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Gearbox Calculation Question

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Elpwnage20 View Drop Down
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    Posted: Nov/14/2014 at 11:14pm
've been trying to figure out our optimal final reduction ratio after considering our CVtech CVT.
It has a max ratio of 3:1 and min of 0.43:1 . So what I'm doing is first assuming we want our top speed of 40km/h at the 3800 rpm mark. I also assuming for the purposes of the calculations, a CVT engagement RPM of 3400 rpm.

So I said that with a 0.43 cvt ratio and 40 km/h needed at the 25" wheels, the final drive ratio should be 26.4! It makes sense because the rpm stays constant while the CVT is shifting, and the ratio of the CVT decreases which causes the speed to increase.

I've seen people posting final ratios of around 9-13 on here.

Should I not assume the speed will be 40km/h at 3800 rpm? I've tried setting my final ratio at 12 to give a ratio of 36 at min speed and 5.16 at max speed. The speeds I'm getting when the CVT is just about done shifting to its max position is 157 km/h.  I'm guessing that the torque has something to do with this and the speed is limited by the weight of the car and friction? Or should the CVT never actually finish shifting fully?

I'm just confused on where to set our wanted max speed of 40km/h. If it's at 3800rpm, the final reduction needed would be way too high.

I've attached a picture of the excel table I made. As you can see, I've manually put in 40km/h for the max speed at the bottom to see what final reduction I need but the rest where found the opposite way where I set the final reduction and solved for speed. Any advice?

BTW, why is the ratio range of the CVT 6.97? What significance does that have to these sorts of calculations? Don't I want to use values from 3 to 0.43 ?




Edited by Elpwnage20 - Nov/14/2014 at 11:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2014 at 2:04am
When we ran a CVTech we would gear it for at least 40mph which is probably part of the problem. I'm not sure if we ever got it to shift all the way to the 0.43 overdrive, which is part of why we ran a lower final drive ratio. We even ran a CVTech with just a 64/9 chain reduction after the pulley which gives a crazy high top speed of like 85mph, but was still our fastest car yet.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elpwnage20 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2014 at 3:05am
But many people don't even reach 40 mph. A typical speed I've read about is around 30mph so we should do 50. But thanks for the info. Yeah I don't think we'll be getting to overdrive either.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jeiB Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2014 at 8:47am
also, your engine will only reach about 3600 when its loaded
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Post Options Post Options   Thanks (1) Thanks(1)   Quote zglazer Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2014 at 8:30pm
You will most likely not get anywhere near the 0.43:1 overdrive ratio with the CVTech. I know the guys from Laval have gotten close, but when we used to run that CVT, I think the lowest we ever measured was around 0.8 or 0.9:1. You can do some searching around the forum for more info on this, since it's been discussed before. Not to mention 40km/h is a pretty low top speed to gear for.

There are a lot of different things you can target with your gear ratio selection, but if top speed is what you're going for, I would suggest gearing a little higher, at least 45km/h or so, and if you can't make any measurements of the CVTech's full shift ratio yourself, make a conservative assumption like 0.8:1. With those numbers, assuming a max engine speed of 3600rpm while loaded like Jeremie said, and a rear tire diameter of 25" like you used in your spreadsheet, you'd need a gearbox reduction of roughly 12:1.

Just to be clear, this is just a very rough suggestion. You should look at the information you have and come to your own conclusions about exactly what ratio you want to go with in the end.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elpwnage20 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/17/2014 at 11:16pm
Yeah I figured that. I don't think we want top speed as much as having more torque. That would warrant a larger gearbox ratio then. But with a two stage gearbox, it seems like anything higher than 12 is pushing "small" size limitations.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zglazer Quote  Post ReplyReply Direct Link To This Post Posted: Nov/18/2014 at 2:09am
Well, that's the thing. It's important to know whether you want more top speed or more torque, and to understand why. I think you're probably wrong in wanting more torque than you'd get from my earlier suggestion, but as far as what the ideal balance is, that's something that takes quite a bit of work and analysis to properly figure out. A good starting point is to draw some free-body diagrams and figure out the dynamics of a baja accelerating in a straight line. I won't go into much more detail than that, but it should be enough to raise a few more questions for you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pj_rohlf12 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/21/2014 at 2:11pm
The best bet is assume that your CVT overdrive is 1:1. Use that to set your speed and work backwards. I know we set 44mph as a top speed and we were able to obtain about 33. There is friction and weight that need to be accounted for. You should reach 75% or so of calculated top speed.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zglazer Quote  Post ReplyReply Direct Link To This Post Posted: Nov/21/2014 at 9:43pm
The fact that you were able to reach 75% of your targeted top speed doesn't mean that rule applies to all situations. By that logic, if you design for a top speed of 100mph, your car will reach 75mph, which is obviously not going to happen.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pj_rohlf12 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/24/2014 at 1:39pm
zglazer, it is practical to apply that when you set your top speed within reason. I am talking baja here not formula 1 cars......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nate Quote  Post ReplyReply Direct Link To This Post Posted: Nov/25/2014 at 1:58am
Keep this thread in mind.

