mab_man20

iTrader: (6)

acloco: You are right about that, but your not taking everything into account. Yes a larger spur gear will create more torque, but to keep the same gear ratio you need a larger pinion.

Right before you hit the throttle the entire drive train is stationary. When you pull the throttle the tendancy for the drive train to remain stationary creates a torque on the pinion. With a larger pinion this torque now also has a longer arm. Also the drag in the drive train will continue to act along this same arm.

Again for practical purposes the two torques cancel eachother out.

Unregistered

Ah, the old spur gear argument.

The only valid point here is the mention of the flywheel affect of the gear it self and even at the high speeds that we run at, the affect is so minute that it becomes insignificant given the weight of the gears.

The geartrain on our cars is nothing more than a simple torque multiplier. I would sugest that anyone who beleives that the diameter of the spur (given the same ratio) has any affect on punch should take a look at any high school physics book for an explanation.

You can compare the affect of spur gear size by looking at the lever arm provided by each gear. As you increase the spur diameter, you are also increasing the pinion diameter. Do the math, it's a wash.

Does spur diameter have an affect on performance? Yes. But it is the change in weight distribution (especially on pan cars) that causes the change in lap times.

lem2

iTrader: (6)

I love this forum!!! Thanks guys for all your input!! It is really appreciated!!

Been preparing for a race this may3!! Its a Big race and I'm aiming for a beetr position! It will be hard, since there will be a lot of racers!(there will be a rafle "kit of choice!")

Again Thanks!!!! - Lem

DaveW

iTrader: (1)

Um... on small high banked oval tracks (oval cars)... going to a larger spur gear DOES allow the car to come up off the corner better... even IF the rollout is the same as being run with a smaller spur. Its called machanical advantage. It is easier for the motor to spin X amount of times MORE on a larger spur gear to move the same amount of weight at the same rollout. But this also takes into account the amount of HP you are utilizing. Admittedley, the numbers are the same from one setup to another... but when a motor spools WITHIN its powerband on a larger spur gear, the car WILL accelerate faster. I think that is the real issue. Not the actual gears alone. I know from running modified in TC, you dont just add a friggin 104 spur to whatever motor you are running. Comparing power bands from one motor to another will make you use a different spur and pinion combo... even when using the same teardown/wind and rollout numbers. Sometimes i use a 96 spur on singles... and a 104 on doubles or triples. Depends on traction available... yada yada yada.... Now as far as breaking this down in mathematical equations... i cannot do this. But the dyno and track results DONT lie. Hope this helps...
- Dave

gotpez

A ratio is a ratio is a ratio. Look at all of the equations on torque multiplication and you will see that it all has to do with ratio, not the size of the numbers involved. A 4:1 ratio with a 100 spur and 25 pinion is the same as a 4:1 ratio with an 80 spur and 20 pinion. N0 matter how you change the number of teeth in the equation, with a 4:1 ratio the motor will spin 4 times for each revolution of the spur.

To keep it simple, yes, the larger spur has a longer lever so it takes less force to make it move. But since the pinion has a longer lever also, it has less power to make the other lever move. These two items cancel each other out in the equations.

As has been stated, the biggest effect any of this has is in weight placement. The motor is one of the heaviest components on any of our cars. Moving it .25 inches can have a big effect on balance.

Unregistered

Dave,

We're laughing at your post here at work...

Rollout is rollout. The number of motor revolutions per inch does not change given the same rollout unless you have some way bend the laws of physics in your favor, this is a constant.

When you change to a larger spur, given the same gear ratio and rollout, there is virtually no change in acceleration other than the change in the location of sprung weight in the rear pod.

Increase the spur size and add bigger tires to get the same rollout and that's a different story. Even the tire diameter change will have more affect than the gear change. Todays gears are so light and efficient that there is no appreciable change caused by larger gears unless you go to an extreme (pinion less than 17).

It's been my experience that some guys can't grasp the concept that when they start moving around weight and axle heights when they change gears, they change alot more than just a simple spur and pinion. Moving the motor changes how the car handles bumps, weight distribution, and most importantly how much tendancy the rear pod/axle has to "axle wrap". These are just some of the things that you change just from that little gear change.

Boomer

Exactly!

the ONLY difference moving from one size spur to another size spur (keeping pinions relative) is the efficiency argument (which John Stranahan mentioned).

Weight is laughable. We're talking minute amounts of weight compared to the power the motors are putting out. negligible.

But absolutely optimal mesh is absolutely vertical - || - and the closer to that you can get, the better the mesh. So, if I use a couple of teeth higher in spur and a tooth higher in pinion, I may have the same ratio but the mesh is going to be better and more efficient.

