Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: Jack Gifford on April 19, 2016, 02:02:09 AM
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My lakester (bought sans engine) has nice 4-wheel torsion bar suspension (but live rear axle, not independent). It's looking like my drivetrain will be too lengthy to use a driveshaft with U-joints. I might forego any rear suspension and rigidly mount the housing to the chassis. But since reading about a rear-engine modified roadster (1956 Hot Rod Magazine) which suspended just the rear of the drivetrain via hinged front engine mounts, I realize that's an option for me. Certainly more unsprung weight than is desirable. But it wouldn't be the worst example of such a configuration- my engine is all-aluminum, the in/out box is light, and the front-QC is compact and quite light (aluminum case). The CrowerGlide and clutch can are heavy, but will be well forward of the rear axle. It seems to me that any rear suspension scheme is highly preferable to none, especially for pavement LSR.
I welcome comments...
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To minimize the effect of unsprung weight, the pivot point should be closer to the unsprung center of mass, at least longitudinally, not fully forward of the engine. Although, this may be more awkward to achieve. In addition, a supplementary structural connection between the axle and engine/transmission would be a good idea--not just the driveline connection.
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This rear engine modified roadster (the same thing as a lakester :evil:) has the type of suspension you are talking about. It has been over 280 mph with no handling issues.....
Thought you might like to know.
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One more that wouldn't post.
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Jack having some type of rear suspension would be a good thing for the pavement. On my converted lakester the rear suspension travel is only one inch I find that to be perfect for Wilmington and Loring.
I gave thought to doing a suspended swing arm carnage arrangement similar to what Dynoroom has shown and what was done so long ago on a number of LSR roadsters and lakesters.
My concern with the swing arm carriage design was how I would keep it tight enough to eliminate any side to side movement. I took the easy way out with a spring and radius rods.
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... To minimize the effect of unsprung weight, the pivot point should be closer to the unsprung center of mass, at least longitudinally, not fully forward of the engine...
... a supplementary structural connection between the axle and engine/transmission would be a good idea...
Thanks for the comments.
I have yet to determine (or closely estimate) the longitudinal position of the center of mass of my drivetrain. But I know that it's considerably forward of the centerpoint, due to the higher mass density (per longitudinal distance) of the engine, compared to the driveline. So I doubt that I would put the pivot point very much behind the front of the engine block, although it will be behind the belt drives (cams & blower).
I agree on the need for robust structural design, to avoid a "flexi flyer". I might be able to keep the existing hairpins that longitudinally locate the rear housing, if their pivot positions coincide with a good "hinge" location.
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Dyno- The success of that roadster encourages me to pursue the idea further. In the photo it appears that his transmission length is mighty close to the same as the length of my in/out/QC assembly- although the pinion support of my differential (Ford 8") is somewhat further ahead than the pinion support of his QC rear.
Ronnie- I'll be keeping the lateral-positioning that's already in my lakester. Shamefully, I can't remember- I think it has a track bar- which should be fine with very limited suspension travel.
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I'm by no means an expert on this but it seems to me that a longer pivot point will not increase unsprung weight significantly because of the small amount of up and down movement near the pivot point.
The longer it gets the less movement in the forward section.
Of course I could be wrong.
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Dyno- The success of that roadster encourages me to pursue the idea further. In the photo it appears that his transmission length is mighty close to the same as the length of my in/out/QC assembly- although the pinion support of my differential (Ford 8") is somewhat further ahead than the pinion support of his QC rear.
Ronnie- I'll be keeping the lateral-positioning that's already in my lakester. Shamefully, I can't remember- I think it has a track bar- which should be fine with very limited suspension travel.
Jack I think your correct about the track bar if memory serves me correctly I have seen your lakester at Maxton or Wilmington a long while back. I was interested in the torsion suspension its simple and effective similar to a sprint car. With the pivot point center line identical to the radius rod center line everything will work perfectly. Compared to the extensive amount of thought and work you put into your engine parts the rear suspension build will be a simple affair.
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To explore and perhaps clarify some of the effects that would be acting in the proposed configuration--
We consider that the engine, transmission, axle, and other related attachments act as a unit, have a particular mass (weight), and for dynamic behavior that mass can be considered to act at the “center of mass”. Since, indeed, the mass is not all concentrated AT the center, the assemblage also possesses a moment of inertia in the longitudinal and vertical (pitching) plane, which is only important if the assemblage is subjected to an angular motion, that is, in the pitching direction. A small moment of inertia would indicate that the object could be “pitched” easily, while a large moment would require more force, or more correctly, more torque to rotate the object. Turning a set of barbells about its axis is easier than rotating it about an axis perpendicular to the bar.
