gigaplex

iTrader: (2)

The math disagrees with you. If you increase the numerator and decrease the denominator or vice versa you're changing the ratio. You can't keep the ratio the same when you do what you said.

oldfool

Bigger gears will mesh better/smoother.
At 4:1 a 25 pinion and 100 spur will mesh better that a 7 pinion and 28 spur.
I've seen 6 tooth pinions in slot cars and that's tough/impossible to mesh.

DirkW

iTrader: (1)

Yes, maybe with such extreme differences. But I doubt it'll make much real life difference whether you have 100/25 or 92/23.

oldfool

The weight shift probably does more.

TheBeave

iTrader: (6)

The only thing I can think of is with a larger spur you get to fine tune your FDR opposed to a smaller spur. You can get a lot more granular with your FDR when using a larger spur but as others have pointed out you also move one of the heaviest pieces of your car with a larger/smaller spur. Maybe find a ratio that works best for your motor/timing combination then pull out the calculator and start punching numbers in if you want to move the weight. I use an 88T spur in every car I own, it's a lot easier to keep the spur gear bag full and consistent when you run the same spur in every car but I have sacrificed that 1/2-3/4 of a pinion tooth by running a much smaller spur gear.

Mig89

Yea, I really don't want calculate the cycle loading of a spur gear. Rest assured, the math shows that a bigger spur is smoother and it better spreads the stresses seen by the teeth.

Robbob

iTrader: (8)

The math may show that but we are racing for 5 minutes (8 in 12scale) and then resting for hours.
A spur costs 10$ and its pocket change when you throw the science out and replace it with driver preference

1spunspur

iTrader: (1)

you need to buy a new calculator.

glennhl

iTrader: (16)

deleted

gigaplex

iTrader: (2)

You either need to re read the comments again or learn how ratios work. If you increase the numerator, you increase the ratio. If you decrease the denominator, you increase the ratio. If you do both at the same time, you increase the ratio further. You cannot maintain the same ratio if you increase one and decrease the other like that comment suggested.

DirkW

iTrader: (1)

Sorry you're just wrong. But since you probably won't ever check your own math (properly) or listen to people telling you, I'll quickly write something up for everyone. I'll use a 90/30 combo as baseline.

I'll show a few examples below:

a common mistake is if you adjust both, spur and pinion, in the same direction but only by one tooth each, with that your ratios change:
{+1 /+1}
93/33 = 2.81818... : 1
92/32 = 2.875 : 1
91/31 = 2.935483... : 1
90/30 = 3.0 : 1
89/29 = 3.068965... : 1
88/28 = 3.14285... : 1
87/27 = 3.22222... : 1
{-1/-1}


What you actually need to do is to adjust both spur and pinion in the same direction (both up or both down), but in the correct overall ratio (here 3:1): this is how the ratio stays constant:

{+3/+1}
99/33 = 3.0 : 1
96/32 = 3.0 : 1
93/31 = 3.0 : 1
90/30 = 3.0 : 1
87/29 = 3.0 : 1
84/28 = 3.0 : 1
81/27 = 3.0 : 1
{-3/-1}


Now if you only ever change one side, in this example only the spur, same pinion - your ratios change:
{+3/+0}
99/30 = 3.3 : 1
96/30 = 3.2 : 1
93/30 = 3.1 : 1
90/30 = 3.0 : 1
87/30 = 2.9 : 1
84/30 = 2.8 : 1
81/30 = 2.7 : 1
{-3/-0}


Same basic thing happens when you change the pinion only: your ratios change:
{-0/-1}
90/27 = 3.33333... : 1
90/28 = 3.21428... : 1
90/29 = 3.10344... : 1
90/30 = 3.0 : 1
90/31 = 2.90322... : 1
90/32 = 2.8125 : 1
90/33 = 2.727272... : 1
{+0/+1}


Everyone still with us? OK, Now I will make the changes to both, spur and pinion but in opposite direction and behold: your ratios change even faster - they do not remain the same ratio.
{+3/-1}
99/27 = 3.66666... : 1
96/28 = 3.428571... : 1
93/29 = 3.206896... : 1
90/30 = 3.0 : 1
87/31 = 2.806451... :1
84/32 = 2.625 : 1
81/33 = 2.454545... : 1
{-3/+1}


Please 1spunspur , provide a single example for your claim that you keep the same ratio when you adjust the spur up and pinion down, or vice versa. Just one example of such adjustment where this works out in your math.


------------------------------

Different from all that (and what some people may confuse): there is one thing that actually stays the same (at least in theory) when you adjust spur up and pinion down (or vice versa) by the same amount of teeth, but it's not gear ratio: the motor position does not change. So, for example if you use 90/30, 80/40 or 100/20 - you will have very different ratios with each, but you should (at least in theory) be able to put all of these gear combos in without ever having to change motor position or even adjust gear mesh at all (manufacturing tolerances may still apply).

glennhl

iTrader: (16)

And here is another tidbit. If you want to keep the motor in the same position so that you don't disturb weight distribution, but you want to change the gear ratio, then keep the total of Pinion teeth plus spur teeth the same. So if you increase the spur gear teeth by two, then decrease the pinion teeth by the same amount.

And here's how you get the perfect gear mesh with 64 pitch gears. Put on a pinion gear that has one more tooth than you want and push the motor hard against the spur and tighten. Now take off the pinion and put the pinion on that has 1 less tooth. Voila, perfect back lash, works like magic.

DirkW

iTrader: (1)

Sounds like what I said at the end of my post.

Viola? Well, at least not the "walla" I see so often. Not quite there yet, though.

oldfool

Even if it works, I hate this idea.

Mig89

one you can calculate and accountant for a 5 minute, 8 min, or whatever time you're going to be running. A spur is only good for an "x" amount cycles as long as you don't shock stress it (ie. getting airborne after hitting a curb). So yes you can design a spur to last a few minutes or an eternality. Obliviously, a spur that will last forever will be heavy and the manufacture will be selling less of them. But I agree, It's a relativity cheap part and tbh, most won't feel the difference. I personally run the biggest spur I can because I've noticed it does influence the way the car rotates. It's small but I prefer how a bigger spur feels. Also, I've noticed, may spurs now lasts months and usually replace it after my pinion has worn out. Like I tell my boss, the numbers don't lie.