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Final Gear Ratio

Iowa Fast Track RC Discussion Board

Post Sun Jan 27, 2013 4:24 pm

Posts: 169
Location: Cedar Rapids IA
OK have a question for on road gearing. On the Final Gear Ratio, the lower the number the faster or is it the higher the number the faster. I have a 78T Spur and a 39T Pinon, 48 pitch = 4.00 Final Gear Ratio. Running in the 17.5 sedan class.
Thanks
Holst RC Racing!!!
Losi Ten SCTE 4WD
Losi XXX-SCT 2WD
Short Course Racing
RULES!!!!!!!!!

Post Sun Jan 27, 2013 7:28 pm
admin Site Admin

Posts: 718
With Final Drive Ratio (FDR), lower numbers result in higher top speeds. An FDR of 4.0 is a good place to start for the 17.5 class.

What Final Drive Ratio means is how many times the motor needs to spin to rotate the tires one time. With an FDR of 4.0, the motor spins four times to make the tires spin once.

Post Tue Jan 29, 2013 1:22 pm

Posts: 169
Location: Cedar Rapids IA
Thank You
Holst RC Racing!!!
Losi Ten SCTE 4WD
Losi XXX-SCT 2WD
Short Course Racing
RULES!!!!!!!!!

Post Sun Feb 24, 2013 8:31 pm

Posts: 148
Location: Wellman, Iowa
So I'm reading about the FDR for the VTA class being 5.0, wanna make sure I'm reading it correctly.
Not too sure where to start out at with the tc3 & the tc5 but right now I have their FDR at 5.14 & 5.12.
Looks like I'm going to have to pick up some more gears, what I have now doesnt give me alot to change to.

Post Sun Feb 24, 2013 9:54 pm

Posts: 207
You're safe with that ratio. Any higher (lower numerically) than 5.0 won't pass tech.

TC3 has an internal ratio of 2.5
TC5 has an internal ratio of 2.0
Here is the calculator I use http://www.tqrcracing.com/rc_tools/gearing.asp

Post Wed Feb 27, 2013 11:42 pm

Posts: 194
Ok so finally after about two months of tinkering I got me old tc3 up and running. I put a 72 tooth spur with a 26 tooth pinion and when Ihook the batt up it seems ok but I have not ran it on the track yet.does this sound about right and if not which direction should I go with it.
Thanks for your help and I cannot wait to run on sunday

Post Thu Feb 28, 2013 12:02 am
admin Site Admin

Posts: 718
Big Joe wrote:
Ok so finally after about two months of tinkering I got me old tc3 up and running. I put a 72 tooth spur with a 26 tooth pinion and when Ihook the batt up it seems ok but I have not ran it on the track yet.does this sound about right and if not which direction should I go with it.
Thanks for your help and I cannot wait to run on sunday


A TC3 has an internal gear ratio of 2.5. So the FDR with a 72 spur and 26 pinion would be 6.92. That would not be right for the classes we typically run. With a 6.9 FDR, your motor will rev high, but your car won't have much top speed.

Which class is this for? VTA, USGT, or 17.5 TC?

For VTA, you need to be at least 5.0, but you want to get as close to 5.0 as you can get. An FDR of 6.9 would be legal in VTA, but slow. For VTA, use a 36 pinion with a 72 spur. You would need to check to be sure the larger pinion would fit. It don't actually know if it will or not.

For USGT, there is no hard rule on the FDR, but 4.0 is usually a safe starting point for most motors. If it fits, I would start with a 44 pinion and 72 spur.

For 17.5 TC, you could start similar to my recommendations for USGT.

If you want to, you can leave your car geared as is. It will work. It just won't have as much top speed. It is really easy to change the gearing in a TC3 at the track and most of us have plenty of spare pinions in case you need to borrow a larger pinion.

Post Thu Feb 28, 2013 9:15 am

Posts: 275
Location: Brighton, IA
admin wrote:
Big Joe wrote:
Ok so finally after about two months of tinkering I got me old tc3 up and running. I put a 72 tooth spur with a 26 tooth pinion and when Ihook the batt up it seems ok but I have not ran it on the track yet.does this sound about right and if not which direction should I go with it.
Thanks for your help and I cannot wait to run on sunday


A TC3 has an internal gear ratio of 2.5. So the FDR with a 72 spur and 26 pinion would be 6.92. That would not be right for the classes we typically run. With a 6.9 FDR, your motor will rev high, but your car won't have much top speed.

Which class is this for? VTA, USGT, or 17.5 TC?

For VTA, you need to be at least 5.0, but you want to get as close to 5.0 as you can get. An FDR of 6.9 would be legal in VTA, but slow. For VTA, use a 36 pinion with a 72 spur. You would need to check to be sure the larger pinion would fit. It don't actually know if it will or not.

For USGT, there is no hard rule on the FDR, but 4.0 is usually a safe starting point for most motors. If it fits, I would start with a 44 pinion and 72 spur.

For 17.5 TC, you could start similar to my recommendations for USGT.

If you want to, you can leave your car geared as is. It will work. It just won't have as much top speed. It is really easy to change the gearing in a TC3 at the track and most of us have plenty of spare pinions in case you need to borrow a larger pinion.


VTA is the easiest to get gearing for. The 36 will fit but some dremeling of the webbing below the pinion may need to be done. Easiest thing to do would be get a smaller spur gear. I had a 60t spur when I ran the TC3. Not sure if the 30t pinion would mesh up with a spur that small though. I had my gearing at 39/60 (before the 5.0 FDR) and had to take a lot of the webbing out. It doesn't hurt the durability of the chassis. It can be a pain to do though.
Losi XXX-SCT/B, TLR 22sct, TLR 22T, TLR 8ight 2.0e

Post Sat Mar 02, 2013 12:43 am

Posts: 194
So. I overnighted a 44 tooth pinion onlt then to come here and read the post about having to get the dremel out t make it fit. Needleess to say the 44 isn't close lol. The largest pinion I have is a 27 and the smallest spur is I ggot is 72. Now I did call boxcar and they got nuttin so I ask you guys am I just gonna have to limp myway through my first race or drive to dsmoines or???
If I were to be able to get the right gearing fom someone or somewhere where should I start. Spur and pinion wise 60/36-7-8 ish or???
Hope I don't have to drive all the way to desmoines where is a closer hobby shop than boxcar that carries stuff like this.
Thanks guys.

Post Sat Mar 02, 2013 3:36 pm

Posts: 194
So I call ed boxcar, hobby corner, and hobby haven with no luck on finding th right gears. Looks like ill be limping around the track for my first race. Oh well

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