So, if we consider the pivot point to be in front of the engine, and consequently in front of the center of mass, when the axle strokes upward, it is simultaneously lifting a proportion of the mass by leverage about the pivot and rotating the assembly, also about the pivot, both actions of which require force to accomplish. That force is then proportional to the “unsprung weight”. (No mass, no moment, then no unsprung weight).
Scenario 1: The pivot is located at the center of mass. In this case, the axle movement is only required to rotate the assembly, not lift the mass, and the force required is proportional to the moment of inertia.
Scenario 2: The pivot is located far in front of the assembly. In this case the axle has to lift the mass virtually the same distance as the axle travel, but since the angular motion is essentially zero, the moment is of no consequence.
Thus, the ideal pivot location, which would minimize the effect of unsprung weight, is seen to be a consequence of the particular mass of the object, the location of the center of mass, and its mass distribution, or moment of inertia.
In most cases this would probably be somewhat in front of the mass center. However, worrying about its exact location is likely more of an academic endeavor than a practical one. Perhaps more important on the salt with its lower coefficient of friction than on a pavement course. Appropriate springing and damping could probably make any set up workable.
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Much more important than merely "unsprung" weight would be the decrease in ratio of sprung to unsprung weight if I take this approach. The ratio would take a big hit. [Unless I add a very large amount of sprung ballast :-D]
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A brief glance at the lakester shows that the rear radius arms' pivot points will lie about 2/3 of the block length behind the front of the block. That's good news if I take this approach- most of the engine mass would continue to be "sprung" mass.
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I'm considering the swing-arm in a tank, the problem I get hung up on is the weight of the engine/trans/rear axle will be greater than the structure 'supporting' it and the 'tail could wag the dog'. So to speak. I'm not entirely sure the terms 'sprung & unsprung' weight really apply here in a normal sense. A good friend built one and has reccommended I do the same, I just can't quite get my head around it.
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Swing arm rear axles were one of the reasons the rear engine modified roadster disappeared from the scene for several years.
DW
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I'm considering the swing-arm in a tank, the problem I get hung up on is the weight of the engine/trans/rear axle will be greater than the structure 'supporting' it and the 'tail could wag the dog'. So to speak. I'm not entirely sure the terms 'sprung & unsprung' weight really apply here in a normal sense. A good friend built one and has reccommended I do the same, I just can't quite get my head around it.
Are you talking about a swing axle rear suspension similar to an old VW or are you talking about mounting the powertrain on a longitudinal frame and pivoting that longitudinally within the main frame? Just remember you don't have to necessarily pivot it right at the front of the subframe. If you pivot it part way back a much larger portion of the weight of the powertrain will be sprung weight. Sprung and unsprung weight definitely apply in this case.
Pete
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Swing arm rear axles were one of the reasons the rear engine modified roadster disappeared from the scene for several years.
DW
thats sounds ominous, can you elaborate?
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When Al built the roadster that i have now, he went with the swing axle design that had been in Hop Up years ago. Still in there now. Al went with torsion bars. And I shortened them considerably to increase the rate. Al also incorporated pretty tight stops so that the tires couldn't tuck under a whole bunch. It seemed to have worked for him. At the speeds that I ran the car it was fine. I think it may be time for the car to find a new home. If someone bought it today they would likely change everything about the rear and frame entirely.
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When Al built the roadster that i have now, he went with the swing axle design that had been in Hop Up years ago. Still in there now. Al went with torsion bars. And I shortened them considerably to increase the rate. Al also incorporated pretty tight stops so that the tires couldn't tuck under a whole bunch. It seemed to have worked for him. At the speeds that I ran the car it was fine. I think it may be time for the car to find a new home. If someone bought it today they would likely change everything about the rear and frame entirely.
Thanks Rich. I keep thinking about the one Marty Strode built, do you remember it? T roadster, driver up front with blown flattie in the rear? The engine, trans & rear were swing arm (I guess I'm using the right term) and he said he'd do it again, said it worked fine.
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I suspect not all went to such lengths to limit travel and/or lift.
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Years ago I used to frequent Tom Beatty's shop off of Glenoaks Blvd, in Sun Valley. We would talk about all kinds of stuff but mostly Bonneville and El Mirage and his Belly Tank which had a swing axle, with only one universal on each side, just like a early VW Bug.
