Wednesday, October 2, 2013

RE-Xtreme RC Drift Bible - Degrees up front

Camber, Castor, KPI and Scrub in more detail.

Here is an awesome bit of gear. Overdose 8 degree Castor Camber hubs with KPI. What and Why?

Well actually they help to explain.

The hub itself is designed with a bit of camber in mind therefore positioning the CVDrive shaft in a flat position which is a bit lower than some others. It also has ball bearings to reduce friction and binding as suspension moves.  It was one of the first to offer the theoretical king pin as inclined or offset. This makes a big difference at full lock.

Don't believe me? try this. and move the object. it behaves completely differently through the arc of movement.

 When the rotation point or king pin is located like this with a lot of camber, when the wheel moves forward or back, the the wheel becomes lower. If there is zero camber, the wheel remains the same height. this can be a problem.

Forgetting castor for a minute and imagine we are still talking about camber. this picture directly illustrates the same point. with the wheel at full lock, the wheel moves downward but it also happens in the forward direction under the camber only scenario.

Castor comes into play to stop the wheels digging in on the forward side of movement. So there is always positive force allowing the wheel to roll properly. now in the above scenario, what do you think happens to the chassis and suspension at full lock. What about the edge of the tyre as it's effected by positive camber. (posican)?

I've since changed my RWD chassis to have a KPI and it now maintains a flat section.

Castor and camber combined, create a complex trajectory for wheel movement through the scrub radius.
The further outward wheels are placed with spacers and offset, the larger the differences become. 5 mm outward can easily mean 5mm of chassis lift if the wrong components are installed.

So moving the hub itself outward and keeping the wheel scrub radius less is a more desired option.
but then you introduce track width and the effect on suspension travel and spring rates... that's another chapter.

Basically if you have no castor and no camber you don't need KPI at all. Everything remains flat through the radius. So whats wrong here. If you have a hub designed for castor and camber and you run less (in this case around 4 degrees.) Then you can also create issues. "posican" is a look that can replicate reality, but not so effective from a geometry viewpoint.

So why add negative camber? Style, I guess.

Race cars can run masses of negative camber to combat chassis roll and keep a flat contact patch for "MORE GRIP". they rarely use more than 10% steering angle unless they are on an ultra slow hairpin where camber probably doesn't even come into play.

Drift cars want to have grip on turn in, then keep the front wheels pointed in a direction to control a slide on full lock. It's a very different philosophy.

There is typically much less reason to run big camber on the front a drift car because the wheels are pointed. The desire is to have the wheels with maximum contact patch at full opposite lock and various amounts of chassis roll. the lead tyre is probably the more important.

This is typically where KPI and castor come into play on a drift car.

The lead drift wheel is controlling most direction and therefore on aft directional lock (left lock on a right hand corner) should be flat.

My DRB in this picture does not have KPI. You can see how much the right wheel is laying over.

Castor goes someway towards getting the left wheel in the right spot on aft lock however the edge of the tyre will maintain camber through the arc. You can still see the camber maintained through the scrub radius even though this photo is on an angle.

You often see drift cars with the outside edge of the front wheels worn. this is castor at work. As the kingpin is slanted towards the rear the wheel will "dig into" the ground on the outside edge. Camber becomes flat and usually past this point to positive. "Posican"Just like in the above pic of my RWD chassis.

My Silver DRB runs 13 degrees of camber and 8 degrees of castor, but in this picture camber is much less at full lock. Actually, when I run a cambered tyre on this chassis, I get a FLAT contact patch on the front at full lock. these tyres were just temporary.
Adding KPI can definitely help negate the camber situation at full lock. My Rear wheel Drive Chassis is testimony to that. tyres remaining flat after changing to a KPI hub.

You will find as the chassis moves and rolls around the KPI on the front will help transition and keep the wheels flatter, reducing camber on full lock as the axle "lays down" and therefore creating grip and control.

note... contact patch will increase so be prepared for that extra grip and corner speed.

10 degrees of castor without KPI and lots of camber is pushing the boundaries of reasonable handling. If you can get the car to turn in at all, trying to maintain control over direction is also a bit tricky.

So without KPI, I recommend running limited camber.

 There are some totally customizable solutions out there. But you need to understand the theory to set them up properly.

 I didn't get to anti dive yet,  but I think that's enough for one day.

1 comment:

  1. Wow,thanks for the effort of writing all this stuff Russ!!!!very helpful.can you please write an article how to keep countersteering on a small cs percentage?that would be very helpful!!


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