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Old 03-28-2007, 09:45 AM   #1
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Default Questions about clutches on nitro cars

OK, I bought my first nitro 1/8th buggy lastnight after racing 1/10 electric for about 6 months.

I had always noticed an odd thing about nitro cars - when people punch their engines, you hear the RPMs rev up quickly but the car somewhat slowly begins to move. What you're hearing does not match what you're seeing. I don't know if this makes sense but I always thought it was oddly more exaggerated than my Honda Accord (imagine cranking my RPMs up to 6000 and then engaging the clutch just to start).

Now that I have a nitro car I understand the reason has everything to do with the use of a clutch. Electric cars have a slipper that performs the same function I suppose. But it seems that in nitro, that clutch slips a whole lot more than the slipper in electric cars.

I'm trying to learn here. My first question is, what happens when your clutch is too tight? I'd guess that if it's too loose your acceleration will be awful and that if it's too tight it will stall your engine. Is that right?

With electric I never had need to adjust my throttle exponentials to make it "softer" around the center of the trigger. I'm wondering if this is a more common practice in nitro to avoid revving the engine up too quickly. Is this the case?

Question 3 - Suppose I punch the throttle from stand-still. Let's say that in scenario A I punch it to 50% throttle. In scenario B let's say I punch it to 100% throttle. Because the clutch will be slipping in both scenarios while the car begins to move, will the initial acceleration be any faster in scenario B?

Thanks!

Last edited by Arboleda; 03-28-2007 at 11:36 AM.
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Old 03-28-2007, 11:10 AM   #2
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I suppose the easiest way to wrap your head around this is to just understand the workings of a centrifugal clutch. A few key points are:

1) As rpm increases, so does the level of engagement. Example: idle is completely free (disengaged), full speed is completely locked (zero slip).

2) Point one implies the clutch works somewhat, but very differently, like a slipper. It slips just enough at low speed to allow the engine to get the car moving without dying.

3) Point two brings up the adjustability of a clutch. Using a clutch with a low rpm engagement point will have a very soft feel. A high rpm engagement will have a very punchy feel, as if the engine has a lot of torque. This is because power increases greatly as rpm increases. For example in a full size car: Just get the car rolling and punch the throttle, versus redlining it and popping the clutch.

With that said to answer your questions:
1) If your clutch is too tight the engine will have to build too many rpm's to engage it. In this case you may wind up with low speed throttle response that is too snappy or in an extreme case, the clutch will never fully lock. That results in burning up the shoes, the clutch bell, and the bearings. Also overall performance suffers greatly.

2) How fast you rev the engine doesn't matter. If the track you run on has great traction, punch the throttle and go.

3) Now that you've read the above, obviously more throttle equals more rpm. More rpm means a higher level of engagement and power output.
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Old 03-28-2007, 11:31 AM   #3
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That was a very, very helpful answer.

The fact that it's a centrifigual clutch (which I had never heard of, but it makes sense) changes my perspective. I assumed we were dealing with a simple friction clutch - which apparently we're not!

Your points and your answers directly addressed my questions and I now believe I understand.

On a related note - I never felt the need to mess with exponentials with electric cars but obviously some people do. Is it any more common to use exponentials in order to soften or harden throttle response near the trigger when dealing with nitro cars?
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Old 03-28-2007, 02:26 PM   #4
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Exponential is all a personal preference, both in throttle and steering. Start out with no expo and learn to drive it first. Then experiment with it and see if you lower your lap times or drive more consistently.

One thing you have to realize while driving a nitro vehicle compared to electric is it is not nearly as linear. With electric, if you pull 1/4 throttle you will go 1/4 speed. Holding in 1/4 throttle on a nitro and you'll be moving along pretty fast as the engine will wind out when it has a light load on it (imagine holding 1/4 throttle on your Accord in 1st gear, you'll redline quickly). However, nitro engines have a significant powerband where power greatly increases with rpm. This in itself is exponential but it works more like a softer response coming out of a hair pin and more punch coming out of a sweeper.
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