2 Speed Gearbox

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fuzzy-hair-man
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Post by fuzzy-hair-man » Wed, 20 May 2009, 18:10

AMPrentice wrote: Do others agree that 2nd and 4th are the best gears for most setups?
eg. 2:1 and 1:1?
It's going to depend isn't it? Image

which motor?
peak torque?
desired top speed?
what diff ratio?
what RPM gives the best efficiency? what is going to be the 'cruising' or average speed?
desired performance, acceleration or top speed?
low speed acceleration or high speed acceleration? (or with 2 gears both)
what peak RPM do you want / can you run?
Are there big hills or steep driveways around your place?

I think a more useful guide would be what is the value (in terms of equivalent gear ratios) of having peak torque available from 0 rpm? if you could definatively say having peak torque available from zero revs means you don't need first gear (assuming the E motor has equivalent peak torque*) then this would give a good starting point, people could map what speed that would get them at peak RPM and or peak power and if they want to go faster or be able to accelerate quicker (at higher speeds) then they need another gear, pick the gear that gives reasonable power at this point but can continue on...

Or if you can't or don't want to change on the fly (whilst moving) pick a high range that allows you to takeoff but also reach the desired top speed, although acceleration might suffer.

Or you could pick the 2nd gear so that at the selected cruising speed the motor is at peak efficiency.

* if your E motor had 2 x peak ICE torque and 2nd was 2:1 you might get away with 1:1, or adjust similarly for whatever E motor you select.

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Post by Squiggles » Wed, 20 May 2009, 18:44

Assuming you have a motor that runs to 5000rpm and optimum revs is around 3000rpm you probably want overall ratios of around 5.5:1 and 4:1.

So with a 4:1 diff ratio, you might want gear ratios of 1:1 and 1.375:1.

Overall you would end up with a "town" gear for speeds up to 80kph and a "motorway" gear for speeds up to 110kph.

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Post by Johny » Wed, 20 May 2009, 18:53

With my soon-to-be AC system running in delta (high speed mode) and a current limited (under-powered) controller and worst case weight (lead) I get 0-60kh in 12 seconds. With a 2:1 gear ratio it brings it down to 6.8 seconds. Gradient climb at 40k/h goes from 1 in 7.3 (about 14 degrees) to 1 in 4 or 25 degrees.
Any greater ratio would not be useful - maybe a fraction less, but 2:1 would be about right.

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Post by Squiggles » Wed, 20 May 2009, 19:04

Johny wrote: With my soon-to-be AC system running in delta (high speed mode) and a current limited (under-powered) controller and worst case weight (lead) I get 0-60kh in 12 seconds. With a 2:1 gear ratio it brings it down to 6.8 seconds. Gradient climb at 40k/h goes from 1 in 7.3 (about 14 degrees) to 1 in 4 or 25 degrees.
Any greater ratio would not be useful - maybe a fraction less, but 2:1 would be about right.


What is you diff reduction ratio?

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Post by Johny » Wed, 20 May 2009, 19:11

Diff is 3.89:1. Motor torque drop-off (in the configuration I gave) starts at just under 3000RPM. Due to controller limitation in this config. I only get about 170NM of torque up to 3000RPM.
I have scope for increasing torque etc. but the gearbox is interesting even with more torque available.

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Post by fuzzy-hair-man » Wed, 20 May 2009, 19:33

Johny wrote:Gradient climb at 40k/h goes from 1 in 7.3 (about 14 degrees) to 1 in 4 or 25 degrees.
Any greater ratio would not be useful - maybe a fraction less, but 2:1 would be about right.
Don't you mean percentage grade rather than degrees?

1 in 7.3 = 0.136 = ~14% grade

rather than atan(1/7.3) = 7.8 degrees

1/4 = 0.25 or 25% grade

atan(1/4) = ~14 degrees

Remembering high school geometry so I could have it all wrong Image

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Post by Johny » Wed, 20 May 2009, 19:44

Sorry - my mistake, brain in neutral - didn't even read it off the speadsheet properly...

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Post by fuzzy-hair-man » Wed, 20 May 2009, 21:53

Johny wrote: Sorry - my mistake, brain in neutral - didn't even read it off the speadsheet properly...


Actually this got me thinking and cleared up some confusion I was having:

In another thread ACMotor said:
acmotor wrote: If you can climb 15deg (slowly) that will cover most situations.
If you can hold 60kmph on 10 deg you are doing fine.
If you can accelerate from any speed on 5 deg that is good.
What is Black Mountain, 10deg ?

