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Old 01-18-2010, 08:51 AM   #1
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Default Gearing Confusion for Newbie!!

ORIGINAL: 122barney

I have the HPI Sprint 2 FLux (5700 brushless by Castle, OEM). PInion is 23, spur gear is 90, 48 pitch. I have seen all the gearing charts (can not find a brushless chart), but they mean nothing to me. I am just looking to gain a bit more top end and not harm the motor. What is the most efficient way to gain top end. LArger (26T?) pinion by a few teeth and keep the 90 spur? What are the basic rules of thumb? Is it true, brushless is different from brushed motor gearing basics? SOmething about brushless getting up to max RPM? Anyway, any guidance is appreciated. Is there a brushless gearing chart?

Thanks
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Old 01-18-2010, 08:57 AM   #2
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Quote:
Originally Posted by 122barney View Post
ORIGINAL: 122barney

I have the HPI Sprint 2 FLux (5700 brushless by Castle, OEM). PInion is 23, spur gear is 90, 48 pitch. I have seen all the gearing charts (can not find a brushless chart), but they mean nothing to me. I am just looking to gain a bit more top end and not harm the motor. What is the most efficient way to gain top end. LArger (26T?) pinion by a few teeth and keep the 90 spur? What are the basic rules of thumb? Is it true, brushless is different from brushed motor gearing basics? SOmething about brushless getting up to max RPM? Anyway, any guidance is appreciated. Is there a brushless gearing chart?

Thanks
First, you need to know that the higher you go with your spur the more acceleration you will have. And the bigger the pinion, the more top speed and. Also if you go down 1 tooth with the spur it will be about 1mph faster. If you go up 1 tooth for a pinion it would be about 2mph faster. So I would suggest going to a 25 tooth pinion but if the motor is too hot then go to a 23 or 24.
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Old 01-18-2010, 09:45 AM   #3
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OUCH...and WHOA !!! That kind of thinking will get you in trouble. Bryan, don't know where you came up with that idea but you should forget it. 1 mph this and that doesn't make a bit of sense and bigger spurs and pinions don't necessarily do what you think. Maybe I can help.

This is all about need and speed and a bit of simple math. 122barney...before we can give you a suggested gear ratio we need to know the "INTERNAL drive/gear ratio" for your car. Your manual should tell you what that ratio is. Every car or truck has one. It's the ratio the vehicle was designed with based on the gears in it's drivetrain that are usually NOT changeable.

When you drive the car...what you see is the result of the "FINAL drive/gear ratio". It's the ratio created after you factor in the spur gear size and the pinion gear size. For instance: On a TC3 chassis the Internal drive ratio is 2.50. If I use the stock 72 tooth spur gear and a 25 tooth pinion gear. To know the FDR I would divide the spur tooth count by the pinion tooth count. 72/25=2.88. Then multiply that number by the Internal ratio. 2.88 X 2.50= 7.2 FDR.

The question is....what the heck does a 7.2 ratio mean to me ?? Does it make the car faster or quicker ?? In general...the larger this number is, the slower the top end speed of the car BUT...it will generally accelerate quicker. 2 key words SPEED and ACCELERATION. Again...another thing you have to learn....what do you want the car to do?? Have higher top speed or accelerate quicker ?? As an example...If I race on a very technical, twisty race course with very short straight sections....would I be more concerned about top speed or acceleration?? Top speed makes no sense because theres no long section of track to use it. Acceleration is more important as I want to be able to make the turns and ACCELERATE quickly to the next one. Soo....I drive my car with that 7.2 ratio and I find its really slow on acceleration....I would then consider changing the FDR to a HIGHER number. Its much more normal to change the pinion gear rather than the spur gear. Easier too. Follow the general rule of thumb...You should make small changes in gear ratios. Change the pinion by only 1 or 2 teeth until you find the appropriate ratio. In this example lets calculate.....72 tooth spur/23t pinion= 3.13. Then 3.13x2.50= 7.82 FDR. Higher number = more acceleration.
This is a pretty basic explanation. It all comes with experience. You develop a sense for what gearing is needed based on the track type. And yes, brushless motors have different gearing requirements. Many manuals don't even mention the suggested gearing for BL use. But...you can compare to the listed motors as a guide. For instance...if the manual shows a 27t brushed motor gearing and you're using a 17.5 brushless system (which is comparable in performance) then start with that number and be prepared to make changes. Generally, you can use less teeth on the pinion to achieve the same performance when you use a BL motor. Run the car for a couple of minutes...evaluate performance AND CHECK MOTOR TEMPERATURE...then make gear changes.
Hope I helped. PM me if you have more questions. If I don't have the answer I'll get you to someone who does.

Last edited by Evoracer; 01-18-2010 at 10:00 AM.
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Old 01-18-2010, 08:05 PM   #4
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Default VERY Insightful!!

Evoracer......NOW this is what I was looking for!! Thanks

Two last questions: Going back to my ten speed bike years, yea a Schwinn Varsity...Big gear in front, little gear in the rear makes you go faster, but your legs sure had to work. (Bad Acceleration, good top end) Is there a point where you have made the pinion TOO BIG that it does not allow the motor to work in it's optimum power band or RPM level? In other words, you can achieve a similar FDR with either a small pinion and big spur or with a large pinion and small spur. I guess this is why you check motor temp?

