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Of the Saleen Tesla gear ratio

Of the Saleen Tesla gear ratio

The Steve Saleen-modified Tesla Model S has been on the news lately and it mentions that he modified the gear ratio to 11.39:1. The factory MS is 9.73:1. This means that the Saleen motor has to turn more to travel the same distance. I want to raise a question on this forum as to why would he do this. The torque is flat throughout, so it can't be because there's a sweeter spot at higher RPMs. It also uses more energy so it can't be beneficial to the range. The articles claim that the acceleration is faster, but to me it doesn't add up because the motor is the same power as stock and the power band is constant from zero until maybe ~10000+ where it starts to taper. Mechanical advantage would be better on the Saleen Tesla due to the higher leverage (torque) but that's the opposite of faster acceleration, unless he increased his overall wheel diameter by quiet a bit.

On a separate note, I don't like like what he's done to the styling. It's bordering on garish and to me the stock style is a perfect balance design by Franz von H. I do like Saleen's cooling improvement. What is it exactly that contributes to the heat build-up? Is it the due to the regen mostly? There could be an aftermarket for that but overall I am discounting the Saleen "improvements".

tga | August 21, 2014

Shorter gear ratios = faster acceleration, trading top speed for acceleration. Works in an ICE or an EV; the physics is the same.

EVino | August 21, 2014

I agree that the gearing math is the same but an EV motor and ICE motor are absolutely not the same physics. An EV motor has a constant and linear power band, ICE (terribly) does not, hence the requirement for variable gearing in ICE. A golf cart has better acceleration than a car, for the first two feet, but I wouldn't pick it over the car for its acceleration. In my take, the Saleen mod has given up many advantages.

Fordy | August 21, 2014

Lets look at it the other way. Would you trade acceleration to say 0-60 in 8 seconds to increase your battery range???? Not me, I like the acceleration. But I wouldn't do what Saleen did.

lolachampcar | August 21, 2014

Horsepower accelerates a car. HP = Torque X RPM / 5200 if I remember correctly. More rpm, more horsepower and more acceleration.

Now for the idea of letting a tuner into the drop gears in my MS............. No thank you. I like Steve but I still do not want him splitting the drive unit on my car.

tga | August 21, 2014

An EV motor has a constant and linear power band, ICE (terribly) does not, hence the requirement for variable gearing in ICE.

Yes, the transmission's purpose is to adjust the ICE RPM to keep it operating in its optimum RPM range, and also to keep the engine RPM under redline at high road speeds. If you are accelerating, you want to keep the engine at close to HP peak for maximum acceleration.

I agree that the gearing math is the same but an EV motor and ICE motor are absolutely not the same physics.

If you separate out the purpose of the transmission and final drive, yes, the physics of the final drive ratios is the same regardless of propulsion.

If you have an "engine" (aka 3 ph AC motor) with a very high redline and a wide, flat torque curve that starts near 0 and extends all the way to redline, then you don't need variable gear ratios to keep the engine RPM in the optimal range and under redline.

Now consider 2 "torque sources": an ICE+transmission (as one unit) and an AC motor, both feeding driveshafts. Both can be adjusted to operate at their optimum RPM for maximum torque output. The ICE needs a transmission with variable gear ratios between it and the driveshaft, the AC motor does not.

Now the driveshaft feeds into a differential with some fix gear ratio (final drive ratio). The mechanical advantage of a shorter final drive still improves acceleration in both cases, regardless of the type of torque source (torque is torque, regardless if generated by an ICE, AC motor, or human with a wrench).

Shorter gear ratio == smaller input gear, larger output gear
Taller gear ratio == larger input gear, smaller output gear

Shorter final gears result in the input shaft spinning faster for a given road speed, higher road speed for a given input RPM, but mechanical advantage increase acceleration at the expense of top speed.

Think about riding a bicycle. Consider the gear selection to be "final drive ratio selection". Low gears (shorter final drive ratio) make it easy to accelerate from a stop (but limit your top speed, base on your max pedal RPM). High gears (taller final drive ratio) are much harder to accelerate, but your top speed is higher.

EVino | August 21, 2014

tga,
I totally agree w/ you on the physics of the final drive regardless of propulsion. My argument was different: the physics behind power generation in an EV motor is very different from a combustion engine. This is why we don't need variable transmissions on an electric motor-powered car. Can you argue that we should put one on a Tesla? Say maybe a CVT? Why or why not?

Grinnin'.VA | August 21, 2014

@Fordy | AUGUST 21, 2014

"Lets look at it the other way. Would you trade acceleration to say 0-60 in 8 seconds to increase your battery range????"

