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This winters project is to freshen up the IRS on my 1971 OTS. Will go through and powder coat all parts except the differential, replace shocks and springs. In researching the parts needed I have noted that both bushings and bearings are available for the lower wishbone. Has anyone made the conversion from the factory bearings to the aftermarket bushings? Any input would be appreciated. Thanks, Rick
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XKE IRS Bushings or bearings?
- Thread starter r67cat
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I thought about converting to bushings, but after talking to a Master of Masters Jaguar mechanic, I decided to stick with the original bearings. I'm no expert certainly, but just thinking about it in practical mechanical terms, with bushings, if no other forces were in play, the frictional forces would be distributed across a surface area equal to the width of the bushing x the circumference. Whereas, with bearings, the frictional forces would be distributed across a greater area due to the additive surface areas of the bearings (the whole idea of bearings I think is to better distribute the frictional forces acting upon two rotational members.
Basil
By the way, here's my nearly completed IRS:
[ 10-26-2003: Message edited by: Basil ]</p>
Basil
By the way, here's my nearly completed IRS:
[ 10-26-2003: Message edited by: Basil ]</p>
PC
Obi Wan
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In theory it's a bit more complicated than that. This would be pretty easy to explain with sketches but I'll try to do it with words.
From a loading perspective you can think of the basic pivot configuration (both bearing and bush) as that of a rod within a cylinder. The rod can rotate freely about its central axis but is constrained from lateral motion (translation). Loading can only be radial from the center toward the shell because any off-center force will result in rotation.
The load will be entirely born by the half of the cylinder that the loading force points toward, the side that it is pressing against. The opposite side will have zero loading. Think of it this way, if you have a weight hanging from a rod sitting in a cylinder and you can cut the top half of the cylinder off you'll have a cradle that will still support all the weight. So a cylinder will support the load on a surface area that is one half of its length times its inner circumference.
The distribution of the force will not be uniform. Since the rod is free to rotate the cylinder can only exert a force on the rod that is normal (perpendicular) to the mating surface. Therefore the force will be greatest at the point radial in the direction of the force and will decrease to zero at the two points to the sides. Mathematically speaking, the cylinder is circular so the actual distribution of the force will be a half-cosine.
This is exactly how the bushing operates. The roller bearing is a special case of the same configuration but instead of being continuously distributed, the load is divided up among the rollers and is born by a series of contact lines. (If they were ball bearings it would be points.)
In the ideal case the contact lines have zero area and so the surface pressure at the contact would be infinite! In the real world the roller and the shell are separated by a film of grease that distributes the load over a finite area and there is microscopic deformation of the metal. Even though the real surface pressure is not infinite it is very high. That's why bearings and races are made from very high strength steel alloys and must be kept greased.
Where roller and ball bearings really show superiority is in friction. With a sleeve (bush) bearing the entire mating surface of the rod and cylinder are slipping, rubbing against one another. In a roller or ball bearing there is no rubbing. The bearings are rolling along one another. It's easy to see why you'll have much less friction force with a rolling bearing verses a slipping bearing.
In a very generalized sense you can say that rollers give lower rotating friction with higher load pressure while bushings give lower load pressure and more friction.
There are a lot of trade-offs in choosing one type of bearing over another but it is interesting to note that the most highly loaded bearings in a car, the engine's mains and rods', are almost universally journal bearings (much like a bushing or sleeve) while the wheel bearings are always roller or ball.
What does all this have to do with Jag suspension pivots? Not too much. That is yet another layer of complication.
Friction is lower in a rolling type of bearing but how important is low friction to a suspension member? Since the forces that move the piece are quite large and the motions are relatively limited small amounts of friction in the pivot will be overwhelmed by the driving force. In actuality, automobiles as we know them today can't operate safely without a very substantial amount of "friction" in the suspension. Friction (energy loss) must be added to a suspension system in the form of shock absorbers (dampers) for a car to handle, ride and respond acceptably. The amount of energy absorbed by the shocks is orders of magnitude greater than in any pivot friction so any difference in friction between rollers and bushes in the pivots is negligible.
A suspension bearing pivots but does not spin freely. It rocks back and forth. Depending on the diameter of the rollers and the angle of suspension deflection you may or may not rotate the rollers in a complete revolution.
What happens to the tires on a car if it sits in one place? They get flat spots. If you rock the car back and forth, never far enough to turn the wheels completely around and always bring it back to rest in the same spot you'll eventually get really severe flat spots. When a roller bearing is loaded heavily and rocked back and forth without spinning a similar effect can occur and the bearings can go out-of-round, lose precision and/or become "notchy". The effect will be severely accelerated by intrusion of dirt or loss of lubricant.
This is the effect that the proponents of bushed suspension pivots cite when recommending their modifications.
Does it actually happen to Jag suspensions in the real world and do the bushings prevent it? Heck if I know. I've never taken one apart, let alone a bunch of them.
PC.
From a loading perspective you can think of the basic pivot configuration (both bearing and bush) as that of a rod within a cylinder. The rod can rotate freely about its central axis but is constrained from lateral motion (translation). Loading can only be radial from the center toward the shell because any off-center force will result in rotation.
The load will be entirely born by the half of the cylinder that the loading force points toward, the side that it is pressing against. The opposite side will have zero loading. Think of it this way, if you have a weight hanging from a rod sitting in a cylinder and you can cut the top half of the cylinder off you'll have a cradle that will still support all the weight. So a cylinder will support the load on a surface area that is one half of its length times its inner circumference.