Would a change from OEM 90/23 (3.91)x2.13+8.33FDR to a
MOD of 90/25 (3.6)x2.13=7.66 be very noticeable in "compromised" Acceleration and Gained top end? Again, I get the entire concern for what you want the car to do, but curious of your thoughts. Another option is 86 Spur and stock 23 pinion for a 7.96FDR. Again, not sure if .3 really means much in performance either way? I know i need to try it, been raining our here in S Cal. I am on a pretty big parking minimal turns (we just aren't skilled enough yet), so top end means a little more at this point. i do know, if you get on the throttle hard, it will slip and slide if you are not careful. (plenty of torque)

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Old 01-18-2010, 08:54 PM   #5
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Hey, glad i could help and hope it wasnt to long winded.
From your questions it appears you have a good grasp on the principle.
Gearing is gearing....no such thing as a pinion thats to big. At least in theory !! But...from a mechanical standpoint the answer is yes...because of limitations at the motor mount or chassis structure...you can run into problems with larger pinions. Especially with brushless. Large pinions can be hard to get..especially in 48 pitch. For instance...I run RCGT class. 17.5 brushless. The ratios required run anywhere between the mid 3's and up to around 5.0. To achieve these on many chassis (particularly older cars and those like the Flux that are built primarily for the fun market vs. true racing) you have to modify the chassis to accept the large pinions.....sometimes as large as 50-52 tooth. Many of the newer cars have motor mounts that allow for this.
But...there are times when simply changing the SPUR size will solve the problem. Most ratio changes are done via the pinion simply because you can normally adjust it over a greater range that way. Given the size of spur gears...there's normally only a small variation available. Shaft drive cars are more difficult in that respect than most belt drive cars.
And yes.... a change in ratio by just 2 teeth can make a big difference. Then again...it depends how far off you start with. If i have a drift car using a 8.0 ratio and I want to change it to a 4.5 ratio for racing....thats going to take more than a 1-2 tooth difference.
If I understand you....your car at OEM is geared at 8.33 fdr ?? I'm not familiar with the 5700kph motor. I'm thinking thats somewhere around a 19-27 turn brushed motor in comparison. If so, I'd suggest making changes 2 teeth at a time. I pulled up the Flux manual and it's useless for gearing info. I wouldn't hesitate to call HPI or Castle for gearing help. Just tell them what you're trying to do. I'm curious why HPI geared the car that way. Then again...it's more of a basher/fun car....I suppose they wanted to keep top speeds down.
Did I help?
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Old 01-18-2010, 09:41 PM   #6
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Very good info there.

Just to add a bit of technical info here:

Final Drive Ratio = FDR, is a number that tells you the number of motor revolution for one wheel revolution

eg: if FDR = 8.5, it takes 8.5 motor revolution for 1 complete wheel spin.

You may heard of the term Rollout. Rollout is the distance travelled in 1 motor revolution. and here is the relationship between rollout and FDR:

Rollout = Pi x TireDiameter / FDR

eg: if your tires are 60mm in diameter, then Rollout = 3.1416 * 60 / 8.5 = 22.17mm
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Old 05-03-2010, 11:41 AM   #7
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Man I love the search button...lol Great info, thanks..
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Old 05-03-2010, 09:18 PM   #8
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Great thread and great info for the new guys!!!!!
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Old 01-24-2011, 12:31 AM   #9
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Evoracer,

You just answered a TON of noobie questions I have on on-road racing gearing! I am into competitive rc rock crawling where gearing doens't play as much as a factor than building your rig to suit the way you drive and knowing how it will behave in certain lines you are trying to clear.

Since it is our off season these past couple of months I've been getting into on-road racing in Houston and it is a blast! Nice change of scenery from driving fast and accurately versus slow and methodically.
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Old 01-24-2011, 12:47 AM   #10
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Really good info here i find gearing blinking
confusing. !!
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Old 07-04-2012, 08:54 PM   #11
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This thread is worth a bump...
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Old 07-05-2012, 12:11 AM   #12
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If you want the best top speed, acceleration, and good runtime without overheating, you'll need to gear your Car around 6.9fdr to 7.1fdr .... Good luck.....
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Old 04-06-2016, 03:55 PM   #13
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Just a question regarding fdr. If I have a 7.5t motor and a 17.5t motor do I still aim for the same fdr if using the car on the same track? or does the turns of the motor mean I should be looking for a different fdr?
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Old 04-07-2016, 12:46 PM   #14
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Quote:
Originally Posted by JPucko View Post
Just a question regarding fdr. If I have a 7.5t motor and a 17.5t motor do I still aim for the same fdr if using the car on the same track? or does the turns of the motor mean I should be looking for a different fdr?
Yes, you should use different FDRs for different motor windings. Less turns = higher FDR, more turns = lower FDR. Doing it wrong can potentially cook your motor and/or ESC.
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