to translate up and down, so essentially the axle seems to act like a torsion bar. Is that the principle ?
 
(quoted from post at 11:22:05 11/15/16) My front axle was "ovaled out" and cracked at the pivot pin, so replacing axle, pin, spacers, and bushing with new.

I noticed that the new bushing has quite a bit of clearance to the new pin (+.020"). That has been discussed in other threads, so won't repeat all that.

What I am confused about is the actual kinematics of the front end with this design. If you have the ball end of the radius rods disconnected, and with the pins in place in the axle at the other end and rotate the axle about the pivot, each ball end wants to translate up and down, so essentially the axle seems to act like a torsion bar. Is that the principle ?

I think you are overthinking this. I don't believe the radius rods are capable of applying any significant torsion in any plane to the axle. The free play in the components is sufficient to allow everything to move more or less freely. Hook up the radius rods and raise the front end with a jack placed under the emgine/axle support. You can effortlessly pivot the axle about the pin. If there is any torsion force being generated it's so small as to be imperceptible. The axle is just a simple pivot that supports the vertical load on the pivot point. The radius rods resist lateral forces generated at the wheels and prevent the axle from deflecting fore and aft.

TOH
 
Your thinking is correct. I can tell you if you remove all the play in the components (tight pins thru the radius rods and no play in the ball sockets) you won't be able to rotate the axle more than an inch either way. Been there.
 
(quoted from post at 14:49:30 11/15/16) Your thinking is correct. I can tell you if you remove all the play in the components (tight pins thru the radius rods and no play in the ball sockets) you won't be able to rotate the axle more than an inch either way. Been there.

JS- Yep, I just got everything back together with new parts, and it will move a couple of inches travel with minimal resistance, but then takes about 50 pounds force at the end of the axle so probably 25 pounds at the tire to bottom it. Near as I can tell, once everything binds you are twisting the axle like a torsion bar.

Also, if you leave the ball sockets end free, that end of the radius arm will translate up about 2 inches (left and right opposite of course). I think visualizing that makes it pretty clear.
that you are twisting the axle at full travel. Obviously it works "good enough" though, just not the best design IMHO.
 
(quoted from post at 20:23:55 11/15/16)
(quoted from post at 14:49:30 11/15/16) Your thinking is correct. I can tell you if you remove all the play in the components (tight pins thru the radius rods and no play in the ball sockets) you won't be able to rotate the axle more than an inch either way. Been there.

JS- Yep, I just got everything back together with new parts, and it will move a couple of inches travel with minimal resistance, but then takes about 50 pounds force at the end of the axle so probably 25 pounds at the tire to bottom it. Near as I can tell, once everything binds you are twisting the axle like a torsion bar.

Also, if you leave the ball sockets end free, that end of the radius arm will translate up about 2 inches (left and right opposite of course). I think visualizing that makes it pretty clear.
that you are twisting the axle at full travel. Obviously it works "good enough" though, just not the best design IMHO.

I guess I am confused. I have always thought the proper setup was enough play that it does not bind.....

TOH
 

TOH- So would you recommend I ream out the pin locations in the radius arms or the axle where they attach to the axle? That is the only place I can think adding "freeplay" would free it up completely.
 

With ball end disconnected at both extremes of droop and bump. I think this also illustrates why folks who did not have the front end unloaded or level when they disconnected the radius arm had it violently release due to suddenly releasing the torsional force from the front axle.

42555.jpg
42556.jpg
 
(quoted from post at 22:16:10 11/15/16)
TOH- So would you recommend I ream out the pin locations in the radius arms or the axle where they attach to the axle? That is the only place I can think adding "freeplay" would free it up completely.

I am not prepared to recommend anything. I understand the geometry involved but I have never seen one so tight it won't pivot a lot more than a couple inches. I have always assumed that was by design and never gave it any thought. If it was not intended to pivot more or less freely why is the axle mounted on a bushed pivot pin with ball ends on the radius arms? I'll have to look mine over to see what's what with it and exactly where the "slop" that allows it to pivot is located and just how much it will move. As best I recall the radius arm pins are right tight .....

TOH
 
(quoted from post at 23:11:57 11/15/16)
(quoted from post at 22:16:10 11/15/16)
TOH- So would you recommend I ream out the pin locations in the radius arms or the axle where they attach to the axle? That is the only place I can think adding "freeplay" would free it up completely.

I am not prepared to recommend anything. I understand the geometry involved but I have never seen one so tight it won't pivot a lot more than a couple inches. I have always assumed that was by design and never gave it any thought. If it was not intended to pivot more or less freely why is the axle mounted on a bushed pivot pin with ball ends on the radius arms? I'll have to look mine over to see what's what with it and exactly where the "slop" that allows it to pivot is located and just how much it will move. As best I recall the radius arm pins are right tight .....

TOH
ind something useful to do with your time…..just bolt the sucker together & use it like thousands have done for the last 65 years!
 
(quoted from post at 00:18:24 11/16/16)
(quoted from post at 23:11:57 11/15/16)
(quoted from post at 22:16:10 11/15/16)
TOH- So would you recommend I ream out the pin locations in the radius arms or the axle where they attach to the axle? That is the only place I can think adding "freeplay" would free it up completely.

I am not prepared to recommend anything. I understand the geometry involved but I have never seen one so tight it won't pivot a lot more than a couple inches. I have always assumed that was by design and never gave it any thought. If it was not intended to pivot more or less freely why is the axle mounted on a bushed pivot pin with ball ends on the radius arms? I'll have to look mine over to see what's what with it and exactly where the "slop" that allows it to pivot is located and just how much it will move. As best I recall the radius arm pins are right tight .....