He told me about the time he was invited to run his belly tank at a dirt track oval in the Los Angeles area. He said in qualifying he was going way too fast into the first turn and said to himself that he messed up, but had to turn the wheel, and to his amazement the car stuck and did not roll as he had anticipated. Ended up setting the fastest qualifying time. As I recall they did not let him race after the qualifying run as the car literally put grooves in the track from the tires. If you think about this scenario the tank body rolled to the right which dipped the right axle and lifted the left axle which made the wheels look like \ \ going around the turn, which would make them dig in, hence the grooves in the track.
He also told me about running at El Mirage, making a U turn after the run and passing the push truck on the way to pick him up.
Nothing wrong with running a swing axle, what you have to look out for is the body you put over it. Some work better than others.
IMO early REMR suffered from short wheel base. 90 - 100 inch wheel base and driver in front. Driver had maybe 48 inches of hood to look down, and could not feel when the car was coming around, then when they realized what was going on it was too late, spin and possible crash.
Tom G.
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The orginal topic here concerned a sprung rear subframe which holds the engine, trans, and a rigid rear axle. Looks like some posts now relate to swing-axle rear suspensions.
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The orginal topic here concerned a sprung rear subframe which holds the engine, trans, and a rigid rear axle. Looks like some posts now relate to swing-axle rear suspensions.
I am getting confused to the terminology. Can you explain what a 'swing axle' is. The term 'sprung rear subframe' is good for me and describes what I am considering to do, is there a different term for it?
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PJ post # 14 describes a swing axle a VW has a swing axel
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PJ post # 14 describes a swing axle a VW has a swing axel
An OLD VW has a swing axle, and OLD Corvair has a swing axle. Bad idea.
Regards, Neil Tucson, AZ
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Swing axle
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Another. Summers Brothers-built quickchange:
(http://i233.photobucket.com/albums/ee171/4-BarrelMike/Streamliners/Lakester03.jpg)
Mike
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Gotcha. Bad juju that.
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... a rear-engine modified roadster... which suspended just the rear of the drivetrain via hinged front engine mounts, I realize that's an option for me...
After more thought, I'm leaning toward this "suspended rear subframe" approach. The scheme that looks attractive to me would pivot the front/center of the subframe on a balljoint anchored to a chassis crossmember about 10" ahead of the flywheel The subframe itself would be an A-frame with the 'point' of the 'A' at the balljoint, and the legs of the 'A' rigidly attached the the rear axle housing about where the present radius arms mount (radius arms would go away but track bar would remain). Torque reaction of the axle housing would be via an aluminum bulkhead between the legs of the 'A' and bolted to the pinion flange. A bulkhead at the inline QC would also attach to those legs to react driveline torque to the subframe. A triangulated-beam structure would extend past the 'point' of the 'A' to provide front engine support (merely support, to avoid any torque stress on the engine block). I like the fact that the existing torsion bar independent suspension would still be functional at all four corners- although I would only provide for a couple of inches of rear wheel travel. Not ideal from an unsprung-weight standpoint, but it would at least provide somewhat of a rear suspension.
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Something to consider: if the 'pivot' point is below the rear axle then when the wheels are driving the car and the car is heavier than the rear subframe then the wheel will try to drive over the pivot point and effectively 'lift' them over the pivot. If the pivot point is at the axle centerline then it'd be 'neutral'. If the pivot point above axle centerline then the wheels would be driven into the ground. In automotive parlance it is referred to as 'Squat' and 'AntiSquat'.
Mechanical ratios also come into play.
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"Squat/anti-squat" are very important considerations for dragrace vehicles. But not terribly significant for my F-class lakester- geared for 200+ MPH (only one drive ratio during runs), with just 182 c.i. for power. Nevertheless, my initial profile-view sketch shows almost neutral "squat" characteristics.
The main drawbacks I see are 1) low sprung/unsprung weight ratio, and 2) increased vehicle weight (a liability for 1.0/1.5 mile LSR).
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While OJ is correct to raise the consideration of squat and lift, the description he gives of how to deal with it is not correct. What he describes applies to an independent rear suspension system, while what is considered here is a solid axle on control arms, the control arms being the rear sub-frame and the suspension instant center being the sub-frame pivot point. The relationship of the axle centerline and the pivot has nothing to do with the squatting characteristics if the axle is attached to the sub-frame.
