Traction Bars

[turboshad]

Here's what I see going on at the instant of torque, no slipage and no forward acceleration. The forces that will lift the car will be FAY from the first set of equations and FFMY and FSY from the second set. This is a lift before any acceleration is taken into account. It definately shows that what is happening isn't a real simple situation, but that what makes cars so interesting. Chime in if I have something off.

[ShelGame]

The only thing I would add is that, due to the compliance of the engine mounts, the center of roatation of the eng/trans assy is not the axle centerline. It's some point in space that's likely impossible to determine analytically. My idea was to use stock side mounts and a poly (or solid) front mount. In this case, the eng/trans center of rotation is at the front mount and your lever arm becomes longer. I'm not 100% sure that's the correct placement, though. I think Ideally, you'd want a linkage that would put the IC at or below the ground so that you can more easily get a rising rate on the linkage.

[BadAssPerformance]

Here's what I see going on at the instant of torque, no slipage and no forward acceleration. The forces that will lift the car will be FAY from the first set of equations and FFMY and FSY from the second set. This is a lift before any acceleration is taken into account. It definately shows that what is happening isn't a real simple situation, but that what makes cars so interesting. Chime in if I have something off.

Nice FBD but Like Shel said below, it would be dificult to calculate the input force.

The only thing I would add is that, due to the compliance of the engine mounts, the center of roatation of the eng/trans assy is not the axle centerline.

You know when I did the FBD in my head I was thinking the same thing... and was wondering if that was in part a contributor to Warren's axle issues? I know his was strapped so the axle angle should not have changed, but if it did, and got greater than the desired ~12° max or so... Hmmmm.

It's some point in space that's likely impossible to determine analytically. My idea was to use stock side mounts and a poly (or solid) front mount. In this case, the eng/trans center of rotation is at the front mount and your lever arm becomes longer. I'm not 100% sure that's the correct placement, though. I think Ideally, you'd want a linkage that would put the IC at or below the ground so that you can more easily get a rising rate on the linkage.

Solid front mount and the engine wont move (at all) to give you the input force. With a poly, wont move much either. Not sure thats the best way tho cuz as the engine rotates back the axle center drops increasing angle on the axles? But if it rotates about eth axle centerline there wont be much displacement... Hmmm...

[ShelGame]

You know when I did the FBD in my head I was thinking the same thing... and was wondering if that was in part a contributor to Warren's axle issues? I know his was strapped so the axle angle should not have changed, but if it did, and got greater than the desired ~12° max or so... Hmmmm.

Couple things I thought about Warrens setup:

1) The input on the sway bar wasn't in the center of the bar. Since the bar has some stiffnes, by offsetting the input, you'll get different Fn at the tire for each tire - at least dynamically.

2) The linkage connection on the trans is below the axle centerline and well below the engine center of rotation (I'm guessing somewhere near the engine mounts). With the (apparent, hard to tell in pics) angle of the linkage, it looks like a dropping rate linkage - the linkage goes toward a toggle point. Of course, it doesn't move enough to actaully toggle, but that's the wrong direction, I think. Would be better as a rising rate linkage, at least that's what my intuition tells me. I could be wrong on that.

3) I think some sort of damping between the engine and body might be a good idea. You have a lot of mass (eng and trans), and a relatively soft spring (roll bar) and no damping. I think you might get a harmonic in there that would load/unload the conatct patch and cause wheel hop. Again, just my intuition. I could be wrong...

[ohiorob]

Where did you mount it to on the LCA? I was looking and thought that making a bracket utilizing the sway bar bolts might work.

I welded a bracket on the frame right above the LCA and mounted the strap to the swaybar bolt. not a good spot. I need to move them out.

[turboshad]

The only thing I would add is that, due to the compliance of the engine mounts, the center of roatation of the eng/trans assy is not the axle centerline.

I definately agree that the engine won't be rotating about the axle. But in an FBD you can sum the moments anywhere and the resultant forces will be the same. In hindsight I would have been smarter to disolve the torque into say the side mount point giving a FTX and an FTY at that point and then sum the moments there. Then I could get rid of two unknowns right off the bat.

Nice FBD but Like Shel said below, it would be dificult to calculate the input force.