In direct drive I find I need 300+Nm to do a reasonable acceleration take-off up a 10deg slope, but only half that to just move off.
1100kg EV, 6.5:1 to rear 15" wheels.
So I went pulling numbers into my spreadsheet and I was getting remarkably good speeds, (which I didn't believe) the numbers I was plugging in were percentage grades but I hadn't read the post carefully enough and noticed the degrees Image it makes a substantial difference Image

Revised numbers suggest my mini would be able to climb a 15 degree, 26.7% slope at something under 10km/hr...
Maybe I need a 2 speed gearbox Image

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Post by Johny » Wed, 20 May 2009, 22:46

I must admit your post made me revisit Path Profiler feeling a little sick that I'd confused percent gradient with degrees (and already paid for my custom motor). Sigh of relief when a saw that PP shows percent grade.

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Post by Thalass » Thu, 21 May 2009, 01:21

So perhaps for off-road fun I would need a low range gearbox (or two, as the case would be). 45 degrees isn't uncommon, and from a stationary start, too.
I'll drive an electric vehicle one day.

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Post by EV2Go » Thu, 21 May 2009, 15:27

I have an idea to float past you all...

Been thinking about this one for a little while now. As we all know automatic transmissions typically consume far more power to drive than a manual gearbox. In the past that was primarily due to the very inefficient stall / torque (call it what you will) convertor. Prior to the introduction of lockup converters (circa 1980) there was always some heavy form of mechanical loss due to the fluid lock up design that made the stall convertor possible.

As I say circa 1980 some bright spark came up with the idea of including an electronic solenoid into the stally that allowed the convertor to be mechanically locked up in top gear significantly improving the efficiency / millage that was achievable with automatic transmissions.

Some of you can probably see where I am going with this... In all gears below top gear there was a mechanical loss through this fluid coupling that had to exist in order for the petrol engine to increase revs to the point where it made decent torque to take off.

Electric motors don’t require stall or slip to take off because of the excellent 0 RPM torque, so what if instead of having a heavy ATF filled torque convertor sitting between the motor and the transmission, you substituted / modified the transmission input shaft to a solid coupling to the motor output...

The only problem that is left to be pondered is the ATF (Automatic Transmission Fluid) pump. The main reason why automatic cars can’t be towed is because they need the motor running to turn the ATF pump in the front of the transmission, there are two tangs on the torque convertor that lock into the pump and turn it when the motor is running.

So why not just put the tangs on the motor output shaft? Because when you take you foot the accelerator off going down the hill the electric motor doesn’t idle like a petrol engine. I am in two minds about the next step...

Theory one: because the electric motor would be directly hooked up to the transmission, even though there was no power being applied to the motor, the main drive shaft would still turn and in turn rotate the ATF pump.

Theory two: some kind of electric pump to substitute the mechanical drive provided by the torque convertor.

Ok so doesn’t this go against what I have said in the past about re-using inappropriate already available parts? Not really because it massively improves the efficiency of the transmission... Instead of only having mechanical lock up on top gear, it has direct engagement like a manual gearbox at all times.

It also removes a massive amount of weight from a fluid filled torque convertor. It doesn’t require a flywheel or flexplate attached to the back of the motor to attach the torque convertor that isn’t there. The biggest plus of all smooth gear changes without a clutch!

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Post by coulomb » Thu, 21 May 2009, 15:49

I'll restate my idea on this, since we seem to be going close yet avoiding it. The torque converter itself is supposed to provide up to about 2:1 torque multiplication, and fades to no torque multiplication at all (i.e. straight through). That sounds pretty much what we want.

So rather than toss out the torque converter and fiddle with an automatic transmission that doesn't understand e-motors anyway, toss the auto transmission and all its weight, and keep just the torque converter.

That leaves the efficiency consideration. But we agree that the lock-up versions are efficient, and can be controlled electrically.

So... use the torque converter only when climbing driveways or other times with extreme torque demands perhaps?

Or... use it only for takeoffs? This is less satisfactory, because in city driving, you would be using the inefficient part of the converter a fair bit. But maybe the weight, simplicity, and space savings make it worthwhile. (Use the extra space and weight for more battery). Then again, modern oil cars use auto transmissions, and don't seem to take a massive fuel economy hit as a result. Or do they?

Maybe someone can kill this idea with a single well placed blow, and I can get rid of it out of my head.
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Post by Johny » Thu, 21 May 2009, 15:55

Thalass wrote: So perhaps for off-road fun I would need a low range gearbox (or two, as the case would be). 45 degrees isn't uncommon, and from a stationary start, too.
I think that if your are planning on these kind of inclines you would be better keeping a full 4 or 5 speed gearbox. The 2 speed doesn't seem to fit your requirements.