TOH
ind something useful to do with your time…..just bolt the sucker together & use it like thousands have done for the last 65 years!

But the curious want to know Jesse :lol: I will waste another 10 minutes today checking to see EXACTLY how much free movement is in the front end on my 8N. After thinking about it I'm pretty sure it's probably a lot less than on the Kubota front axle which doesn't have radius arms. Until now I never once thought of the the 8N front end as a torsion bar design :roll:

TOH
 
(quoted from post at 22:22:51 11/15/16)
With ball end disconnected at both extremes of droop and bump. I think this also illustrates why folks who did not have the front end unloaded or level when they disconnected the radius arm had it violently release due to suddenly releasing the torsional force from the front axle.

I don't seem to have anything close to the binding and torsional forces you are reporting. I would call this tractor "tight". No discernible "slop" in the radius rod pins or yokes and ball caps snug to the rod ends with just enough clearance to move freely. When jacked up under the axle support the front end easily pivots 8" measured at the wheels before any significant resistance is felt. If there is any torsional force being generated it is so small as to be negligible. Interestingly the axle will easily bottom against the axle support on the right side but the linkages bind a 1/2"or so before it touches on the left side. It takes considerable force to push it the rest of the way down against that side and I can't see any reason for the difference. Sitting on the ground or jacked up as shown there is no torsion released and the rod ends don't jump out of their sockets when the radius rod caps are removed.

I have always assumed this is the way it was designed to work and that the relevant clearances on this tractor are close to new. I m still unclear how you can get it setup so tight it springs apart when you remove a radius rod cap. That sounds like something is bent or twisted and I don't know what to tell you other than repeating what Jesse said. I'll call this a personal learning experiment and won't count the 10 or so minutes I spent looking at the tractor as wasted ;-)

TOH

WP_20161116_10_43_50_Pro.jpg
 
(quoted from post at 09:26:39 11/16/16)
(quoted from post at 22:22:51 11/15/16)
With ball end disconnected at both extremes of droop and bump. I think this also illustrates why folks who did not have the front end unloaded or level when they disconnected the radius arm had it violently release due to suddenly releasing the torsional force from the front axle.

I don't seem to have anything close to the binding and torsional forces you are reporting. I would call this tractor "tight". No discernible "slop" in the radius rod pins or yokes and ball caps snug to the rod ends with just enough clearance to move freely. When jacked up under the axle support the front end easily pivots 8" measured at the wheels before any significant resistance is felt. If there is any torsional force being generated it is so small as to be negligible. Interestingly the axle will easily bottom against the axle support on the right side but the linkages bind a 1/2"or so before it touches on the left side. It takes considerable force to push it the rest of the way down against that side and I can't see any reason for the difference. Sitting on the ground or jacked up as shown there is no torsion released and the rod ends don't jump out of their sockets when the radius rod caps are removed.

I have always assumed this is the way it was designed to work and that the relevant clearances on this tractor are close to new. I m still unclear how you can get it setup so tight it springs apart when you remove a radius rod cap. That sounds like something is bent or twisted and I don't know what to tell you other than repeating what Jesse said. I'll call this a personal learning experiment and won't count the 10 or so minutes I spent looking at the tractor as wasted ;-)

TOH

TOH- Thanks for the input. I am going to look into it further. BTW my "bicep meter" is obviously way off, as the force required to completely bottom the suspension on each side is close to 100 lbs when measured with a scale.

The springing of the rod end with the cap removed would only happen when it is at or near full travel, like with one tire up on a block and the other hanging free (for extreme example). This has been reported in other threads with guys cautioning that they got hurt not realizing it would spring up with such force.
 
(quoted from post at 13:06:34 11/16/16)
(quoted from post at 09:26:39 11/16/16)
(quoted from post at 22:22:51 11/15/16)
With ball end disconnected at both extremes of droop and bump. I think this also illustrates why folks who did not have the front end unloaded or level when they disconnected the radius arm had it violently release due to suddenly releasing the torsional force from the front axle.

I don't seem to have anything close to the binding and torsional forces you are reporting. I would call this tractor "tight". No discernible "slop" in the radius rod pins or yokes and ball caps snug to the rod ends with just enough clearance to move freely. When jacked up under the axle support the front end easily pivots 8" measured at the wheels before any significant resistance is felt. If there is any torsional force being generated it is so small as to be negligible. Interestingly the axle will easily bottom against the axle support on the right side but the linkages bind a 1/2"or so before it touches on the left side. It takes considerable force to push it the rest of the way down against that side and I can't see any reason for the difference. Sitting on the ground or jacked up as shown there is no torsion released and the rod ends don't jump out of their sockets when the radius rod caps are removed.

I have always assumed this is the way it was designed to work and that the relevant clearances on this tractor are close to new. I m still unclear how you can get it setup so tight it springs apart when you remove a radius rod cap. That sounds like something is bent or twisted and I don't know what to tell you other than repeating what Jesse said. I'll call this a personal learning experiment and won't count the 10 or so minutes I spent looking at the tractor as wasted ;-)

TOH

TOH- Thanks for the input. I am going to look into it further. BTW my "bicep meter" is obviously way off, as the force required to completely bottom the suspension on each side is close to 100 lbs when measured with a scale.

The springing of the rod end with the cap removed would only happen when it is at or near full travel, like with one tire up on a block and the other hanging free (for extreme example). This has been reported in other threads with guys cautioning that they got hurt not realizing it would spring up with such force.

Yes - sitting as you describe even on mine the rod ends would be under enough force to pop out of the socket which is not particualry surprising. I do have that same level of resistance as you describe near the bottom out point on one side (although not the other) but there is a LOT of "free play" before it reaches that point. Perhaps we are talking past each other - I had the impression you have none.

TOH
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top