PHB vs. Watt's Link

[Chevy350]

This thread is over one year old...
I have that issue of Hot Rod, and thought the '67 Chevelle was bad ass the second I saw it!

[Mean 69]

Hoiw ironic, the last time I replied to this thread, I was putting a motor in my 69 Camaro. Today, I was putting a motor, in my 69 Camaro.
M

[parsonsj]

the last time I replied to this thread, I was putting a motor in my 69 Camaro. Today, I was putting a motor, in my 69 Camaro

Geez, Mark, how engines does that Camaro need?

With me, last year I was working out all the final bits and pieces I needed to finish my car. This year is different: I'm working out all the final bits and pieces I need to finish my car.

jp

[Mean 69]

Geez, Mark, how engines does that Camaro need?

Only one. The one that doesn't blow up! No more factory blocks in performance cars for this guy, Bowtie time.
M

[trackrat79]

There is another option. You could use what is called a wishbone track bar. It gives you the function of a watts and the weight of a strait bar. The only thing is I don't know what type of lateral load can be put on them. A center pivot is mounted directly over the pumkin and one on the inside of each frame rail usually towards the center of the car. It is nice because ther is no laterall movement in the system and the axle can still pivot on the drive shaft centerline.

[Norm Peterson]

I doubt that making it strong enough would be as much a problem as the fact that it generally leaves you with a roll center that's either several inches too high for best handling on pavement (top of pumpkin mount) or too low for unrestricted street use if it's attached to the bottom of it (read: marginal ground clearance to at least the low point of the center pivot's stud).

Either way has worked in competition, but I think usage follows the venue; high for dirt and low for paved road courses.

Norm

[David Pozzi]

I came across this while looking for something else. I thought it would be a good topic for the Tech section, so I moved it here.

[hssss]

[ProStreet R/T]

I'll come back to this shortly... looks interesting.

[hssss]

I have more info that can be posted. I am interested in using in somewhat extreme Lotus 7 with 3 link.

[g-roadster]

hsss:
The Mumford linkage looks very interesting. It looks like the instant roll center will be at the intersection the right and left axle to "Mumford linkage" links. It seems to have all the advantages of the frame mounted watts linkage with increased ground clearance, a lower roll center and less complex attachment to the axle housing. Please post more info.

[hssss]

http://locostusa.com/forums/viewtopic.php?t=77
http://texaslocost.homestead.com/Suspension.html
http://www.project-seven.goof.com/rear_suspension.htm
http://not2fast.wryday.com/chassis/mumford.shtml

I am planning a 392 Lotus 7 the best solid axle setup seems to be a 3 link with a Mumford link and adjustable sway bars. I have a 06 Mustang 8.8 coming. I'm not an engineer and too far into therory makes my head hurt but I like the looks of this setup. This should be enough info to get started.

[wendell]

If you're working from a blank slate I would think about a lateral locating system that offered an adjustable height. It's pretty tough to beat a pbar for simplicity, weight and adjustablilty. With the exception of a low insprung weight, the mumford doens't look very attractive with no adustablilty and the weight of two bell cranks and a full chassis cross member (unless you use the cross member to mount your fuel cell)

[hssss]

The typical Lotus frame has that cross member anyhow. Once the ride height is set with this car it would probably not be changed though adjustability is via the two long links.

[wendell]

With the two bell cranks fixed, I see no method of adjusting the rear roll center.

[Mean 69]

This is a pretty trick setup, and as pointed out, the main benefit is that the roll center can be set really, really low. On the typical heavier front engine sedans that folks in this forum typically build, there is without question a rear roll center height that is indeed "too low." One strong consideration for these cars is the roll couple, which is basically the imaginary intersection line between the front and real roll centers: on these cars the most desireable condition is to have the line sloping down from rear to front, it gives the best "feel" for the car going around turns. These cars typically have high-ish centers of gravity (17 to 20" or so, depending upon ride height, engine selection, etc), and front roll center heights that can vary all over the place to put it nicely. A good practical front roll center height on a well thought out modified front suspension system is of the order of 2 - 4", give or take. "Probably" the lowest you'd want to go for the accompanying rear roll center height is 8", but this is not a hard-fast rule. Most of the cars have roll centers well above this, typical PHB cars are around 13" because for some reason the suspension suppliers in this market (most, not all), put the bar at the same height as the axle centerline. Anyway, a frame bellcrank mounted Watt's can be packaged to get to this suggested target RRCH and still have good ground clearance.

For your Lotus, comparison bets are off. It's a WAY lighter car with a far lower center of gravity (I'd guess, I have never owned one), and you can approach more of the purpose built race car "numbers" for things like roll center heights, roll couple, etc: it's a different animal. If you find that you really CAN benefit from an ultra-low RRCH, this looks like a fine way to go. But I'd suggest doing your homework (and maybe you already have, not to imply you haven't)on the overall suspension system including looking at the dynamics of the front before you commit to this structure. For certain, if you are going to be restriced to a small range of RRCH adjustment, I'd plan on an adjustable rear torsion bar as well, as when the roll centers drop, the tendancy for roll angle increases (all things being equal).

Fun project!
Mark

[Norm Peterson]

With the two bell cranks fixed, I see no method of adjusting the rear roll center.

This arrangement works differently than the Watts link that I suspect you're comparing it to. If the roll center were indeed specified by the two bellcrank pivots (rather than just their influence on link inclinations) you'd have two competing "roll centers", very little roll, and lots of unexpected/unwanted roll stiffness.

Mark - with a car as light and as low as a 7 or any of its clones, parameters like roll moment vs track and mass moment of inertia in roll (or pitch, for that matter) are quite different from the typical ~3200# ponycar or 3500# intermediate. For the same amount of roll, relatively less ride rate and/or sta-bar is required, and the shocks are not forced to work quite as hard in damping either pitch or roll motions during transients.

If I'm not mistaken, the front suspension has a slightly falling rate in bump . . . something else to ponder

Norm

[Mean 69]

Indeed, I saw an outline of the parameters for his project on another forum earlier today. Quite, quite impressive from an overall standpoint, terribly light, lots of power, this car is going to be a rocket once sorted, that is for certain. Daytona Prototype power levels, F1 weight.

I'd be concerned about a falling rate front setup, but of course depending upon the rate of decline. Not much travel needed on this car, so perhaps in practice in this example it might not be too much to fret over, but again, not optimal IMHO.

M

[hssss]

Here is a pic of the tow vehicle.

It's taking a nap while I gather additional parts.

Mean 69
I think the project is quite dooable and have most of the mechanicals on hand. I am somewhat commited to the front suspension but the only thing set is the spindles. Basicly a MII sourced front is used on a LOT of cars. One of the problems seems to be the U&LCAs are too shortso since they will be fabbed I had figured on extending the LCA to 15" and the UCA to whatever it needs to be. One of the things I have seen mentioned was that the spindle was a bit too short so I figure the pivot point can be lowered 1". What else should be done? I am frugal/cheap so wherever possible I intend to use production parts but not be silly about it.

[wendell]

Norm,
That is what I was thinking. What defines the roll center in a Mumford linked suspension?