[racerunner]

The orange blur in the pic above is a Slot-it Porsche going around a home-made 45degree banked corner at 5m/s (16.4feet/sec). To my surprise it makes no difference if you remove the magnet. It still goes around at 5m/s!

This got me thinking. How much lateral friction do you need to go around a banked corner?

I crunched a few numbers and got a theoretical answer: in the above scenario you need a coefficient of friction of 0.6

This is a very low number. Even if you double it, it’s easily achieved. Decent tyres give more like 2.0

So why did I spend so much time gluing steel wires under the copper foil? A waste of time. Live and learn!

Here’s a graph showing some other data points. The data point for the Porsche is arrowed.

Thanks for reading!

 

[racerunner]

Decent tyres give more like 2.0

Correction. I just checked some newly-trued MJK rubber tyres on a Scalextric Nissan and I'm getting a coefficient of friction close to 1

 

[racerunner]

I'd say they would have been no faster with aerodynamic wings.

Avus 1955. From Wikipedia.

 

[racerunner]

If the banking is very steep the cars will tend to bog down

Yes, if the banking is very steep you get the sense that the car is pushing very hard into the track and the clickety-clack of my plastic track gets a little too loud for comfort.
The lesson for me is: it's better to have the 45deg banked corner without steel under the copper tape so the magnet cars have less bogging down. The rest of the track can have rails that attract magnets. A hybrid layout is best.

If the cars have decent tires 15-20 degrees of banking is enough.

Agreed. I have moved the arrow on the graph below to the 20deg line which shows that my Slot-it Porsche, with magnet, could still go full beans around the banking.

 

[racerunner]

My method for finding coefficient of friction

Keep tilting the track till you find the point where the rear end starts to slide. The coefficient is the ratio of rise over run, a.k.a. gradient, a.k.a. the tangent ratio of the banking angle.
E.g., when rise and run are equal the coefficient is 1
if rise is twice run, the coefficient is 2
if the pic above was where sliding began, the coefficient would be around 0.5

The mass of the car makes no difference. Put the body on the chassis and you get the same answer.

What other methods are there for measuring level of grip?

 

[racerunner]

there are other forces in effect not just the grip level between tyres and track.

Agreed. My number crunching took account of grip, weight and the force of the track pushing up as the momentum drives the car into the track surface (i.e., Normal force) ["Normal" as in "90 deg to the surface"]

A circular test track is the best as it gives you a reading for the coefficient of friction under dynamic conditions.

Nice! That would be ideal. Any idea how practical c.f. readings compare to theoretical, esp. for plastic track like Scalextric?

You can also measure the forward thrust of the car by having it push on a bell crank to load digital scales.

Is that with wheels spinning or a static load situation?

 

[racerunner]

All the ones I have bought have been urethane

Thanks for that! All this time I thought I was buying rubber!
I can't see anywhere on their website or catalogue that says what material they use. Plus I wouldn't be able to tell the difference anyway.

 

[racerunner]

Usually we're looking for tyre grip to act downwards and stop the car from spinning out. On a wall of death you're looking for tyre grip to act upwards.

I've added a 85deg line on the graph below. All of the friction needed is negative - it needs to act upwards.

The place where you need zero lateral tyre grip is like where the arrow is pointing; i.e., the speed is "just right" so tyre grip is needed neither up nor down the slope! At the arrow, 45deg bank, my Porsche at 2.5m/s could take the bend even if it was made of wet ice.

 

[racerunner]

Scott B: Thanks for the pics!
pic 1: The backlighting and reflection off the track gives a kind of mystical quality. Could Gandalf step out from among the trees and ask for a go?
pic 3: That's one happy looking little LMP - getting a run outside in the sunshine!

Physics. Yes, there's only one direction you need lateral grip in an angled straight and that's UP the track!

I make a living out of teaching Physics, so Physics and I are friends most of the time.

 

[racerunner]

both higher downforce and gravity type cars are slower through the banking than they are in the straightaway.

Normal force can become massive. Referring back to the orange Porsche in the pic at the start, the Normal force would be around 3.5 times the body weight of the car when magless. Add the magnetic force to that if you've got a magnet. And Normal force rises exponentially with speed.

Gripping: Thanks for the interesting info and ideas.

 

[racerunner]

Very interesting. Maybe RichD could use an "anti-decking rail" on the Purple Mile.

To be pedantic, it rises as the square of the speed

Quite correct Sir! Sometimes pedantic is good.