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Post by Squiggles » Thu, 21 May 2009, 16:19

This is for the FWD converters among us
Why don't we just pool resources and;
1. Select a common existing easily obtained transaxle/gearbox that we can use the diff and synchromesh parts out of.
2. Design a two ratio box, with two ratios that are generally usable and I guess with a reverse.
3. Get the new casing (without huge bell housing) cast in asia.
3a. A simple adapter plate could be used to match various motors
4. Get the new input shaft and gears manufactured....somewhere.

The gear change can be solenoid operated, meaning a smart controller could auto change up at say 75kph and down at 40kph

End result easier conversion for all in future! One less perplexing decision to make. Much lighter gearbox, weight saved is a good thing.
I have a relative who used to design gearboxes at BorgWarner, I could get an opinion from him.

Come on rip my idea to pieces :)

edit: there is a chance some of the gears (at least one) off the output shaft could be used a well.

edit: as an example there is an Accent 1600cc box (maybe not common) that has a 4.06:1 final drive and 3rd gear 1.37:1, 4th gear 1.03:1 meaning that with on 175/65 14 tyres you are going to get approx 57 & 76 kph at 3000rpm and 86 & 114 kph at 4500 rpm.
Last edited by Squiggles on Thu, 21 May 2009, 06:54, edited 1 time in total.

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Post by EV2Go » Thu, 21 May 2009, 16:32

Coulomb It sounds like you want to throw the baby out with the bath water.

The torque convertor is really the weak link in the an otherwise good system. It is designed to be inefficient (slip) so the revs can be brought up independent of the gearbox speed to take off. If you are chasing torque multiplication it would be far easier to just use lower diff gears at no additional mechanical loss.

The only real reason for the torque convertors existence is to get the motor into a sweet spot for the petrol engine, since the e-motor produces maximum torque at 0 revs you don’t really need torque multiplication. The other thing is the auto gears are comparable to the manual counterpart so a transmission without a torque convertor would be like drive a manual.

Squiggles that doesn't sound that absurb of an idea Image

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Post by fuzzy-hair-man » Thu, 21 May 2009, 18:18

coulomb wrote: So rather than toss out the torque converter and fiddle with an automatic transmission that doesn't understand e-motors anyway, toss the auto transmission and all its weight, and keep just the torque converter.

That leaves the efficiency consideration. But we agree that the lock-up versions are efficient, and can be controlled electrically.

So... use the torque converter only when climbing driveways or other times with extreme torque demands perhaps?

Or... use it only for takeoffs? This is less satisfactory, because in city driving, you would be using the inefficient part of the converter a fair bit. But maybe the weight, simplicity, and space savings make it worthwhile. (Use the extra space and weight for more battery). Then again, modern oil cars use auto transmissions, and don't seem to take a massive fuel economy hit as a result. Or do they?

Maybe someone can kill this idea with a single well placed blow, and I can get rid of it out of my head.
That sounds like a good idea to me, I've often wondered whether my spreadsheet mini would climb all the hills I want it to and what grades I'd really find in say peoples driveways or the odd really steep hill, having the system you suggest could mean I can go direct drive and select the diff ratio that performs best for the majority of the driving I expect to do, on the odd occasion I do find something steeper that I'm not able to get up I can unlock the torque converter and use it to climb the hill and immediately lock it back up again. If the torque converter doesn't have any losses whilst locked up this system in normal use would be more efficient that a 2 speed gearbox IMO.

The ability to to choose optimum gear ratios without having to accommodate the occasional steep hill might allow your EV to achieve even higher efficiencies or performance in the general case. For example my spreadsheet tells me that a 9:1 diff ratio gets me to 100km in 22sec, a 5.2:1 gets me to 100km in 9.7 secs. Without 'hill climb' mode Image I might be forced to select a diff ratio of closer to 9:1 and live with the slower 0 - 100km time.

BTW it is slower with a 9:1 diff because the revs get above the point where they produce peak power and power starts to drop off, acceleration to 60km/hr is better with the 9:1 diff.

Great thinking!

EDIT:
I did some more thinking, how does a torque converter handle having to work in a not locked up (not the electronic lock up) state for a reasonable period of time? ie while you climbed the hill. It would seem if the torque converter was made to perform a similar role in an auto gearbox as the clutch does in a manual it wouldn't spend much of it's time in the non locked state, and perhaps asking it to climb a hill would be asking a bit much? Mind you it might only need to work for the takeoff up the hill and then it could lockup again. Image

Right I'm off to find out how torque converters do their thing. Image
Last edited by fuzzy-hair-man on Thu, 21 May 2009, 08:32, edited 1 time in total.

