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Discussion Starter · #1 ·
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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.

Rectangle Slope Plot Font Parallel


Thanks for reading!
 

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· Rich Dumas
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The banked corners on a slot track do not have to be very steep to allow the cars to run flat out. If the cars have decent tires 15-20 degrees of banking is enough. If the banking is very steep the cars will tend to bog down and would probably run slower than they would in a corner that is not banked as much.
 

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Discussion Starter · #8 ·
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.
Rectangle Slope Font Parallel Circle
 

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Discussion Starter · #9 ·
My method for finding coefficient of friction

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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?
 

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

You can also measure the forward thrust of the car by having it push on a bell crank to load digital scales. That will give you the c.f. too.
 

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Discussion Starter · #12 ·
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?
 

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Once you get in to banked track there are other forces in effect not just the grip level between tyres and track. If it was just down to tyre grip walls of death, the ultimate banked track, would never work
Although, on walls of death, tyre grip is still, apparently, important. I recall an interview with the guy running what was, I think, at the time, the last remaining one in the UK, where he mentioned that spectators spilling drinks down the wall could cause serious problems for the riders.
 

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Discussion Starter · #16 ·
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.
Slope Rectangle Font Parallel Plot

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.
 

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All I know is when I was running my LMP cars on the Carrera banking in the summer those with anything less than very grippy tyres (no magnets) would "fall" down the slope. Hence the short strip of border to nudge them back on.

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When I added a straight section between the curves it was obviously much more notable, but I soon learned - in the interests of not having to get up and re-slot the car - to keep a good speed up in the banking and hope physics was my friend.

Great fun though.
 

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Discussion Starter · #18 ·
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.
 

· Rich Dumas
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Here is an HO track with a very serious banked corner. I believe that it is 56 degrees.



This is the HO version of the famous 1/24th scale Purple Mile Sovereign tracks. It is located at LenJet Raceway and shares a building with Modelville Hobby that I think has the last 1/24th Sovereign.
I have only raced on the HO version a few times, but both higher downforce and gravity type cars are slower through the banking than they are in the straightaway.
There is a difference between a continuous banked corner and two banked corners that are connected by a banked straight section. The cars will tend to slide out more in that straight section.
 

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


I have no idea what the theoretical c.f. Is but on Ninco track with NSR ultra grips and no traction magnet I measured an apparent c.f. of 1.75

Is that with wheels spinning or a static load situation?
You can measure both depending on the power you use. I got a slightly higher reading than with the same car on the circular track because there were no bumps and track joints to upset the grip.

I suppose the ratio between the two readings is an indication of the efficiency of the chassis to cope with track irregularities - possibly a useful tuning indicator?
 
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