There is another argument. There is a limited amount of adjustment space in the TC3. . .I've gone down a couple of teeth in my spur in certain circumstances to get a little more room. . .

DaveW

iTrader: (1)

Unregistered...
Glad to amuse you and your friends. But i guess i should have been a little more clear. I was referring to making rollout the same... by making a tire size change... not a pinion change. Yes i also know that this (tire change) affects other variables... and if you go back and read... i also stated that the numbers ARE the same, ratio to ratio. YES, moving the weight of the motor around makes an affect on the handling of the car. THAT was not a debate. But answer this... if there is no need for gear changes from one size pinion/spur combo to another, because a ratio is a ratio... then WHY are these options made? Why even have the options? All types of vehicles could wonderfully run on ... lets say... an 84 tooth spur gear. No you say? Well why then? Now, i know im just comparing apples and oranges... and being simplistic and silly. But hey... i AM here for your amusement.
- Dave

Boomer

Hey Dave, I figure that's my primary role in life too, so welcome to the club!

Mike D

Spur gear size - The same discussion has come up hundreds of times and the same things are always said. Trust me, nobody will win.

BTW - The reason so many spur gear sizes are available is because people want to buy them. It follows the whole "make it out of aluminum and anodize it" marketing technique. No science involved...

RichChang

Dave - you actually answered your question for yourself.

(btw: Unregistered mentioned just him and his co-workers. Who know if they are his friends. )

Different size spurs are around because some vehicles only accept certain size spurs (and also because of what Mike D said).

Also, as you agreed with , different size spurs move the motor around and that affects handling (especially in pod based cars -- like 1/12th scales).

In regards to roll-out, "roll-out is roll-out" (it has gear ratio as a piece of the roll-out formula) whether you keep the pinion the same, the spur the same, or the tire size the same. So, as long as you end up with the same roll-out, it doesn't matter which parameter you keep the same, or if you change them all. For example, when speaking about just gear ratio, we are assuming the tire size is the same (for a 1:1 relationship). We really are also talking about roll-out, but we are being 'sloppy' and dropped the tire size factor.

-Rich

Brian McGreevy

iTrader: (10)

Then why would companies like Losi make different spur sizes for their XXX-S? You can't tell me that you need a pinion so big that you'd need to use anything smaller than the blue 128 tooth spur. Hmm ponder that guys...

Boomer

Hmmmm, what would that be in 48p?

I likely agree with you.

I run Pro10 and use 84 or 85 (depending on what is available, I hate Kimbrough gears) no matter what my pinion is. . .

I also run my TC3 and the only time I change my spur is if I'm gonna drive it at Rev (1/8th scale track) and don't mind burning up the motor. . .

RichChang

As has been mentioned many times before, it affects motor placement which affects weight distribution either fore or aft (ie:XXX-S) or side to side (ie: TC3).

On a TC3, the bigger the spur gear, the farther to the right side of the chassis the motor is located. On the XXX-S, the motor is pushed fore or aft depending on the size of the spur.

If you don't think motor placement is that important, ask the Losi folks. There is a thread on Trinity Tech Talk -- "Ask Todd Hodge." On one of the last few pages on his thread someone asked him why Losi doesn't offer a lowered motor mount on the XXX-4 (as they did for the XX-4). He said the reason is that on the XXX-4 the buggy works better with the motor higher.

If the motor placement didn't matter and didn't have an effect (in this case up or down and affecting C.G.) then they wouldn't care if the motor was lowered. Thus, it extends into being able to affect the car fore or aft, left or right.

Also, this discussion is going on in 2 threads on here. Read the other TC3 thread for some other responses.

-Rich

Kevin K

iTrader: (32)

I see what a lot of you guys are talking about and from my personal experience when I run a larger Spur gear the car always accelerates harder then it does with a smaller spur with the same FDR and same tires. And when I race on a larger or longer track cars seem to like a smaller spur with the same FDR....why is this? They just seem to be faster out of the hole with larger spurs and have more top end with smaller spurs with the same FDR and tire size. How much of this has anything to do with trans ratio? I know that in the old off road days when Losi had two transmissions for the buggy and truck they would sometimes run the truck trans in the buggy so they could get up to enough speed so they could clear jumps that they could not with the other trans and all gear ratios. So this would lead me to think that a car that has a higer(numaric) trans ratio would be quicker off the line then a car with a lower(numaric) trans ratio if both cars have the same motor, tires, gearing(FDR)....and the Car with the lower trans ratio would reach a higher top speed then the other car.....is this true what do you think?

You could test this with a car that has an adjustable trans ratio like the Schumacher Mission....I have raced that car in the past and what Im talking about is exactly what I found with the car.