The distribution of the force will not be uniform. Since the rod is free to rotate the cylinder can only exert a force on the rod that is normal (perpendicular) to the mating surface. Therefore the force will be greatest at the point radial in the direction of the force and will decrease to zero at the two points to the sides. Mathematically speaking, the cylinder is circular so the actual distribution of the force will be a half-cosine.
This is exactly how the bushing operates. The roller bearing is a special case of the same configuration but instead of being continuously distributed, the load is divided up among the rollers and is born by a series of contact lines. (If they were ball bearings it would be points.)
In the ideal case the contact lines have zero area and so the surface pressure at the contact would be infinite! In the real world the roller and the shell are separated by a film of grease that distributes the load over a finite area and there is microscopic deformation of the metal. Even though the real surface pressure is not infinite it is very high. That's why bearings and races are made from very high strength steel alloys and must be kept greased.
Where roller and ball bearings really show superiority is in friction. With a sleeve (bush) bearing the entire mating surface of the rod and cylinder are slipping, rubbing against one another. In a roller or ball bearing there is no rubbing. The bearings are rolling along one another. It's easy to see why you'll have much less friction force with a rolling bearing verses a slipping bearing.
In a very generalized sense you can say that rollers give lower rotating friction with higher load pressure while bushings give lower load pressure and more friction.
There are a lot of trade-offs in choosing one type of bearing over another but it is interesting to note that the most highly loaded bearings in a car, the engine's mains and rods', are almost universally journal bearings (much like a bushing or sleeve) while the wheel bearings are always roller or ball.
What does all this have to do with Jag suspension pivots? Not too much. That is yet another layer of complication.
Friction is lower in a rolling type of bearing but how important is low friction to a suspension member? Since the forces that move the piece are quite large and the motions are relatively limited small amounts of friction in the pivot will be overwhelmed by the driving force. In actuality, automobiles as we know them today can't operate safely without a very substantial amount of "friction" in the suspension. Friction (energy loss) must be added to a suspension system in the form of shock absorbers (dampers) for a car to handle, ride and respond acceptably. The amount of energy absorbed by the shocks is orders of magnitude greater than in any pivot friction so any difference in friction between rollers and bushes in the pivots is negligible.
A suspension bearing pivots but does not spin freely. It rocks back and forth. Depending on the diameter of the rollers and the angle of suspension deflection you may or may not rotate the rollers in a complete revolution.
What happens to the tires on a car if it sits in one place? They get flat spots. If you rock the car back and forth, never far enough to turn the wheels completely around and always bring it back to rest in the same spot you'll eventually get really severe flat spots. When a roller bearing is loaded heavily and rocked back and forth without spinning a similar effect can occur and the bearings can go out-of-round, lose precision and/or become "notchy". The effect will be severely accelerated by intrusion of dirt or loss of lubricant.
This is the effect that the proponents of bushed suspension pivots cite when recommending their modifications.
Does it actually happen to Jag suspensions in the real world and do the bushings prevent it? Heck if I know. I've never taken one apart, let alone a bunch of them.
PC.
Online
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by PC:
In theory it's a bit more complicated than that. <hr></blockquote>
Yes it is. I was just trying to use the KISS principle
My bearings are 35 years old and still in relatively good shape. I've changed them out just because, well, I have the IRS out now so may as well renew everything.
In theory it's a bit more complicated than that. <hr></blockquote>
Yes it is. I was just trying to use the KISS principle
My bearings are 35 years old and still in relatively good shape. I've changed them out just because, well, I have the IRS out now so may as well renew everything.
Bob Stevenson
Senior Member
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The needle bearings on the inside are fine but the tapered rollers in the carrier can and do cause problems. They move such a small amount that if the pre-load is not correct or with any wear they will basically stop turning and beat an indent into the race. I've seen this many times and have had a set of what I call plastic bearings in the carriers of my 64 for the past 12 years with absolutely no problems.
Bob
Bob
Online
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Bob Stevenson:
<snip> if the pre-load is not correct <snip><hr></blockquote>
This is very true - a friend of mine who is a Master Jag mechanic (actually a master of masters) tells me that those bearings must be properly pre-loaded per the manual.
Basil
<snip> if the pre-load is not correct <snip><hr></blockquote>
This is very true - a friend of mine who is a Master Jag mechanic (actually a master of masters) tells me that those bearings must be properly pre-loaded per the manual.
Basil
Thanks to Basil , PC and Bob. Lots of information. Classic Jags are pretty positive on their bushing replacement, they recommend 3000 mi.lubrication interval. Their products look to be first class. I would also be interested in what Bob has done with his IRS. When I tear down the wishbone and inpect the races,which have been lubed often, I think I will have a better idea which way to go. Thanks for all the input.
Bob Stevenson
Senior Member
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I doubt that you will find any problem with the inner bearings which are needles but if there is any trace of markings on the taper bearing races, and I'd be VERY surprised if there isn't, replace them. Actually I have the "plastic' bushings for both the inner and outer and bought them from a place, I believe in Washington that sold Jag rear ends mainly for street rods. That was years ago and now most venders sell something similar. I was not happy with the inners as the overall length was slightly to long and I had to have a few thousands machined off before they were useable. If I hadn't doubled checked and dimensions the lower control arm would have be rotating on the bushing instead of the bushing rotating on the shaft.
Online
For what its worth, after discussing this with a friend who also happens to be a Master Jaguar mechanic, per his recommendation, I am going to have him install factory bearings in my hub carriers, which he will properly pre-load for me. I'm sure the Classic Jaguar product is first rate, but I am sticking with the factory setup.
Basil
Basil