Are there any contributors to this forum with enough engineering knowledge to estimate the tradeoff between HP and acceleration, top speed and range if we vary the gear box ratio over the range of 8:1 to 11:1? If so, please share such information with us. Thanks.

I've read many comments on the Tesla forums that claim that there is no such tradeoff, which doesn't make sense to me.

Ron :)

Red Sage ca us | August 21, 2014

It should be a straight mathematical calculation, all things being equal, I'd think...

Olof | August 21, 2014

@JC85: you are right that the torque curve is flat, but the power curve is not flat. You need power to accelerate and the faster turning motor will bring on the power earlier in the Saleen car. Only if you can prevent the wheels from spinning of course.

When the torque tapers off at Autobahn speeds the Saleen car will have to eat the dust of a regular S...

mgboyes | August 21, 2014

The torque curve is flat but it's measured at the motor not the wheels. To get wheel torque you multiply by the reduction ratio, so the Saleen will have considerably more, so it will be able to exert a greater force against the road surface and accelerate faster (traction permitting).

renwo S alset | August 21, 2014

Does any of this matter if you have traction control on?

DonS | August 21, 2014

You can use the 416 HP to calculate force of the tire to the ground. A P85 is traction limited up to about 35 mph, and then is torque limited. Saleen's lower gearing probably gets another 5 mph that is traction limited at the expense of lower top end before hitting the motor redline.

renwo S alset | August 21, 2014

Don. I thought no matter how much power you dumped on the rear tires, traction control would limit it. The stock reduction is more than enough to spin the tires, what would putting more there accomplish?

Red Sage ca us | August 21, 2014

Saleen said they changed the automatic braking system, traction control, and added limited slip differential. So there is bound to be a completely different response at launch. Likely the push in the back will be a bit different when exiting turns on a track as well.

renwo S alset | August 21, 2014

Red. My S doesn't have limited slip differential? Who would have thunk it.

DonS | August 21, 2014

Traction control limits torque because torque is force, i.e. the maximum force the tires can stick to the ground. Power is force x speed. So if power is constant (as limited by motor and inverter current), force goes down as speed goes up.

renwo S alset | August 21, 2014

Don. I'm not an engineer (obviously), but if my S, without traction control on, can spin the tires "til the cows come home, what does lowering the reduction gear accomplish other than I can say I have viola "a lower reduction gear"?

DonS | August 21, 2014

@renwo S alseT
If you turn the traction control off and make a run down the drag strip, by 40 or 45 mph the tires will stick. The 21" tires are pretty sticky so most of the car's weight gets onto the rear tires at launch, meaning you can get close to 0.9g's off the line and hold that acceleration until the car is power limited.

Note that this isn't exactly the same as cruising at 40 or 45 mph and accelerating because the weight transfer is different. Also, just smoking the tires until the speedometer says 45 mph while the car barely moves isn't the same thing either. If anyone wants to try these cases as an experiment, I'd be interested in actual data. I have no problem changing the assumptions in the light of measured data. However, I'm not going to try it because my tires wear fast enough already.

DonS | August 22, 2014

Does anyone have data from a performance data recorder in a drag run? The G-force graph would be definitive information as to what speed the car transitions from traction limited to power limited.

nickjhowe | August 23, 2014

My 2 cents:

Most people commenting above are kinda right, but maybe the explanation gets confusing.

Fundamentally, it is wheel torque, car mass and tire-road friction that matters.

The torque at the wheel is twisting the wheel against the road surface. The higher the torque, the faster the wheel can be accelerated - until the torque is too high and the tires start to slip excessively. Maximum acceleration is achieved when the tires are slipping slightly.

The high weight of the car helps the traction, but the higher the weight the more torque is needed to achieve a given acceleration. Strip all the seats out of the Model S to lose a few hundred pounds and you'd see the 0-60 time go down.

As the car accelerates, we have to look at how the torque varies with increasing RPM. The higher the torque and the higher the RPM, the longer we can keep accelerating strongly.

In the Model S, we have the benefit of a flat max torque curve from zero to 5000 rpm (0-41 mph in the standard S, 0-35.5 mph in the Saleen). After that the torque starts to decrease. This is the exact opposite of an ICE car, where torque is essentially zero at zero RPM, and builds to a max around 4000 RPM, holds flat for a while then tails off.

Power is a measure of torque times RPM as @lola said. ICE owners get fixated about power, but that is mainly because of the severe limitations of the torque curve of an ICE engine.

