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Braking & The Physics Of Sidewinders

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Why can some drivers leave their braking to the last possible moment?

Is braking too early worse than braking too late?

Does the wheelbase of a car affect braking?

Why do some cars brake better than others?

 

In the words of the late, great Professor Julius Sumner Miller, "Why is it so?"

 

A recent thread on SlotForum posed the question "Anglewinders. Why are they so good?" The link to the full thread is here

 

In that thread is a little physics and maths, and a very easily read explanation, from Ecurie Martini. Within that is a secret about sidewinders and anglewinders. A very big secret.

 

In a nutshell, the secret is this:

 

When an anglewinder or sidewinder slot car is braking, the net of all forces applied by motor, axles, chassis, wheels etc act to force the guide downward into the slot. When the same car accelerates, those same forces net out to apply lift at the guide.

 

Note that this does NOT apply with an inline car. Sidewinders & anglewinders apply forces in the line of travel, whereas inlines apply motor-related forces across the direction of travel.

 

The technical explanation follows later and if you are interested I commend to you the Slot Forum thread referenced earlier.

 

How can YOU use this in your racing?

 

Firstly, find the last possible moment you can brake -- this maximises this downforce advantage on the guide. Caution is NOT your friend here because if you brake too early, you'll need to apply power (accelerate) through a turn, which the physics show us generates lifting force acting at the guide.

Also, if your controller or track offers variable braking, experiment with it to help your car coast through turns. Use any practice to find your late braking points.

 

Secondly, a sidewinder should be more stable under braking and acceleration than an inline. This difference should become more pronounced with more powerful motors. So, if you have the choice, pick sidewinder or anglewinder over comparable inline chassis.

 

Thirdly, all else being equal, a short wheelbase car is going to brake better than a long wheel base car assuming relative motor shaft to guide dimensions. There is not a significant factor here with 1/32 cars but keen racers seek every advantage.

 

The technical explanation

 

Reproduced here with kind permission of the author, Ecurie Martini, is his excellent explanation:

 

"Here is a thought experiment: Assume a motor with infinitely flexible leads. Hold the shaft fixed and apply power. The motor will rotate - in the case of the point of view noted above (Ed. viewed from the right-hand side), in a clockwise direction. Hold the motor and the shaft rotates counterclockwise. Simply because the motor does not move relative to the chassis does not negate the force. In point of fact, there is a torque exerted by the motor case which is exactly equal to and opposite in direction to the torque exerted by the

shaft. That torque acts on the front of the car. A countervailing torque is exerted by the rotation of the rear axle. The magnitude of this force can be seen in the case where the rear axle is held and the motor operated - the chassis will lift.

 

If we consider the case of zero tire slip, the ratio of those forces can be expressed , where Tm is the motor torque in arbitrary units, Rt distance from the rear axle to the guide, Rg is the distance from the center of the motor to the guide and M is the gear ratio (and the gear set is frictionless):

 

T clockwise= Tm/Rg (downforce on guide)

 

T counterclockwise = Tm X M /Rt (lifting force on guide)

 

Clearly, the counterclockwise force will be greater but the clockwise force is not eliminated. Although the torque applied to the axle is multiplied by the gearing, it is also acting through a longer lever arm.

...

Of course, any loss in the gearing or tire slip will reduce the "T counterclockwise" term and increase the proportional effect of the sidewinder motor."

 

 

To see how downforce at the guide varies with different chassis and gear ratios, I built a very simple spreadsheet. Allowing columns for Rt, Rg and M and assuming an arbitrary 100 for Tm, you can see how different chassis dimensions should change the downforce applied to the guide by calculating both forces and comparing the difference.

 

Cheers


There are 10 types of people in the world. Those who understand binary and those who don't

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It all makes sense...

 

I have a side winder Madza 787B Lemans in 1/24 scale that I race out at Hornsby.

 

This supposedly is the fastest car they have seen out there in it's class.

 

I like to set my 1/24 cars up with very short gearing 5:1.

The car is very very light so it is possible to pop it out if the loud pedal is floored too aggressively.

 

I also have my own body mounts that offer too scale suspension movement which includes lateral, braking and acceleration pitch.

 

Re the late braking, this car is the best example I have driven, plus the body shape seems to create some downforce, so with the above formula and the added downforce of the faster you go the more grip it has, it's a very quick car.

 

But!!!

 

There's a down side with my car, the gearing that provides the massive brakes and acceleration also makes it very very unforgiving and the limit sneaks up unannounced. Basically mid turn you find yourself trying to balance the car on neutral throttle in the mid to high end of the power band.

 

Forgive me, but the only way to descibe driving this car fast is that it's a bitch!

 

Very rewarding tho, if you can get it right for 8 x 2 minutes (done that a few times)

or 8 x 3 minutes ( messed that one up big time).

 

I let a few guys try it after the recent Nat's and most walked away from it saying it was undrivable as they prefer a more flowing style of car.


Cheers for now

 

Roland

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