Anti roll bars

[jdincau]

In order to improve straight line stability I want to put an anti roll bar
on a 4 link, Jacob's ladder, coil over suspended solid rear axle. The front
end is a transverse leaf spring which is very stiff. How do I make a guess
about the spring rate of the bar?

[bobadame]

I think generally what you would do is to determine how much body roll you can tolerate. The center of gravity height near the rear axle and the distance between the springs compared to your track width determine how much roll resistance you already have.

I guess you might start by loading the chassis with weight directly over the area where you plan to attach the bar end.  Just load one corner of the car. Measure the degree of lean for the given weight. From this you can roughly work out a spring rate needed to counter balance the car.

I'm curious, do you plan to use an open differential?

[ddahlgren]

Your problem is in the front not the rear. Any well designed chassis has more roll resistance in the front than rear and you have none in front at all. To be candid if you put a modern front suspension in the issues will go away. The straight axel while simple in concept is just no good for anything short of a horse drawn wagon as that is where they came from. Sure you can make them work at times but it is a chore and comes with a lot of poor compromises. The fact that a bump on one wheel upsets the other is a start and they get worse from there.
dave

[bobadame]

I have more questions aboout your car. What class was it built to run in? Where is the engine? What is the weight distribution? How is the front axle located? split wishbones? how long? paralell? What is the center of gravity height front and rear? Is the front axle tubular or I beam? What is the wheel base length?

[jdincau]

Bob
     The car is an 1800 lb 100 inch wheel base street roadster. The engine is set back 10 inches. The weight is distributed 43% front 57% rear. The tube front axle is located by 30 inch long wishbones and a 24 inch long 5 leaf transverse spring with 2 inch shackles. The wishbones are parallel to the ground and the 30 inch long drag link. There are provisions for a pannard rod but pulling side ways on the chassis shows little movement from the shackles. I know that the combination of tube axles and wishbones binds on one wheel bump but the suspension travel is one inch. The good side to this is that it increases the roll resistance at the front. The CG at the front is approximately 17 inches off the ground at the rear it is about 20 inches. The tires are 24.5 inches OD front and 28.5 inches rear. The track widths are 55 inches front and 56 inches rear.
     I would like to keep the roll to less than an inch so the tires can be close to the fenders. I can put a spring scale on the roll hoop and see how much force it takes to roll it. Given the manufactures chart of stiffness for a given bar and arm length I should be able to do the geometry and math to calculate the desired moment.
     Dave's comment about the antiquity of the design is of course correct. I know that the front roll stiffness should be greater than the rear to avoid over steer. That being said, for a vehicle designed to go in a straight line on a relatively smooth surface, the anti squat, anti dive, toe change on bump and all the other sundry adjustments required for getting the right amount of under steer for a driver to produce the quickest lap times are moot. I will again look at the possibility of putting the bar on the front however there is extremely limited space available and I don't want any of it to hang out in the air turbulence.
Thanks for your time,
Jim

[bobadame]

With that  kind of weight distribution I'm guessing you must have some ballast in the rear. If you could relocate that weight below the rear roll center, that would be a really good thing. I don't know if that is practical. One of the problems with split wish bones, as you know, is that they can cause roll steer. So it's probably worth figuring out a way to control roll at the front. Formula cars use some high rate, short little bars that they can tuck in just about anywhere. Maybe you could place a bar either under or over the frame behind the radiator and run the levers forward. Maybe linked to the wishbones. Probably would need stronger wishbones. Since you only have an inch of travel, the bar would need to be really stiff or the levers quite long.

I know from very limited experience that using a bar on the rear only can completely change a cars handling and it isn't all good.  

I'm really new at this land speed stuff but it seems to me that some of the fastest cars in the world use Henry Fords front suspension.

[russ jensen]

. The straight axel while simple in concept is just no good for anything short of a horse drawn wagon as that is where they came from.
dave

Really-maybe you should clue in the sprint car guys- everything I build has one in the front, by throwing away the K member and its associated garbage I can save around 150 lbs{GMC} On my Esex  it seemed to work well @ 150 .

[ddahlgren]

It is straight line racing until the car starts to get out of shape then you are cornering trying to either get it back straight or pulling the chute. As for comparing a sprint car to a 200 mph roadster on a flat smooth track, how can you what might they have in common? Where is the 'berm'?? I fail to see as well how you could save 150 lbs as a well designed a-arm suspension does not weigh that much and you have to put something back as well.
with a a-arm setup
you have 4 a-arms (tubes with heim joints)
2 coilovers
2 spindles
2 brakes
2 wheels
1 crossmember

With a straight axel you have
2 coilovers
2 spindles
2 brakes
2 wheels
1 crossmember
1 straight axel
1 locating rod ,panard,jacobs ladder something
2 wish bones..

What am i missing here??
Dave

[russ jensen]

With a straight axel you have
2 spindles
2 brakes
2 wheels
1 crossmember
1 straight

What am i missing here?? you assume to many parts, your premis was a strt axel is only good for wagons  simplicity - reliability are worth a lot -havent seen to many K whopers or petes with anything else either.
Dave[/quote]