Separating ride from roll...?

[exwestracer]

Jaybob (and everbody else...) here are some pictures to mull over. This is where the car stands now.


Above shot from the front of the car showing the basic Mumford setup with cross link and 2:1 crank arm for the ride shock. At upper left is the "top" mount for the coilover.

LF corner of car showing pull rod and bell crank for ride shock. Behind the aluminum cap on the pivot is a needle and thrust bearing arrangement. In the background is the engine mount plate. Notice how the Chevy bolt pattern is rolled 30deg to the left.

RF corner. Back side of pull rod spud on axle is threaded for the roll link. End of roll shock shaft is just visible under upper frame rail

From right side of car. Roll shock and adjustable mount at top of picture.

Closer view of double end roll shock and mount. The shock will be held in place by a spring perch adjuster nut on either side of mount. Moving the shock side to side will change cross weight or "wedge" on front axle. The small spring on the far end of shaft represents how the roll springs will be mounted on both ends. The spring force will balance against the shock body to hold the car upright at static ride height. Keep in mind the car will have about 66% of the total weight on the left side tires.

[arue333]

So any news? Updates?

[exwestracer]

So any news? Updates?

Bought another car that was supposed to be "bolt together and go". It turned out to be a parts pile, but I'd spent the money, so now I'm finishing it first.

Short answer, no.

[BrianP]

Why wouldn't adding a coil over setup just shove the axle to the side until the other linkage is at the limit of it's travel?

[cornfedbill]

Ray,

I did some sketching of the Mumford link suspension. If I am not mistaken, mounting the pivots to the frame, as in your photos, causes the roll center to move towards the outside tire under roll. Isn't it more desirable for the roll center to move inboard during roll?

If the pivots were located on the axle (not desirable for unsprung weight) the roll center could be made to move inboard on a corner and thus cause the body of the car to begin rolling downward, reducing the center of gravity (slightly) and making the car more stable.

We did this on a student design project years ago. It was for the SAE Mini-Baja. The front and rear roll centers were very low and moved inboard significantly during roll (which was much more than a street car). The body would actually begin to squat under hard cornering. It was nearly impossible to roll even with the relatively high roll center and modest track width.

What do you think?

[exwestracer]

Ray,

I did some sketching of the Mumford link suspension. If I am not mistaken, mounting the pivots to the frame, as in your photos, causes the roll center to move towards the outside tire under roll. Isn't it more desirable for the roll center to move inboard during roll?

If the pivots were located on the axle (not desirable for unsprung weight) the roll center could be made to move inboard on a corner and thus cause the body of the car to begin rolling downward, reducing the center of gravity (slightly) and making the car more stable.

We did this on a student design project years ago. It was for the SAE Mini-Baja. The front and rear roll centers were very low and moved inboard significantly during roll (which was much more than a street car). The body would actually begin to squat under hard cornering. It was nearly impossible to roll even with the relatively high roll center and modest track width.

What do you think?

Bill, your theory is correct, however there isn't enough roll motion in these cars for it to be a concern.

I did Mini-Baja for CSU Fresno for 3 years back in the late 80s. Formula SAE as well, although I never made it to a competition. Great training!

[cornfedbill]

Bill, your theory is correct, however there isn't enough roll motion in these cars for it to be a concern.

I did Mini-Baja for CSU Fresno for 3 years back in the late 80s. Formula SAE as well, although I never made it to a competition. Great training!

Yes, you are probably correct. It is fascinating to see how effective moving the roll center towards the inside tire can be at stabilizing the car in hard cornering.

It is amazing to see how many fast cars break all of the theoretical "rules".

I do wonder if idealized roll center movement would be effective in a multi-purpose PT car. Maybe roll stiffness could be reduced to give a more compliant ride without a measurable sacrifice in cornering. It may even be effective when cornering on an uneven surface to prevent the tires from bouncing, thus keeping more rubber on the road.

Just thinking out loud.

[exwestracer]

Typically, we'd like the roll center migration to be as small as possible. Again, it's one of those things that would be different for each corner we went around, so predicting where the R/C would be and how it would affect cornering becomes mind-boggling.

I've had thoughts about the effectivness of the "modified" Mumford on the rear of a P.T. type car. It would need a LOT of bar, as the R/C is MUCH lower than any other type of lateral locator (except a WOB link) would give us.

I guess in my mind it's not really "necessary"...

[mitch_04]

Awesome thread, thanks for sending me to it. I'll reply with more in a couple weeks when I understand what's going on!

[AMC Racer]

Sorry if this is a bit off topic, but with the Mumford link, if you put the chassis mounts and axle mounts higher, can you get a roll centre height more typical for a PT car? Should give better ground clearance and maybe require less substantial bracing than a typical watts?

Thanks.

[exwestracer]

Sorry if this is a bit off topic, but with the Mumford link, if you put the chassis mounts and axle mounts higher, can you get a roll centre height more typical for a PT car? Should give better ground clearance and maybe require less substantial bracing than a typical watts?

Thanks.

You can change the R/C height by raising (or lowering) the chassis mounts. This involves taking the included angle out of the drop links. I haven't mapped this out, but I believe the Mumford wants a certain minimum amount of angle in the links. If the links become too close to horizontal, they have to "stretch" to allow any body roll. As I said, I'm not sure where that definite point is.



The mounting can also be reversed (as can a Watts link) to put the pivots on the axle housing. With a Watts, one mounting gives us a R/C height fixed to the chassis, the other gives us a R/C fixed to the ground. With the Mumford, the R/C is still moving with the suspension, but the direction of motion is reversed. With the pivots mounted to the chassis, the R/C drops with suspension compression. With the pivots on the AXLE, the R/C raises with compression.