Any time forward tractive effort is applied (at ground level), the reaction will be to raise the pivot point, and consequently the chassis. The only way to mitigate this reaction is to lower the pivot point, giving zero lift when it is at ground level. Likewise, any rear braking will induce squat to the chassis.
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I see the 'rear subframe' as nothing more than an extended 4 link.
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After thinking things thru I see it is more complicated than I thought. It is quite a bit different from a 4 link, the problem as I see it the 2 lower attachment points to the chassis are more a hinge. Hmm.
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The thing it's closest to is ladder bars with the engine mounted on the bars. Depending on where the engine is mounted in relation to the pivot point more or less weight will fall in the unsprung category. My feeling is it's way too much work for the dubious results that may occur.
Pete
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:cheers:
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On my Lakester, it is solid in the rear. My thinking is; No engineering is better than bad engineering. :wink: Wayno
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... The thing it's closest to is ladder bars with the engine mounted on the bars. Depending on where the engine is mounted in relation to the pivot point more or less weight will fall in the unsprung category. My feeling is it's way too much work for the dubious results that may occur...
I agree with all of that. Except that in my case the "way too much work" is offset somewhat by the fact that the nicely designed torsion bar suspension is already in place for the rear wheels. So the only "work" I'd be left with is design & fabrication of the subframe and its frame-anchored pivot. [While avoiding the "work" of mounting the rear axle rigidly]
With an A-shaped subframe, it's closest to ladder bars with their front pivots moved together. This would preserve the non-binding independent travel of each rear wheel.
As for "dubious results", I've seen a number of examples of satisfied landspeed racers using some form of suspended rear subframe.
Thanks for all the comments.
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Although I'm usually not a fan of swing axles, it looks like I should at least consider them. It has the attraction of true independent suspension, and limited travel would keep the tires reasonably "upright". And by doing quite a bit of the machining myself it might even be inexpensive to convert my 8" Ford center section- depending on what "works" for axle U-joints.
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Jack;
Have a look at a Dedion rear suspension, it should be better than a swing axle (think old VWs).
https://en.wikipedia.org/wiki/De_Dion_tube
Regards, Neil Tucson, AZ
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I was just about to suggest the same, the deDion axle design. Seems to me that a deDion axle might be a better choice IF the 1/2 shafts are set such that they're level at speed (CVJ's running with no angle) to keep parasitic losses to the bare minimum. I would place the deDion tube behind the1/2 shafts so that I could use the pin/bearing in a slot (that is attached to the diff housing) method for lateral location. This would allow servicing the diff w/o having to disassemble the whole rear suspension too.
I did read 8" Ford, right? Just a slightly smaller & lighter 9" in design. A friend built a 9" housing narrower than Currie said it could be done. He literally cut the housing so that normal semi-float bearing ends fit up against the diff adjusters with *just* enough clearance should the diff be really offset in the drop-out. Application was different, but the approach worked then and still is working in that IFS 4WD truck 20-odd years later. I think it was Summers Bros who made the flanged axle shafts for him.
I would use the aero version of this CVJ boot: https://www.pegasusautoracing.com/productselection.asp?Product=1472 (https://www.pegasusautoracing.com/productselection.asp?Product=1472) or this aero boot: https://www.pegasusautoracing.com/productdetails.asp?RecID=5549 (https://www.pegasusautoracing.com/productdetails.asp?RecID=5549) depending on the CVJ size chosen. Could even attach a simple aero fairing to the deDion tube to enclose the outboard CVJ's and and 1/2 shafts.
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Thanks for the reminder of the DeDion option.
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Spent the last few weeks cramming all the "stuff" into the lakester. As expected, only room for a 3" driveshaft. So, for expediency, I'll try running without any rear suspension. To "freeze" the rear axle in place I simply replaced the shocks with heim-end rods.
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I've always been somewhat partial to using some sort of elastomer in the suspension such that you have almost a solid mount suspension but there's a little give to aid traction. It may be worth thinking about as in many cases it can be self damping.
Pete
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Spent the last few weeks cramming all the "stuff" into the lakester. As expected, only room for a 3" driveshaft. So, for expediency, I'll try running without any rear suspension. To "freeze" the rear axle in place I simply replaced the shocks with heim-end rods.
In my lakester in place of such a short driveshaft I simply used a four wheel drive transfer cash H yoke thats for the front driveshaft. This gave me two U joints placed about 2 -3/4 inch on center along with the transmission slip yoke it worked perfectly at Maxton ,Wilmington and Loring with my rear suspension.
Ronnieroadster