# Crown Gear diameter vs. Gear ratio??



## [email protected]

Is there a difference in the gear ratio of a (7T) pinion and (23T) crown if the diameter of the crown gear is different?? By eyeball the Wizzard crown has about 25% less diameter than the BSRT crown. With all eles being equal does that affect anything besides clearence?? Thanks for your sage advice. mj


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## Hornet

A 7/23 = a 3.29 :1 ratio,tooth counts are usually the way you factor gear ratio's.Divide the number of pinion teeth into the number of crown gear teeth
Theoretically the bigger diameter gear should give you slightly more torque and a slightly smoother car,but that's a point for argument.
Personally i worry more about mesh then diameter.
Just have to watch the BSRT gears especially his "Poly" gears for track clearance and chassis clearance.
If you run your cars slammed on the rails and they make a funny noise,check the crown gear and make sure it isn't physically hitting the track.
Slottech sells some nice crown gears,that are a small diameter,if you do have track clearance issues.


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## johnbalich

counting teeth is the only way........its the RATIO of one to another.


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## [email protected]

I switched out my BSRT gear set for the Wizzard gear set....... it ran like a dog........ switched back to some fresh BSRT she ran like a champ. mj


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## afxgns

I have had this argument with others before.
I think that the dia. does make a difference, a big difference.

Flame on!
Tim


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## [email protected]

afxgns said:


> I have had this argument with others before.
> I think that the dia. does make a difference, a big difference.
> 
> Flame on!
> Tim


In this case the BSRT (larger dia) ran much better than the Wizzard (smaller dia) but these are two different gear sets and that brings in other vairables other than diameter.
What is the difference in you opinion Tim?? thx, mj


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## AfxToo

The diameter of the gear definitely affects the mesh between the pinion and crown, i.e., the angle that the pinion strikes the crown. I'll mix gear brands just to get the mesh I like. The setup I prefer is to configure my rear ends with multiple spacers so I can run either a 7T or and 8T pinion. I set it up so with the 8T one of the spacers is outside the bearing. I recommend using an axle retainer in this scenario because you don't want a marshal to ruin your setup by mashing the axle back in with the spacers not in the right place.

As to brands, the Tyco 7T pinions are soft and mesh well from the start. They just don't hold up well and need to be glued on the shaft or else they will move. I really like Wizzard Pro Predator gears for both pinion and crown, but the Slottech crowns work very well too but it is expensive. The Wizzard 8T pinion is a fabulous gear, not too large like other 8 tooth pinions. The Wizzard pinion gears are really tough so you can run your crown right at the edge and it will hold up just fine. BSRT crown gears are very good too as long as you don't go too low in the rear, say under .434 on a Storm. They are large, .400 or slightly over. If you experience bizarre handling when running a BSRT crown, mark it with a silver Sharpie to see if it is hitting the rails. I'm not a fan of Tomy or BSRT pinions, but I'm sure if you spend the time to set them up right they will be as good as any of the others. 

If you switched out a BSRT crown for a Wizzard crown and were not happy with the mesh, you should also switch to a Wizzard pinion too.

It's all about the mesh. Take the time to get it right and any gear set will serve you well.


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## [email protected]

AfxToo said:


> If you switched out a BSRT crown for a Wizzard crown and were not happy with the mesh, you should also switch to a Wizzard pinion too.


Yeah it actually was a wizzard pro predator 7t pinion and a Wizard 23 crown. i was suprised how badly it ran......... so i switched back to the BSRT pinion and crown and she was rollin'. Maybe i got a bad set. yes slottech is also good, and lighter i have read. mj


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## AfxToo

How are you setting the spacing on the gears? Spacers or gear boss?


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## [email protected]

AfxToo said:


> How are you setting the spacing on the gears? Spacers or gear boss?


Spacers. I actually just assembled a car and i decided to give the Wizard gears another chance (since i bought three sets) and it hooked pretty well. Smooth and pretty fast....... needs a little tweakin'. I had a funny feeling about the first pinion it seemed to go on the shaft a little funny. Maybe that was it. Takes a bit more spacer to get the mesh right. Any tips you have are welcome. mj


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## AfxToo

Nothing really to add. Once you find a combination that works for you, stick with it and see what it do for you on the track. I've had very good luck with all the three major brands of gears. I still use Tyco pinions on occasion, but always put a dab of superglue or green locktite on the tip of the arms shaft before mounting a Tyco pinion. I got on a Slottech crown gear kick for a while because I loved the way they mesh and liked their smaller diameter. Then my supply dried up and I switched to the BSRT gears and they served me well in 21T-23T. However, once my cars started grooving and running lower I ran into problems with their large diameter. I sanded several of them down and that helped a bit. I only recently started trying the Wizzard gears and they are working very well for me. I was initially turned off by the Wizzard gears because the white ones that come with the stock rollers are ... not very good. The white crown gears are better than the stock Tyco crowns however. I still run all brand gears regularly. 

When I buy tires I always get 2 pairs. I mount up one pair on an axle with a 22T crown and another pair on an axle with a 23T crown. I then use spacers to get the mesh set correctly for both 7T and 8T pinion gears as I mentioned earlier. This gives me 4 combinations for every tire height I run, which is generally from .430 to .438 for Storms and .426 to .434 for G3s. I have a smaller number of 21T and 24T crowns mounted up, but the 22T and 23T crowns seem to work for around 90% of the tracks I run on. (This is all for stock and superstock.) 

