Leading Arm vs. Trailing Arm

[budawe]

Well put Steve. I don't have a tool like Mitchel's program, but strings and measuring tape have told me the same thing.

[sabre1fv]

I too have seen numbers similar to what Steve mentioned using two dial indicators on a bump steer plate and dialed in my front trailing arm (in front of the axle) mounting points prior to welding them on. Minimal bump/roll steer change is achievable as both Brian and Steve have mentioned with the arm leading or trailing. So is one better than another? Mounting in the back would seem better from a packaging standpoint (better aero and more possibilities for the shock) but the reaction forces under braking may tend to lift the back end of the car. Some of this may be controlled with your shock settings. Forward mounting points would tend to place the brake reaction forces near the center of the car - is this better? I don't really know.

Then as Brian H. brought up, maybe some roll/bump steer is useful. This can be tested more easily when the mounting points are in the back and you have the vertical posts for mounting points.

I will add that when I checked the bump steer on my car when it had the original Lynx style front mounting points it was really bad!

-Jim

[budawe]

Many designers use a bit of roll steer to reduce understeer. The original Lynx (caracaL) zero roll update kits included pickup points that induced roll steer. I personally did not like the feel of the car and learned how to dial it out by reducing the rear pickup point. Essentially, I approached it by making sure the arms were parallel to the axle tubes just like I do with tie rods and the roll steer was gone.

If one gets on their hands and knees and eyeballs the trailing arm vs the axle you will be able to tell if there is an issue. Then take a string parallel along side the rear wheels. Jack up one side of the front end. If the distances to the string become unequal you have roll steer. Check if it steers in or out. Steering inward will increase understeer and steering out will induce an oversteer.

Super modified oval dirt cars have a huge amount of roll steer designed in to get the car to rotate. You can actually see the inside rear tire move forward as the car goes into the turn. All oval cars have some linkage to get roll steer. In most cases, it's much more subtle.

[Hardingfv32]

I like a little toe-in from my roll steer or jack steer to reduce oversteer. Is there any downside to such actions? Could drag from the toe in be of significance in say a flat out sweeper?

Brian

[Daryl DeArman]

I like a little toe-in from my roll steer or jack steer to reduce oversteer. Is there any downside to such actions? Could drag from the toe in be of significance in say a flat out sweeper?

Brian

IMO Brian, the drag from a small amount of toe through a sweeper is not of much consequence IF it is only the amount of toe-in you need (nothing more) to get through the corner flat and it returns to zero at whatever rake angle you have "accelerating" (it is a FV) down the straight.

[S Lathrop]

If you check the toe change of both rear wheels you will see that net change between the wheels is not measurable. You will have the outside rear wheel toeing in and the inside toeing out by nearly the same amount. Or you can have the reverse. Toe in or toe out will produce different effects.

Unfortunately the toe issue is interrelated with squat/jacking under acceleration and braking. In the end you have to make choices on what you want.

I have built cars with roll toe in and pro squat and roll toe out and anti squat. Both can be made to work well. I have not been able to get roll toe in and anti squat.

[Daryl DeArman]

I have not been able to get roll toe in and anti squat.

You just need to get that CG lower

On a related note, how do you determine the IC on a 2 link swing axle ?

[budawe]

First, its really great to have Steve with us. He's designed some of the best handling vees on the planet. It took a rain race at RA to realize how good his cars are.

That said, I would never be able to engineer out the same issues challenging us all. What I have concentrated on is to slow the rate at which all interactions occur.

Many years ago the D-13's showed up with VW steering dampners on their early zero roll. When I saw that I almost wet my pants! I learned from motorcycles that head dampening was critical and when I saw that set up I had to try it.

While not as critical at RA, a car that would rotate quickly and stablily, was what worked at Topkea. I've run an adjustable m/c head dampener on my car ever since.

[S Lathrop]

You just need to get that CG lower

On a related note, how do you determine the IC on a 2 link swing axle ?

The geometry of the suspension gives the potential for anti spuat/lift. The CG relative to the IC gives the amount of that effect.

The VW rear end is a special situation for a standard 4 link rear end like and vintage FF. The inner ends for the upper and lower links are the same.

[Daryl DeArman]

The geometry of the suspension gives the potential for anti spuat/lift. The CG relative to the IC gives the amount of that effect.

Understood. Which is why I said (with a smiley) you just need a lower CG to get both anti-squat and roll toe-in as roll toe in would require the link to be level or down hill towards the front of the car.

The VW rear end is a special situation for a standard 4 link rear end like and vintage FF. The inner ends for the upper and lower links are the same.

So the IC on a VW is calculated much the same as a ladder bar suspension? Thanks for the info.