If you plot torque vs RPM, it is the area under the graph that really matters (plus mass and tire grip)

As has been said, the reduction gear acts as a torque multiplier. The torque at the wheels will therefore be greater in the Saleen car than in a regular Model S. It therefore should accelerate faster than a regular S, at the expense of top end speed. Whereas an S will limit at 133 mph, the Saleen will max out at 113 mph.

The problem - as @DonS said - is that tire-road friction is the limiting factor with Model S. I've had my S at a drag strip and traction control was fighting wheel spin all the way up to 50 mph. Increasing the gear ratio would in theory decrease the 0-60 time because wheel torque would increase, but it would depend very much on the tires being used, the road surface, and - as @Red Sage said - if Saleen have added a limited slip diff and improved the traction control.

Sooooo.....on its own, with stock tires and stock TC, the higher reduction gear probably wouldn't improve acceleration but would hurt top end speed. With wider, stickier tires, and/or better TC, and/or a limited slip diff, 0-60 times would come down - but still at the expense of top end speed.

Makes sense?

Bikezion | August 23, 2014

Has anybody tried some drag slicks on their model S?

procarl | August 23, 2014

@Grinnin'

I, too, am interested in trade-offs, specifically, one in particular: gear ratios compared with range. Why wouldn't a higher ratio gear, one provided by any of several means---i.e.,direct drive, a continuously variable trans system, a fixed ratio gear introduced automatically or thru manual linkages---necessarily provide a longer range? I, and thousands of other current and potential owners I would bet, would gladly trade either or both acceleration and top end for increased range. This is particularly true, I would think, as we look ahead to bolster the mass market attraction of the Model 3.

I raised this question on another thread, but the discussion was soon enveloped by talk of acceleration, fixed torque, top speed, etc, etc. Likewise the current thread---all relevant topics, of course. But. . .why isn't increased range (albeit a trade-off loss of acceleration and top end) an easy by-product of higher gear ratios?

Bikezion | August 23, 2014
lvaneveld | August 23, 2014

@procarl
Because the efficiency of Tesla's electric motor is essentially constant through most of the RPM range, range will not be significantly affected by any halfway reasonable gear change including this one.

The reason it helps with ICE engines is they typically have efficiencies that are strongly dependent upon engine RPM and they have the best efficiency at low RPM, well below their peak power band. That is why they use high top gears to drop cruising RPM as low as 2000 or less. That also explains why many ICE cars cannot even achieve their top speed in top gear, not enough torque at the drive wheels because the engine RPMs are too low.

lvaneveld | August 23, 2014

By the way in the interview I saw they said it should have better range because they used a more efficient gear design. I would expect this to have relatively little effect.

They might be using straight cut gears which I believe are slightly more efficient but might also create more noise, kind of a high pitched gear howl if I recall right. Not at all sure a out the gear stuff, I am not a gear design or machine design guy.

Bikezion | August 23, 2014

Ivaneveld, I believe you are right, look at the pic of the gears in the link I posted, definitely straight cut! No wonder they mention the sound! Those will whine like crazy.

carlk | August 23, 2014

@mgboyes's explanation makes sense.

Red Sage ca us | August 23, 2014

nickjhowe: +1 UP! Precisely. These are some videos that cover the Saleen FourSixteen, and give details on what they actually did to the car:

Saleen Tesla Car with Steve Saleen and William Wieting Pebble Beach Concours d'Elegance (Part 1, 9:34)

4sixteen Model S Tesla SALEEN Launch Steve Saleen William Wieting MustangMedic (Part 2, 7:58)

procarl wondered, "I, too, am interested in trade-offs, specifically, one in particular: gear ratios compared with range. Why wouldn't a higher ratio gear, one provided by any of several means---i.e.,direct drive, a continuously variable trans system, a fixed ratio gear introduced automatically or thru manual linkages---necessarily provide a longer range?"

Hmmm... OK, it isn't about the gear ratio in an electric car. It is about power, or more specifically, energy consumption. The means by which you consume the available stored energy determines your range. You must understand the difference between how this happens with an electric car, as opposed to an ICE.

Many performance ICE cars have a gear I that is good up to 60 MPH. If you floor it to sixty, over and over and over again in traffic, your stored energy, the gasoline in the tank, will get used rather quickly, especially if you don't switch to another gear. However, the fuel will last longer, even if you leave it in gear I all the way up to 60, if you aren't dropping a lead boot on the the accelerator from a dead stop, and instead have slow, controlled acceleration to that speed point. Leaving the car in gear I with cruise control locked at 60 MPH, will use the fuel more quickly than it would in one of the overdrive gears, so your range would be reduced significantly, though the car is capable of maintaining the speed.