I think the input torque would be the easiest to get an educated guess on. You could either use a 60ft and get an average acceleration, and then use the mass to get the force at the wheels and back calculate. Or you could use a dyno curve and estimate the torque with launch RPM, boost and overall gear ratio assuming a no slip condition on the slicks.

You know when I did the FBD in my head I was thinking the same thing... and was wondering if that was in part a contributor to Warren's axle issues? I know his was strapped so the axle angle should not have changed, but if it did, and got greater than the desired ~12° max or so... Hmmmm.

Straps make the FBD even more confusing, though I think it would be an internal stress and not transfer more energy to the wheels. It would just limit the CG rise.

Solid front mount and the engine wont move (at all) to give you the input force. With a poly, wont move much either. Not sure thats the best way tho cuz as the engine rotates back the axle center drops increasing angle on the axles? But if it rotates about eth axle centerline there wont be much displacement... Hmmm...

If the axle cenerline drops it should be decreasing the angle since the initial angle is on a downward slope from the engine.

Couple things I thought about Warrens setup:

1) The input on the sway bar wasn't in the center of the bar. Since the bar has some stiffnes, by offsetting the input, you'll get different Fn at the tire for each tire - at least dynamically.

I agree. It will have a larger bias towards the drivers axle since there would be less sway bar angular deflection. By chance is this the axle that was breaking?

2) The linkage connection on the trans is below the axle centerline and well below the engine center of rotation (I'm guessing somewhere near the engine mounts). With the (apparent, hard to tell in pics) angle of the linkage, it looks like a dropping rate linkage - the linkage goes toward a toggle point. Of course, it doesn't move enough to actaully toggle, but that's the wrong direction, I think. Would be better as a rising rate linkage, at least that's what my intuition tells me. I could be wrong on that.

It is hard to tell from the pics but I agree that you would want a rising leverage rate if you could do it.

3) I think some sort of damping between the engine and body might be a good idea. You have a lot of mass (eng and trans), and a relatively soft spring (roll bar) and no damping. I think you might get a harmonic in there that would load/unload the conatct patch and cause wheel hop. Again, just my intuition. I could be wrong...

The damping might have an adverse effect since you are relying on the shock load from the engine going straight into the wheels before the body has a chance to react and lessen that force. A damper may cause the process to take too long and the body will react before you have a chance to fully take advantage of it.

I welded a bracket on the frame right above the LCA and mounted the strap to the swaybar bolt. not a good spot. I need to move them out.

Well, that sounds exactly like how I saw it. What size straps were you using? I was thinking they would have to handle at least 1000lbs per side with it being inward of the spring center line and there being an element of shock. So I was looking at the 4X4 limiting straps good for a 5000lb working load. Thanks for the feedback

[BadAssPerformance]

I think the input torque would be the easiest to get an educated guess on. You could either use a 60ft and get an average acceleration, and then use the mass to get the force at the wheels and back calculate. Or you could use a dyno curve and estimate the torque with launch RPM, boost and overall gear ratio assuming a no slip condition on the slicks.

It is an instantaneous hit, way sooner than 60'...

Straps make the FBD even more confusing, though I think it would be an internal stress and not transfer more energy to the wheels. It would just limit the CG rise.

Hmmm...

If the axle cenerline drops it should be decreasing the angle since the initial angle is on a downward slope from the engine.

At rest, my axles go from the trans upwards to the hubs... MP lowering springs

[dodgeshadowchik]

engineers.


are.


fun!

DJ: your drawing gave me a slight headache.

/end worthless post. back on topic!

[thx138]

I hate to bring this topic about straps back from the dead but: If the downside of straps is the shock when they limit, could a person limit the spring instead. For example, if the spring has say 6 coils, run a strap/cable around 4 coils leaving 2 open. You would still have full compression, but the spring 'lift' would be limited to the 2 coils out of 6. This should be more transitional without the impact of hitting a limit strap and should limit the lift to a inch or two at most (depending on how many coils are bound). Kind of like getting the benefit of a spring clamp only on extension. What I get out of all of the prior posting is that straps work, but axles suffer because of the shock.

Just playing around with an idea.