For tires, I run all of the brands you mentioned. I really like the new black rimmed Wizzard ATFs and the venerable BSRT ASTs (when I can find them). But I also have some of Tom Sheppard's tires, Rabbit Racing, Slottech, one-off brands that I've picked up at shows, and off course several generations of Wizzard tires in various compounds. For slip-ons I also have a number of axle setups with different crown gear sizes, different diameter dual flange (Wizzard and BSRT) hubs, and two sets of every SuperTire size that I need to dial-in the right height combination for the rim sizes I use.


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## [email protected]

Thanks AFX, You got me to think of trying those Wizard gears and that car is running sweet with them. Are the ATFs soft or firm? mj


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## AfxToo

The black ones on the 340 hubs are soft. You need to lightly scuff them with a scotchbrite pad, but they do last.


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## [email protected]

AfxToo;You need to lightly scuff them with a scotchbrite pad[/QUOTE said:


> i'm going to try them. what exactly does scuffing do? i haven't been scuffing mine. I just run them in a few laps. mj


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## afxgns

*The diameter of the gear definitely affects the mesh between the pinion and crown, i.e., the angle that the pinion strikes the crown. I'll mix gear brands just to get the mesh I like. *


I still say that the bigger gear has a different ratio than a smaller gear.

I don't see anyone debunking. It's a math problem. Simple Pi if I'm thinking right.

Tim


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## Hornet

Can't have Tim,the gear ratio determines the amount of turns the crown gear makes compared to the pinion,not the diameter of the gears.
The force applied to the axle to turn it is another story,the bigger gear should theoritically have more torque to apply a twisting motion,but personnally i think the gain in torque is offset by the extra weight of the bigger gear.


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## AfxToo

Yes, the larger diameter gear has a longer moment arm and thus greater torque for the same applied force. From a racer's perspective you are equating this change in torque with the same resultant effect you feel when changing gear ratios with the same diameter gear. Yes, changing gear diameter can absolutely be used as a way to tune the performance of the car to better match your driving style, the performance curve of the car, and the track/power conditions. The gear mesh directly effects the efficiency of the power conversion from the arm shaft to the axle and also has an impact on the tuning equation. 

Yes, gear ratio is just a simple mathematical formula. But tuning a race car is far from simple and requires insight and recognition into all of the static and dynamic variables that come into play. I think afxgns has this kind of insight and has a racers perspective of what is happening on his race cars. The resultant effect of changing gear diameter is similar to that of changing gear ratios. However, the root cause is actually quite different.

No need for flames.


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## Hornet

And who's flaming who,at times your "All Knowing " posts could be construed as flaming,every once in awhile you do have some good info,but alot of times you're just pounding your own chest
I spent too many yrs twisting wrenches and working in the oil and gas industry to have much respect for engineers Dave
So Dave,we aren't that stupid,and your theory doesn't factor in the rotating mass of the bigger gear.
Outta curiousity,how did you get along with fellow employee's when you were working Dave,your attitude of "it's my way or the highway" musta made you very popular with fellow employee's,just my .02


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## AfxToo

Tim's first post.



> I have had this argument with others before.
> I think that the dia. does make a difference, a big difference.
> 
> *Flame on!*
> Tim


My post, in response to Tim's posts:



> *No need for flames.*


Hmmmm???


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## afxgns

Hornet said:


> Can't have Tim,the gear ratio determines the amount of turns the crown gear makes compared to the pinion,not the diameter of the gears.
> The force applied to the axle to turn it is another story,the bigger gear should theoritically have more torque to apply a twisting motion,but personnally i think the gain in torque is offset by the extra weight of the bigger gear.


I thought the difference in pitch diameter of the two gears in question determined the ratio, not the tooth count
Does the ratio change if the ring gear changes diameter?


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## SwamperGene

No Tim, it is impossible. Gear ratio is simple math, size has no bearing on ratio. 

Take a 7/21 setup, which is 3:1. The small gear has to make 3 revolutions to turn the big gear once. No matter what size either gear is, the same mathematical fact remains for all 7/21 setups.


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## afxgns

SwamperGene said:


> No Tim, it is impossible. Gear ratio is simple math, size has no bearing on ratio.
> 
> Take a 7/21 setup, which is 3:1. The small gear has to make 3 revolutions to turn the big gear once. No matter what size either gear is, the same mathematical fact remains for all 7/21 setups.



Soooo,
You're telling me that the 15 tooth gear in a t-jet (.315 .o.d) will be the same ratio as the 15 tooth gear in a specialty chassis. (.260.o.d)?

Funny, When i use these two gears, and I turn the rear wheels one time exactly. The t-jet gear turns the motor 3 2/3 times.
When I use the specialty gear(same tooth count) it spins the motor 3 1/3 times.

But there's really no difference?


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## Hornet

Not gonna argue here Tim,but no way in hell can that happen ,on a pinion and crown gear assembly,unless you shift the gear during turning.I'm pretty sure the post started out about a normal pinion/crown set-up,at least that's what i was talking about,how it got turned into a T-jet post is beyond me.
On a 3 gear T-jet set-up yes the differant diameters will do what you're talking about,but not on a pinion and crown set-up


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## hankster

First, I would suggest that if anyone has a "personal" problem with someone else, they take it to email.

Second, if the tooth number is the same, the ratio will be the same. The PD (pitch diameter) of the gear is what determents the diameter of the gear. You should NEVER mix different PDs with the spur and pinion as the gears will not mesh properly and your efficiency will go way down.

http://en.wikipedia.org/wiki/Gear_ratio


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## Hornet

Actually i owe AFXII an apology,i mis-interterpted your post Dave,sorry about that,i thought you were accusing me of flaming Tim,and it wasn't my intention to flame anybody at all.
Sorry Dave
Rick


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## SwamperGene

Hankster's link pretty much sums it up. :thumbsup:


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## AfxToo

> Sorry Dave


Accepted and appreciated.


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