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Post by Johny » Thu, 21 May 2009, 18:52

This tends to put one off Torque Converters. Read the "Capacity and failure modes" section as well. Am I being too harsh?
Wikipedia on torque converters
BTW The Torque Converter on my Vogue was the only thing that failed that put me off the road in such a way that I couldn't be the one that fixed it - don't like 'em.

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Post by woody » Thu, 21 May 2009, 19:08

Wikipedia wrote:resulting in the violent dispersal of hot oil and metal fragments over a wide area.
Image
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Post by fuzzy-hair-man » Thu, 21 May 2009, 21:10

woody wrote:
Wikipedia wrote:resulting in the violent dispersal of hot oil and metal fragments over a wide area.
Image
Certainly sound like they can go out in a blaze of glory...Image

So long periods of slippage could be a problem...Image

Looking around at the HowStuffWorks Article and the links torque converters seem to be used pretty heavily by drag racers, those guys tend to break stuff and engineer things pretty close to the bone, I'm wondering if some of that bit on breakages comes from these sorts of applications, I don't really hear of torque converters breaking regularly, but then why would I? Image either way they are still used in autos all over the place so they must be pretty reliable. I guess it's a matter of if they are reliable for the sort of use we might want them for? at a guess an EV might also have more cooling available, they wouldn't have to be inside a bell housing or attached to a gearbox and ICE making heat.

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Post by fuzzy-hair-man » Thu, 21 May 2009, 21:38

It seems we have been down the torque converter road before:
Previous topic on manual vrs auto gearboxes and torque converters

EDIT: grammar and:

The idea was more to get off the line and not as a second gear but still...
Last edited by fuzzy-hair-man on Thu, 21 May 2009, 11:44, edited 1 time in total.

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Post by antiscab » Thu, 21 May 2009, 22:12

EV2Go wrote: The biggest plus of all smooth gear changes without a clutch!


Rob Mason had a Borg Warner TH-40 in his mustang for a while (minus the torque converter). Take offs were brisk, but i wouldn't say smooth. The shifting between gears wasn't particularly smooth either.

The torque converter, as well as doing torque multiplication, also "averages" out the torque either side. so when you shift up, the momentum in the motor gets depleted (due to the rpm change) over a second or two, rather than in ms. without the torque convertor, it does take ms, and u get a bit of a lurch.

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Post by Johny » Thu, 21 May 2009, 22:26

Really good point antiscab.
That would explain why torque converter lock-out is always disengaged BEFORE the gearbox changes down a gear. I hadn't thought about it before but had always been in awe that my 40 year old Borg Warner 35 did such smooth gear-changes - the torque converter absorbs the "thump".
BTW. My donor is an auto - that's why I'm going down the Direct Drive path.

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Post by EV2Go » Thu, 21 May 2009, 23:24

The TH400 (Turbo Hydromantic) is an incredibly strong box, during my drag racing days we had one stuck behind a 1200HP Top Alcohol Door Slammer. I can vouch from firsthand experience when the convertor let go they create heat like you would not believe. My hands were literally cooked through a pair heat proof gloves, and even after I whipped the gloves off I could still feel my hands burning for about 10 minutes after.

The TH400 is not what you would call a smooth shifting box at the best of times, but you would be flat out finding a transmission with more optional parts. You can buy from mild (stock OEM replacement parts) to wild (billet parts and full manual valve bodies) where the box won’t change unless you manually shift gears.

I had a TH400 in both a HQ Statesman and a HQ 2 door Monaro, if you put a shift kit in them it sometimes can actually improve the shift quality by taking less time to change gears, but your right about the torque convertor effect it basically acts like a big fluid spring between gear changes.

Again if even if you did a direct hook up to the motor output shaft you could buffer some of that harshness with a rubber coupling between them much like an early mini uni joint.

BTW the electronic lockup was only ever used on top gear for a reason... it is basically just a roll pin that locks it up and can’t take large amounts of torque, so subjecting it to loads like a gear change could break the convertor real easy. Once you reach a certain performance point you have to revert back to a non a lockup convertor.

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Post by gttool » Fri, 22 May 2009, 02:03

I am willing to do some development and mods to do an auto box If someone wants to try it
I beleve that the roughness of changes could be reduced by reducing the line pressure to the clutches ??

with the pump either add an accumulator to store pressure whilst the vehicle is stationary?

or an external pump to keep pressure up

why not use a sprung centre on the coupling ?


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Post by Squiggles » Fri, 22 May 2009, 02:20

Are clutchless gear changes really a problem when using an electric motor?
I always thought that the shock springs in a clutch plate were there because of the impulse nature of the ICE power, where as the electric motor has a much smoother characteristic, add to that the virtual lack of mechanical resistance when in power off mode.

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