Straight Runner rear wheel alignment

[toddh1971]

I am setting up my sons alignment and was wondering if it is best to run rear negative camber on a four wheel straight runner? Is this better than zero camber? I have about a 3/4" COM, running on an aluminum track. I know a rail runner would be better but I'm a little late on that for this year.

[Stan Pope]

If it is 3-wheeler, the diff between straight running and a rail guided is a bit of toe-in on the DFW and assuring rear alignment keeps the rears off the rail.

If you have time to align it to run straight, you have time to align it to rail guide!

For both, I would use negative camber on the rears.

[toddh1971]

Can I make it a RR with no narrowing of the body? This is what scares me. It runs straight now with rear wheel negative camber and positive camber on the front. The car is complete and the wheels are on it. It tracks straight with the rear wheels going out to the axle heads. Worried the rear wheel would hit in a RR.

[Stan Pope]

If the rear wheels would strike the rail when rail guided, then they will strike with at least that vengence when running straight! To address your fears, tape a yard stick (meter stick if you are on the continent) down a slightly tilted flat surface and allow the car to roll down the slope straddling the stick. Watch rhe rear wheels. If the wheel behind the DFW rubs, you can add toe-out to it and toe-in to the rear opposite! "Just a dab will do you." Then repeat the test!

[Stan Pope]

Then, if you want to be sure that the rear axles/wheels are aligned with each other, you can temporarily bias each rear wheel, in turn, and show that the rear wheels follow the same path, regardless of which rear wheel carries the most weight (and is, therefore, dominant.)

I wrote this all out in detail at http://www.stanpope.net/bentaxlealign.htm.

[toddh1971]

Thank you Stan. I will give it a try tomorrow. How much drift should I have? I have read 4" in 4 feet? Is this about right?

[Stan Pope]

Thank you Stan. I will give it a try tomorrow. How much drift should I have? I have read 4" in 4 feet? Is this about right?

Drift depends on how rough the track is ... the rougher the track/rail, the more drift desired. On a good smooth Freedom track we ran best at about 1.75" in 30" ... with rather short CM location and mediocre wheels.

The shorter your CM location, the more toe-in is needed to hold the rail, which may account for excellent results with less extreme CM locations!

[ngyoung]

Just the difference of negative cant rears and positive canted DFW will keep the back off the rail.

[Stan Pope]

Just the difference of negative cant rears and positive canted DFW will keep the back off the rail.

Depends!
Assumes that the rears track true, i.e. that the axles are perpendicular to the car body! A fraction of a degree error and a rear is rubbing the rail!

[toddh1971]

I tested the car last night. The rear wheels stay off they rail. I have a drift of about 4" in 4 feet.

[Stan Pope]

I tested the car last night. The rear wheels stay off they rail. I have a drift of about 4" in 4 feet.

Great! That drift should be okay if your DFW carries very low weight, e.g. 0.5 or 0.6 ounces. As its weight increases toward 1 ounce, the optimal drift decreases, especially if you are racing on a longer track.

Were you able to perform the parts of the rear alignment test that confirmed that the rears were in agreement about their direction?

[toddh1971]

Yes, the wheels gravitate outward running forward and backward.

[Stan Pope]

Yes, the wheels gravitate outward running forward and backward.

If rear axle camber doesn't exceed 2.5 degrees, then that test is sensitive enough. Sounds like you are "good to go!" Show 'em your taillights!