same ratio dfferent gears

Flange are you saying that 27/9 = 3 will have better braking than 33/11 = 3 ?? I have to disagree with you on that, sorry. Or are you saying 27/9 will give more braking than 27/11 ?? In that case yes. Mechanical advantage is mechanical advantage. Its not changed since Leonardo da Vinci invented the pulley system.

OK had a think about it and yes the 33/11 will possibly have less chatter under brakes as the +/- deviation from a mechanical advantge of 3 will be less. The 27/9 will possible lock up as the +/- deviation from a mechanical advantage of 3 will be greater. Without sitting down to do the calculations it may go as high as 3.05 and down to 2.95 as the gears rotate for the 27/9 set and the 33/11 set may be 3.02 down to 2.98. These are just guesses as Id have to sit down and do som trig maths to work it out.

cheers
rick1776

Stoner,

The simple answer is that a mechanical advantage of 3 is a mechanical advantage of 3. This is whether it is achieved using 27/9 or 30/10 or whatever other combo you want. Anyone that says something different failed year 11 high school physics. Remember though that this is the AVERAGE mechanical advantage, as the each individual tooth engauges and dis-engauges this value will oscillate by a small amount both above a mechanical advantage of 3 and below a mechanical advantage of 3 (by a very small amount). The larger the tooth count the less the mechanical advantage will vary from 3.

Any one that says they can notice the difference in acceleration using a spur gear that weighs 1.2g compared to 1.1g is pulling there short one too hard. A while back it was fashionable to use hollow axles because they had less rotational inertia. I did some calculations and on the assumption that the hollow axle was 1g lighter, if you removed 0.003g from the outer edge of the rim/tyre it reduced the rotational inertia by the same amount. Smoke and mirror stuff. Not disputing the axle weighs less and that this could increase the accelleration, but the gain would not be due to less inertia!!

cheers
rick1776

QUOTE (stoner @ 26 Apr 2012, 08:38) <{POST_SNAPBACK}>the best ratio for a neo drag car is 14-54- 64 pitch. you can use a smaller pinion and gear to get the same ratio,s the drag boys do hundreds of gear changes to get the right combo, so why go with the bigger heavier gears, when weight is a big issue in drag cars, same as 1/32 and 1/24. what i.m getting at does a large pinion with more leverage slow a motor down, or is it such a small amount in the leverage ,not make any difference. ive seen old 16-24 pinions, why make them if theres no advantage. john

We seem to be going around in circles. If I understand you correctly one of the questions youre asking is whether there is for example, a difference between a 6.5mm 9T pinion coupled to a 19mm 27T spur compared to say a 5.5mm 9T pinion coupled to a 18mm 27T spur. On the assumption that they mesh properly then both MUST provide the same mechanical advantage which will be 3. The leverage as you call it will be the same in both cases, and that is 3. Now granted that one gear set may have a better mesh and so have less inherant losses (less friction) compared to the other. But that is a different question altogether.

The other question is, if I unerstand it correctly, why a particular ratio and not another? That really depends on the torque of the motor, the rpm, the wheel diameter, weight of car, is top speed important or is acceleration, short track, long track etc. You choose a ratio that gives the best performance for your situation. Sometimes its as simple as the physical room you have that dictates the gear sizes used.

Why make a 16T pinion? Well the motor may be a low RPM type with huge torque, so you can afford to go to a 16T with say a 36T spur. Put that in a Scaly motor with about 90-100 g.cm of torque and the car will be very sluggish.

cheers
rick1776

QUOTE (stoner @ 26 Apr 2012, 15:25) <{POST_SNAPBACK}>that wasn,t the question rick take a 14t pinion 9mm in dia and mate it to a 42 gear 3-1 then take a 6t pinion 5mm in dia and mate it to an 18t gear. whats the advantage, does a big pinion drop motor torque or does a small pinion increase motor torque.pretend that wheel, ground clearance isnt an issue. why would you go for the different pinions and gears to achieve the same ratio 3-1. there must be a very good reason judging by the racers and scratch builders setups. i hope this is a bit clearer. john

Not sure if youre winding me up John? Ill assume your not. Both 42/14 = 3 and 18/6 = 3 have the same mechanical advantage (you calling it leverage). That is for example if the motor produced 100 g.cm of torque then at the rear axle the torque would be multiplied by 3, so it would have 300 g.cm available. What is the penalty for having a mechanical advantage? The speed will be reduced by 3.

