Gearbox Calculation Question |
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Elpwnage20
Welding Master Joined: Apr/12/2014 Status: Offline Points: 225 |
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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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Richie_Dagger
Baja Godfather Joined: Sep/25/2011 Location: Las Vegas, NV Status: Offline Points: 615 |
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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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Elpwnage20
Welding Master Joined: Apr/12/2014 Status: Offline Points: 225 |
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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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jeiB
Baja Godfather Joined: Jul/17/2009 Location: Montreal Status: Offline Points: 604 |
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also, your engine will only reach about 3600 when its loaded
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Jeremie B.
McGill Baja Racing 2009-2011 Captain minibaja.mcgill.ca |
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zglazer
Baja Godfather Joined: May/27/2012 Location: Metro Detroit Status: Offline Points: 593 |
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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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Zack Glazer
McGill Baja Racing Alumnus 2009-2013 http://baja.mcgilleus.ca/ 2012-2013 Drivetrain Designer |
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Elpwnage20
Welding Master Joined: Apr/12/2014 Status: Offline Points: 225 |
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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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zglazer
Baja Godfather Joined: May/27/2012 Location: Metro Detroit Status: Offline Points: 593 |
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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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Zack Glazer
McGill Baja Racing Alumnus 2009-2013 http://baja.mcgilleus.ca/ 2012-2013 Drivetrain Designer |
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pj_rohlf12
Milling Master Joined: Jun/09/2014 Location: United States Status: Offline Points: 53 |
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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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UW-Stout Baja VP
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zglazer
Baja Godfather Joined: May/27/2012 Location: Metro Detroit Status: Offline Points: 593 |
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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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Zack Glazer
McGill Baja Racing Alumnus 2009-2013 http://baja.mcgilleus.ca/ 2012-2013 Drivetrain Designer |
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pj_rohlf12
Milling Master Joined: Jun/09/2014 Location: United States Status: Offline Points: 53 |
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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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UW-Stout Baja VP
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Nate
Welding Master Joined: Jun/16/2014 Location: Ames, IA Status: Offline Points: 285 |
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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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Iowa State University Cyclone Racing 2013 - 2016
Tech Director, DAQ, Media, Manufacturing, Radios 2016 - S&T, Maneuv, & Endurance Driver "Because BAJA" "Because RACECAR" |
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Hitesh arora
Bolt Sorter Joined: May/16/2016 Location: jaipur Status: Offline Points: 17 |
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Can you please upload the excel file or explain what formulas you used for calculating the table?
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RLM
Baja Godfather Joined: Nov/15/2012 Location: Hamilton, ON Status: Offline Points: 1258 |
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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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McMaster Baja Racing (09-Dec 2015)
Team Captain 2012-2015 Suspension Lead 2015-2016 All spoonfeed PM requests will be billed at $10 USD per reply, payable via paypal. |
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Hitesh arora
Bolt Sorter Joined: May/16/2016 Location: jaipur Status: Offline Points: 17 |
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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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Nick914
Organizer Joined: Jun/16/2010 Location: U Laval Status: Offline Points: 256 |
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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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Nicholas Lefebvre
Universite Laval |
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Hitesh arora
Bolt Sorter Joined: May/16/2016 Location: jaipur Status: Offline Points: 17 |
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with software? Please explain what data to check for.
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RLM
Baja Godfather Joined: Nov/15/2012 Location: Hamilton, ON Status: Offline Points: 1258 |
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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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McMaster Baja Racing (09-Dec 2015)
Team Captain 2012-2015 Suspension Lead 2015-2016 All spoonfeed PM requests will be billed at $10 USD per reply, payable via paypal. |
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sujandinesh
Baja Godfather Joined: Dec/27/2013 Location: Enschede Status: Offline Points: 494 |
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How fast is your car? I mean in seconds for a certain run.
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RVCE BAJA: 2011-2015
General Motors: 2015-2016 Tyre Testing - University Racing Eindhoven: 2016-2017 Tyre Dynamics - TASS International: 2017-2018 Tyre Engineer - Apollo Vredestein The Netherlands |
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Nick914
Organizer Joined: Jun/16/2010 Location: U Laval Status: Offline Points: 256 |
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If you read the clutch tuning handbook it's pretty straitforward.
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Nicholas Lefebvre
Universite Laval |
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Hitesh arora
Bolt Sorter Joined: May/16/2016 Location: jaipur Status: Offline Points: 17 |
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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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sujandinesh
Baja Godfather Joined: Dec/27/2013 Location: Enschede Status: Offline Points: 494 |
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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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RVCE BAJA: 2011-2015
General Motors: 2015-2016 Tyre Testing - University Racing Eindhoven: 2016-2017 Tyre Dynamics - TASS International: 2017-2018 Tyre Engineer - Apollo Vredestein The Netherlands |
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Swappy
Double Secret Probation Joined: Jun/18/2017 Location: maharastra. Status: Offline Points: 4 |
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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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Swappy
Double Secret Probation Joined: Jun/18/2017 Location: maharastra. Status: Offline Points: 4 |
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The spur gear design is on the basis of speed or torque ?
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