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Old 11th April 2015, 15:33   #1
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Default Why do cars struggle to climb a gradient (slope)?

My son has a RC toy car that sprints at 10 to 15 km/hr on a level ground. But when I take it outdoors on uneven ground, I notice two things -

- It struggles to climb short (lengthwise) but steep (probably 30 degrees) gradient from standstill. It can manage to climb over this only when the car has momentum.

- It struggles to go over long stretches of very small gradients (probably a 5 degrees incline) and eventually stops. And once it stops (no momentum), it doesn't move further.

Now all of have experienced this with real automobiles too (especially trucks). Questions:

What does a gradient do to an automobile? How does it strangle momentum of a vehicle? Does a gradient somehow increase the "effective weight" of the automobile?

I welcome both logical (Eg: It's gravity, stupid) and mathematical (Eg: calculus) answers.
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Old 11th April 2015, 16:26   #2
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Default re: Why do cars struggle to climb a gradient (slope)?

We all know that by Newton's first law, every body continues to be in a state of rest or of uniform motion unless and until it is acted upon by an external force. If an automobile is moving on a level track with a velocity (say 15 kmph), and the frictional resistance in the opposite direction is the only resisting force, then the car engine has to develop only so much power to exceed the frictional force so as to keep the vehicle moving at that particular velocity. The Engine has to exert additional power only if it has to accelerate the car to a higher velocity.


Now consider a block (or a car) moving up an inclined plane

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In the earlier case, the only force in the horizontal direction to be overcome to keep the vehicle moving at a constant speed was the frictional force opposite to the direction of motion. But check the above figure. In addition to the frictional force, there is a component of the weight (sine component) that is actually acting opposite to the direction of motion and hence even to keep the vehicle at 15 kmph, the engine has to exert extra power to overcome frictional force+sine component of weight of the vehicle and hence the climb becomes strenuous.

Regarding the momentum, it is the product of mass and velocity. So even if engine is not capable of climbing a gradient, the initial momentum while it was travelling on the plains will help it climb the gradient to a certain extent. But since the sine component of the weight opposes the motion in the gradient, the car loses its momentum rapidly (as the mass in the direction of motion is lesser) and a point will come when the engine will have to do the job of pulling the car up the gradient and if its not capable, its pretty much done.
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Old 11th April 2015, 16:50   #3
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Default re: Why do cars struggle to climb a gradient (slope)?

Quote:
Originally Posted by RiGOD View Post
there is a component of the weight (sine component) that is actually acting opposite to the direction of motion
So essentially, the weight of the car - instead of acting downwards - actually acts opposite to the direction of motion. This makes things easier to comprehend. And it also explains what happens when a vehicle goes downhill. Thanks!

Internet research on the topic led me to discover the street with the steepest gradient - Baldwin Street in New Zealand
http://en.wikipedia.org/wiki/Baldwin_Street

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Old 11th April 2015, 16:53   #4
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Default re: Why do cars struggle to climb a gradient (slope)?

As Jeremy Clarkson would have said, "more power!" is the answer. No problem with slopes.
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Old 11th April 2015, 17:44   #5
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Default re: Why do cars struggle to climb a gradient (slope)?

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Originally Posted by VCheng View Post
As Jeremy Clarkson would have said, "more power!" is the answer. No problem with slopes.
Jeremy got himself in quite a pickle over a slope!

http://www.mirror.co.uk/tv/tv-news/t...ccused-3253312


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Old 12th April 2015, 08:08   #6
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Default re: Why do cars struggle to climb a gradient (slope)?

Not directly related to your question. But the topic does remind of it. You might like to read one of a very very interesting phenomenons called "gravity hill" - more of an optical illusion that makes it appear defying gravity.

Attaching one video that explains it. There are many of them, as well as wikipedia links would explain the phenomenon.




There is one in India, too. This page has a list:
http://en.wikipedia.org/wiki/List_of_gravity_hills

Last edited by mayuresh : 12th April 2015 at 08:15.
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Old 12th April 2015, 09:16   #7
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Default re: Why do cars struggle to climb a gradient (slope)?

hi,

Possibly the RC toy might be having only one gear combination which is designed for more speed and less power for considerably good amount of speed in level ground.

If the no.of teeth in driving gear(motor) is more than the no. of teeth in driven gear(wheels)you will more speed and less power. If you interchange the driving gear and driven gear you'll get more power and will climb steeper slopes but it would not take pace and would be like driving your car in the first gear only.

Try installing a CVT in the RC car. Then you can get power according to the conditions
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Old 12th April 2015, 20:45   #8
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Default re: Why do cars struggle to climb a gradient (slope)?

Unlike internal combustion engines, electric motors and external combustion engines (like steam locomotives) generate the maximum design torque from the get-go. The motor in your RC car has enough torque to make it go on level ground, but not enough to make it go up an incline.

What kind of battery does it use? If you're using alkalines, try rechargeable Nickel Metal Hydrides (NiMH); the best are Low Self-Discharge (LSD) models like Eneloops that are available at Amazon.in and Flipkart. Unlike alkalines where the voltage drops immediatley on use, NiMHs maintain power almost till the end.

See this image for an example.
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Old 13th April 2015, 08:16   #9
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Default re: Why do cars struggle to climb a gradient (slope)?

This thread gave me a new information - "gravity hill". Never knew about it before. Any idea about such roads in India? It's funny that how optical illusions can defy scientific explanations.
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Old 13th April 2015, 09:47   #10
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Default re: Why do cars struggle to climb a gradient (slope)?

Quote:
Originally Posted by sajusherief View Post
This thread gave me a new information - "gravity hill". Never knew about it before. Any idea about such roads in India? It's funny that how optical illusions can defy scientific explanations.
We have something called magnetic hill in Ladakh.

You can read about this in the lovely compilation (Leh, Ladakh and Zanskar - The Ultimate Guide) by tsk1979
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Old 13th April 2015, 13:05   #11
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Default Re: Why do cars struggle to climb a gradient (slope)?

Quote:
Originally Posted by VCheng View Post
As Jeremy Clarkson would have said, "more power!" is the answer.
Or more precisely, torque + short gearing + no turbo lag.

Related threads for additional reading on the topic:

Hill climbing techniques (Hill Climb Technique - For Cars (Front Wheel Drive))

FWD vs RWD on slopes (FWD climbed a hill but RWD failed. Reason ?)

Stopping on an incline (Stopping on a steep incline?)
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Old 13th April 2015, 13:40   #12
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Default Re: Why do cars struggle to climb a gradient (slope)?

Quote:
Originally Posted by sajusherief View Post
This thread gave me a new information - "gravity hill". Never knew about it before. Any idea about such roads in India? It's funny that how optical illusions can defy scientific explanations.
My post already answered that. See the link at the bottom of the post.
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Old 13th April 2015, 15:57   #13
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Default Re: Why do cars struggle to climb a gradient (slope)?

Basically a couple of T-reasons:

1. Torque - Been mentioned and explained earlier.
2. Traction - You can have a Torque-Monster, but if it doesn't have enough grip, the wheels will keep spinning away, without moving the vehicle forward an inch. The key to this, is the Tread and Weight Distribution across the Axles.
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Old 13th April 2015, 16:47   #14
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Default Re: Why do cars struggle to climb a gradient (slope)?

Interesting topic. Thanks to RiGOD for explaining the science of it. I think smartcat's RC car shows that just like in any other field of life, it's very difficult to get to the top. You need to have sufficient inner drive or you won't make it. Good luck (momentum) may carry you up the slope to an extent, but eventually luck will run out if not accompanied by work. But, it's very easy to go downhill
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