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Old 02-20-2003, 03:52 AM   #31
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How long a car spins for is a pretty crude test. There should not be much in it between the top cars. When your drving you are usually going at least 5 times as fast, so you have much more reistance in the drivetrain and that is when the drivetrain needs to be loose. something like a belt which has low tension at low speeds but really tenses up at high speeds will allow the car to freewheel for ages but be comparatively poor(er) at high speeds.

just my 2 penceworth.

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Old 02-20-2003, 05:39 AM   #32
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Isn't it just the opposite?! A belt driven car becomes more efficient at higher speeds dye to the centrifugal forces reducing the axial forces between the pulley and the belt. However too loose belts will cause vibrations resulting in drag.
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Old 02-20-2003, 06:36 AM   #33
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Quote:
something like a belt which has low tension at low speeds but really tenses up at high speeds will allow the car to freewheel for ages but be comparatively poor(er) at high speeds.
what does he means??? Is loose belt better or tighter better??
which ones??
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Old 02-20-2003, 09:18 AM   #34
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well my point was not nailed at specific transmissions in particular it was a generic statement.

As the pulley's spin faster, the tension in the belt increases.

For the techie amongst us:
as the belt wraps round the pulleys it is in effect changing direction (accelerating). As the speed of the pulleys is increased the rate of acceleration increases. Its actually F=MV^2*R, where F is the force, M is mass, V is the speed of movement and R is radius.

Now to stick that into laymans terms:
as the belt wraps round the pulley and the speed is increased, the belt tries to come away from the pulley, thus trying to stretch the belt, this in turn causes tension in the belt.

with a shaft drivetrain you don't get this problem, but it has other drawbacks (i.e. the drive is shifted thru 90 degrees, which is the major inefficiency)

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Old 02-20-2003, 09:38 AM   #35
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Quote:
Originally posted by Nightbreed
All has been done except for a hard titanium wheel axles. There must be a demand before someone will introduce them to the market. Heck I would pay more to get a titanium axle like the steel ones.
They would make a difference.

In Japan there is two companies that make Ti Drive Axles but they cost about 7000 Yen or about 65$ for a set and they only make them for the TC3 and the EVO III

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Old 02-20-2003, 09:38 AM   #36
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Default How free is your drivetrain?

Nightbreed;

I've used the Graphite spray at work before. It is normally used as an Anti-Seize coating and is very slippery. There are a couple different companies that sell it.

Titanium is lighter, but not as strong as Hardened Steel, probably closer to a high grade of Aluminum. Axles are subjected to very high loads and subject to damage from hits. TI and AL axles would bend very easily in a T/C (IMHO). In a Pan car the weight is much less, so the whole chassis moves very easily when hit.

schumacher;

I understand EXACTLY what you are saying. You can see it happen on machinery with full size belts. It is most noticeable during acceleration under a load.
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Old 02-20-2003, 11:38 AM   #37
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It's true that a belt try to get of the pulley, while speed increases. But this is actually an advantage, since it reduce the powerloss between belt and pulley.

At lower speeds shaft cars are the most efficient. They got that nice low bottom acceleration. But at higher speeds, belts are the most efficient. So they are really good on large open tracks.

So this simple freewheel comparision, aint giving a true picture.
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Old 02-20-2003, 12:30 PM   #38
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Nightbreed - TI Racing make TI CVD's for most cars.
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Old 02-20-2003, 12:45 PM   #39
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I hate to break it to everyone... but the fact that a car freewheels for an extended period of time means next to nothing on the track - what we are all looking to gain is the efficiency of the drivetrain under load - NOT freewheeling... Take for example a driver who has all of the heavy steel components on the car... it will spin a long long time - because the rotational mass is high, but this doesn't mean anything on the track.... why?? because of the higher rotational mass, the acceleration will not be as great and that driver with the car that could spin for 30 seconds in the pit will be left sitting at the corner while the car with the lighter drivetrain and shorter freewheel time is out of the corner - less force was required to move the car with the lighter drive train.... what does this mean to us?? The fact that the lighter car had to use less force from the motor, less heat will build up, keeping the motor faster for a longer period of time, the battery will retain its punch for a longer period of time.... etc, etc.

SO - I ask this question, can someone truly demonstrate the benefit of having a car freewheel for a longer period of time, or are we just talking about pit racing?


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Old 02-20-2003, 12:48 PM   #40
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Preach on brother Wilde
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Old 02-20-2003, 07:48 PM   #41
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hehe....... what schumacher .... and jessieT said is right on ...

well it mite be only noticable on bigger machines but if u look carefuly u will also see it on the rc car .... just that most of the time u will see the belt tightening on acceleration and then hopping wildly loosing tension upon deceleration ....
and when its at 0V it seem slaging a little ...

IMHO about the freewheeling part ..... i personally think its no use measuring the freewheeling without the motor on ...
why ? because most of the acceleration and stopping forces is generated from the motor itself and other forces like the friction from bearing and driveterain seems to be quite insignificant.....

btw ... my kawada sv10 II freewheels for 12 secs
and my 414m for also around 20 secs but the kawada seems faster everywhere aorund .... expecially noticable on the acceleration down the strights ( sling shot foward ) but not on the 414m it does not have the sling shot effect ....
( all equieptment are the same ...for both cars )

and FYI the internal drive ratio for both cars is 2.248 for kawada and 2.13 for 414m ...
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Old 02-20-2003, 08:26 PM   #42
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Everyone that says that freewheel time is not a good indicator of acceleration and top end on the track is correct.

the most pertinent intermediate variables are the force required to accelerate the drivetrain to a given rotational velocity and the force required to maintain said velocity. these are both directly related to the mass of the drivetrain and the composite frictional coefficient of the drivetrain. Obviously the mass of the drivetrain is essentially a constant. Friction however changes due to many variables: differential tension on the belt, applied force at every point that two surfaces meet(belt to pully, axle pin to out drive, gear to gear, etc), temperature(and therefore kinematic viscosity of lube in bearings and diffs). Some of these variables provide more resistance as speed and increases and some will drop. In the end, the only good measure would be to mount a loadcell in a rotational orientation and spin the drivetrain up and capture the resistance force vs rpm. it could be done and there are setups to do it.

anyone out there want to cough up an encoder, strain guage and data aquistion board?

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Old 02-21-2003, 05:43 AM   #43
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spot the engineer (petzl)

I got access to a strain gauge but how do you measure it? its nigh on impossible to measure the belt tension while the transmission is still int he car.

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Old 02-21-2003, 06:54 AM   #44
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are you familiar with a brookesfield viscometer? if so, you need to measure the resistance to rotational force in the same manner. let me see if I can draw it in ascii




**
* *
* *
******O*****
* *
* *
**

now imagine the outer ring is a a tube with two connecting aluminum struts to drive the centers shaft. the center shaft drives the pinion gear. the strain guage is attached to one or both of the struts. signal is sent via a rotating coupling to the wheatsone bridge. force applied is calculated. additionally, you would need to attach an encoder wheel to measure rpms. I am sure that there is a test fixture out there that does this. actually this is basically a driven dyno with a much higher sensitivity.



does this make sense? haven't had my caffeine yet
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Old 02-21-2003, 07:02 AM   #45
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HAHAHAHA NOW THAT DID NOT TURN OUT THEY WAY I WANTED. LOOKS RATHER PORNOGRAPHIC NOW


try this
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