http://forums.bajasae.net/forum/absolute-top-speed-of-competition-ready-car_topic1037.html

There's not much point in designing for a higher top speed than your engine can produce.

From what I've read, some people design so they see the most efficiency from their CVT. Aka: find what speeds you normally operate at, then gear so that the efficient ratio is in that normal speed range.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hitesh arora Quote  Post ReplyReply Direct Link To This Post Posted: May/16/2016 at 10:32am
Can you please upload the excel file or explain what formulas you used for calculating the table?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: May/16/2016 at 11:15am
Google is your Friend. These are the most basic gear reduction formulas used. day 1 of mechanical engineering 

https://en.wikipedia.org/wiki/Gear_train
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hitesh arora Quote  Post ReplyReply Direct Link To This Post Posted: May/16/2016 at 11:54pm
yes, i have been reading various books on transmissions , aaen cluth tuning handbook , book by gisbert lechner but I am not able to find answers for my baja atv. We are using a CVT (6" primary and 8" secondary , centre to centre distance = 10") coupled with an FNR gearbox ( it has gear ratios : 1st - 31.5; 2nd-18.7; 3rd-11.4; 4th-7.35; reverse-55) but we are not able to get a decent acceleration and topspeed is only 30kph at 1700 idling and 3800 max governor setting as in  the rulebook. While testing we increase the rpm to 5500 and idling to 2500, that makes the atv fast and fun. Please suggest some tweaks, I have searched alot but still not found the relevant solution.
ATV's weight is around 240kg without driver( is this too high and killing the speed) 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nick914 Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2016 at 8:06am
Have data acquisition on the engine and gearbox and see if it matches what the clutch tuning handbook says.
Weigh definitely doesn't help
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hitesh arora Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2016 at 8:42am
with software? Please explain what data to check for.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2016 at 8:46am
try reducing weight, and check for other efficiency losses. the CVT sounds like a gaged from that description, try playing with the center distance a bit. it's not exactly 10". if you put it at 10" it will not spool out fully. that could be some of your problem. check that your brakes are properly aligned, check CVT alignment left/right. could be a combination of a number of things. mostly, if you want a faster car, reduce weight. a 530 lb car is a very heavy car.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sujandinesh Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2016 at 11:06am
How fast is your car? I mean in seconds for a certain run. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nick914 Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2016 at 8:15am
Originally posted by Hitesh arora Hitesh arora wrote:

with software? Please explain what data to check for.

If you read the clutch tuning handbook it's pretty straitforward.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hitesh arora Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2016 at 1:28pm
hello team helios, our atv covers 50m in 8+ seconds, we keep on experimenting with the idling and throttle system , sometimes it works 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sujandinesh Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2016 at 10:34am
For that time, considering we had run a similar weight car couple of years ago, looks like its not a weight issue.

As discussed earlier, you have got to tune your CVT and check for losses in your transmission. Also try varying the tire pressures and also as said the throttle system. 

But 8+ seconds is too much for that weight for 50m. It definitely is not a weight problem.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Swappy Quote  Post ReplyReply Direct Link To This Post Posted: Jun/18/2017 at 1:44am
In designing gear box !
The position of gears may effect in speed ?
I design these gears in 30 & 60 degree angle arrangement !?? uploads/4340/TMPDOODLE1497764631664.jpg
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Swappy Quote  Post ReplyReply Direct Link To This Post Posted: Jun/18/2017 at 1:51am
The spur gear design is on the basis of speed or torque ?
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