With an electric car, the assigned gear ratio relates only to the top speed you can attain and how quickly you accelerate. It has no particular adverse effect on the consumption of energy because you get 100% torque at 0 revolutions per minute no matter how quickly it spins up. Also, it only turns because the motor tells it to.

The control of the flow of energy is from the inverter, between the battery and the motor, not the drive reduction gear, between the motor and the wheels. You control the inverter directly with your right foot on the accelerator. Since that is typically a 'drive by wire' system on an electric car, a specific rate of energy release is tied directly to the position of the accelerator. So rather than adding complexity in a physical mechanism, it is more beneficial to modify the software that directly effects the ratio of energy applied at different accelerator positions.

That is why there is a software limitation on the top speed of most EVs that cuts in well before the physical limits of the drivetrain. By limiting the top speed of a car you manage two issues at once. First, you know the maximum amount of energy leaving the battery at any given point, so you can predict its discharge more easily. Secondly, you limit exposure to the forces of drag and without a higher top speed you know the aerodynamics of the vehicle do not adversely effect range.

Once again, no matter the vehicle, flooring the accelerator uses energy more quickly than slow, steady, gentle application of power. Being able to accelerate quickly in an electric car does not forego the ability to drive economically. Given the same amount of energy, an electric vehicle traveling 40 MPH will go further than one moving at 60 MPH, which will go further than one speeding at 120 MPH. Changing the gear ratio will not effect the distance traveled, with a given amount of energy, in either case. All the change in gear ratio can affect is a change in the level of enjoyment of the journey.

nickjhowe | August 24, 2014

@procarl - short answer - no.

ICE engine efficiency ranges from 15-35% over the rev range and under different load conditions, therefore ensuring the engine is at the optimum point on the efficiency map (BSFC chart) is paramount to energy consumption and therefore range.

The Model S motor is roughly 95% efficient over the majority of the RPM range and therefore as @lvaneveld says it doesn't matter what RPM the motor is at - unless it is at the extremes of the rev range. Changing the gear ratio so that the motor is at 9000 rpm or 11000 rpm at 80 mph instead of 10000 rpm would have zero impact on energy use and therefore zero impact on range.

But it would affect the acceleration at speed, and the maximum speed.

DAB | August 24, 2014

Interesting write-up from Fred over on Reddit: http://www.reddit.com/r/teslamotors/comments/2egltj/is_saleen_lying_in_o...

"Is Saleen lying in order to sell its version of the Tesla Model S?"

renwo S alset | August 24, 2014

From the beginning the Saleen mods didn't pass the smell test.

procarl | August 24, 2014

Thanks for the explanations.

EVino | August 24, 2014

I stand corrected on my initial post. As tga said, the Saleen mod has shorter gearing so it's a mechanical fact that it will have faster acceleration. The difference may not be significant. There may be some good ideas to get from the Saleen method, such as improved cooling and the limited slip diff. I'm curious how he got to the software API in order to do the software mods as he mentioned in the interview. I have friends with a Shelby 427 kit car and they are proud of their software that allows them to tweak everything from volumetric efficiency (fuel/air ratio) to suspension settings.

What Saleen did is open the possibility that the Tesla car is really a modular platform, where there could be a potentially huge market for intercoolers, gearing, suspension, software tuning, motors, etc. We just have to come up with a new term for hot-rodding.

Red Sage ca us | August 24, 2014

Hot-swapping...?

Brian H | August 25, 2014

Hot-wiring?

Grinnin'.VA | August 25, 2014

@procarl | AUGUST 23, 2014
"@Grinnin'"

"I, too, am interested in trade-offs, specifically, one in particular: gear ratios compared with range. Why wouldn't a higher ratio gear, one provided by any of several means---i.e.,direct drive, a continuously variable trans system, a fixed ratio gear introduced automatically or thru manual linkages---necessarily provide a longer range? I, and thousands of other current and potential owners I would bet, would gladly trade either or both acceleration and top end for increased range. This is particularly true, I would think, as we look ahead to bolster the mass market attraction of the Model 3."

Several posts on Tesla's forums claim that changing the gear ration would have zero, meaning absolutely no, affect on the efficiency and range of an MS for any given situation and driver use of accelerator and brakes. They claim that only the acceleration and top speed would change for a different ratio. I suspect this isn't quite correct.

Here's where I am in my attempt to figure this out. Assuming basic physical principles, I've made a spreadsheet to calculate/estimate the change in acceleration and top speed that would be caused by a given change in gear ratio. This simplified model assumes:

* Constant torque for the motor at all rpms
* No change in the efficiency of the gear box
* Acceleration formula: force = mass X acceleration
* Air resistance proportional to the square of the velocity

The inputs for the spreadsheet are:

* Mass/weight of car and driver: 4800 pounds
* Initial acceleration (0-1 mph) of the MS
* Maximum speed of MS without software limitation.
* Gear ration of MS = 9.73:1
* Alternate gear ratio = 7, 8, 9 or whatever you wish.