So youre question is why bother making or using 42/14 if 18/6 does the same job? Right? A couple of reasons spring to mind. The physical space you have to play with. Maybe the bigger bulkier gears wont fit, or vice versa. Another reason, the bigger gears will tend to give a nicer smoother mesh and as a result have less frictional losses. Try for example getting a 12T spur to mesh nicely with a 4T pinion!! 12/4 = 3 so you get the same mechanical advantage but the mesh would be horrible. Going to the other extreme 300T spur and 100T pinion. The size would be huge and the weight would start to be significant by then. The third advantage is that when I say the mechanical advatage is 3, that is the AVERAGE ove one complete rotation of the gears. The instantaneous advantage will fluctuate slightly so that at any one point in time it might the advanteg will be 2.95, then 3.05 etc. This is due to the way the teeth engage and disengage. But the average over one rotation will always be 3!! This fluctuation becomes less pronounced the more teeth we have. So 36/12 = 3 fluctuates less than 27/9 = 3. But we are starting to split hairs at this stage.

With respect to the typical ratio of say 3:1 that is used for a lot of slot cars, assuming the mesh is good, then I really dont think there is a lot of difference between 27/9, 30/10, 33/11, 36/12. They should give the same result. Assuming each meshes well the frictional losses will be about the same, there will be very little difference in the weight so inertial losses will be about the same etc. It then comes down to personal preference and what will physically fit.

OK if I still havenet answered your question Im officially giving up.

cheers
rick1776

Dont worry about me John, I was having a bad hair day. If youre mostly into vintage 1/24th stuff then a lot of the gears that were used were from "where ever" people could source something. Im also mostly into 1/24th stuff of late. I prefer to use brass pinions with delrin (plastic) spur gears. The wear rate will be too high with metal to metal gears in 1/24th. Take extreme care to get the gear backlash to nothing more than a bee's dick otherwise youre asking for trouble. The cars are usually 150-220g in weight so they have a lot of inertia. Heavy cars with back lash and metal on metal gears will kill the gears real quick.

Also with the vintage stuff inertia starts to have an affect. The difference in performnce between a gear weighing 1.1g and 1.2g is unmeasurable (in my opinion). Howe ever the diference between 1.1g and 8-10g is measurable. Scale auto look like they make nice spur gears in plastic. I would not use their plastic pinions though. They are marginal. I use a lot of MJK Engineering stuff, especially their stainless steel chassis, they are really quite simple in design but work very well. Their gears are OK with a bit of work. Below is an MJK Eng Can-Am chassis kit.







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rick1776

Thanks smurfen. Yes the lexan bodies are typically painted from the inside. Cant help but get a nice looking finish, even if you hand paint it!! The injector stacks were painted on the outside. The black lines were done using a fine tipped marker pen. The interior base coat is applied on the under surface and then you paint the driver on the top surface. Decals need to be applied on the inner surface which is a real pain. Next time I will use peel and stick vinyl.

Heres a couple of more pics of my car, the 2011 season Can Am championship winner, as well as some of the other guys in the club. Lot stiffer comp this year and I doubt I'll retain the title.









cheers
rick1776

QUOTE (John Cahill @ 20 May 2012, 23:44) <{POST_SNAPBACK}>re: "all gears work on a sliding action thats how the pinion moves them"

with respect, that's not true...and that's one of the points I was trying to make earlier on. Any gear that's designed with the classic Involute profile (such as a sidewinder with pinion and spur,
as well as properly shaped contrates on inline setups) are specifically shaped to eliminate frictional losses from sliding contact. That's the whole point of the profile...it's by definition, the shape
which allows only "normal" reaction forces at all points of contact. You might think of it as "rolling" contact at all times.

That's why I'm saying that weight (inertia) is one problem...but going too small to save weight and sacrificing a well-formed gear profile isn't good either. There's a balance
that involves lightness, quality of profile and material stiffness for optimum performance. Sorry to repeat, but something like a thin, Aluminum, precise slot-it gear is a good example.

John

Which leads into the concept that a 3:1 ratio (ie 27/8 = 3) is only the average mechanical advantage over a complete revolution. At any point in time the actual advantage may be less than 3:1 or more than 3:1. This is due to the fact that the pinion does not slide against the spur. The "rolling" action sees the point of contact of the pinion on the spur move from the land of the spur down towards the base. This effectively changes the mechanical advantage. Then as that tooth disengages the following tooth engages and the whole process starts again.

cheers
rick1776