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Author Topic: Lakester rear suspension  (Read 14445 times)
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Jack Gifford
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« on: April 19, 2016, 01:02:09 AM »

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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« Reply #1 on: April 19, 2016, 09:58:13 AM »

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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Dynoroom
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« Reply #2 on: April 19, 2016, 10:29:04 AM »

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.


* From shop 907b (Large).jpg (143.08 KB, 1024x768 - viewed 382 times.)

* From shop 908b (Large).jpg (138.26 KB, 1024x768 - viewed 343 times.)

* From shop 905b a(Large).jpg (160.88 KB, 1024x768 - viewed 346 times.)
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Michael LeFevers
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Dynoroom
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« Reply #3 on: April 19, 2016, 10:31:04 AM »

One more that wouldn't post.


* From shop 906 b(Large).jpg (151.68 KB, 1024x768 - viewed 391 times.)
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Michael LeFevers
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ronnieroadster
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« Reply #4 on: April 19, 2016, 05:08:07 PM »

 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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Jack Gifford
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« Reply #5 on: April 19, 2016, 11:01:01 PM »

... 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.
« Last Edit: April 19, 2016, 11:18:14 PM by Jack Gifford » Logged

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Jack Gifford
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« Reply #6 on: April 19, 2016, 11:06:55 PM »

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.
« Last Edit: April 19, 2016, 11:24:14 PM by Jack Gifford » Logged

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« Reply #7 on: April 20, 2016, 06:15:31 PM »

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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ronnieroadster
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« Reply #8 on: April 20, 2016, 08:12:28 PM »

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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Working in the shop I use the 'F' word a lot. No not that word these words Focus and Finish go Fast and Flathead Ford!
 ECTA  XF/BGRMR Record 179.8561
 LTA    XF/BGRMR  Record 186.946
 SCTA  XF/BGRMR Record 192.448
 SCTA  XXF/BGRMR Record 216.131 plus a Red Had
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« Reply #9 on: April 20, 2016, 10:04:32 PM »

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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Jack Gifford
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« Reply #10 on: April 21, 2016, 12:19:28 AM »

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 grin]
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Jack Gifford
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« Reply #11 on: May 09, 2016, 12:37:59 AM »

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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« Reply #12 on: November 06, 2016, 12:17:07 PM »

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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dw230
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« Reply #13 on: November 06, 2016, 01:28:45 PM »

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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« Reply #14 on: November 06, 2016, 04:23:14 PM »

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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