Preliminary results based on SWAG inputs:

Max Speed & Acceleration Times:
Ver. Gear Ratio Max Speed 0-30 mph 0-60 mph 0-90 mph
MS 9.73 130 mph 2.55 s 5.41 s 9.23 s
Alt 7.0 110 mph 3.56 s 7.79 s 14.63 s
Alt 8.0 118 mph 3.11 s 6.71 s 12.00 s
Alt 9.0 125 mph 2.76 s 5.89 s 10.22 s
Alt 10.0 132 mph 2.48 s 5.41 s 8.92 s

Besides inaccurate inputs, these results suffer from a couple of problems:
1. At low speeds traction control limits the application of torque to prevent wheel spinning. The difficulty is that this varies for different tires. Of course, given the same tires there should be no change in acceleration for different gear ratios until the car reaches a speed which allows the full application of torque to accelerate.
2. The torque is assumed to be the same at all speeds. Reportedly the MS motor's torque starts dropping off when it rotates faster than 5000 rpm. This is estimated/calculated by someone at speeds faster than 41 mph. If the reported dropoff is true, the corresponding rpm limit for maximum torque should vary inversely with gear ratio. Consequently, for a gear ratio of 7, the speed limit for maximum torque would be 41 X 9.73 / 7 = 57 mph.

Questions:
* Does anyone know the maximum speed for an MS for spinning its tires for any particular type of tire and operating environment?
* Can someone confirm the rpm and speed limits before torque dropoff?
* Does anyone have information about the torque curve vs rpm? Or how fast does the torque curve drop off at high rpms?

None of the above deals with the core issue: How efficient is the MS drive train as the car accelerates? Or is the efficiency really flat? Or nearly flat? I think I saw a curve on how the efficiency of the motor varies as the current varies. But I don't know how to apply that data to the core issue. Can someone tell us how the current flow is related to the torque being used to accelerate the car under various conditions?

Thanks for whatever technical insight you may have to contribute toward clarifying this technical puzzle.

Ron :)

Haggy | August 25, 2014

@fordy,

Would you trade acceleration to say 0-60 in 8 seconds to increase your battery range????

Lots of people would. I don't know how many of them would buy a Model S in the first place, but if you owned a Subaru, you might comment on how peppy that car is that can make it from 0-60 in 8 seconds.

Range anxiety is a big issue, and most people don't have cars that come close to the performance of an MS. Eight seconds is quite respectable for a typical car.

So all this might not be very relevant, at least beyond the theoretical, for current owners. But it should be a real world concern for Tesla, given that at some point they will want to go after that market segment too. I'm hoping it will be a moot point by then and free charging stations will be widespread and convenient enough that people won't care.

Grinnin'.VA | August 25, 2014

@Haggy | AUGUST 25, 2014

"I'm hoping it will be a moot point by then and free charging stations will be widespread and convenient enough that people won't care."

I share your hope that Tesla will go after the mass auto market soon.
However, I expect "free charging stations will be widespread and convenient enough that people won't care" to happen at approximately the same time as "free beer being available convenient enough."

Ron :)

P.S. Listen to the economists who say: "There is no such thing as a 'free' lunch".

nickjhowe | August 25, 2014

@Grinnin' - I thought I'd covered many of your questions in my earlier post.

A couple of specific answers. The following graph is speculation on my part, but I think it is pretty accurate:

This graph shows the motor torque. It is the official numbers from Tesla. I've included a Porsche for comparison.

Both graphs are taken from my book Owning Model S

Haggy | August 25, 2014

By "free" I really mean "included in the purchase price." It's not free now and won't be free in the future, but "free" has become such a marketing buzzword that people gladly pay for things as long as they are free.

Red Sage ca us | August 25, 2014

I will have to research this a bit to make sure, but I think something is being overlooked here. The Tesla Model S uses a Reduction Gear, so the wheels turn slower than the motor by a factor of 9.73:1... ICE vehicles use a transmission/transaxle that has an Amplification Gear as the Final Drive Ratio, meaning that the drive shaft/differential spin faster than the motor. That is why Saleen's modified drivetrain uses a larger number 11.39:1 to achieve faster acceleration. In an ICE faster acceleration is achieved with a lower factor. At least, that's how I have always understood it. I could certainly be mistaken, of course...

mgboyes | August 25, 2014

Yes you're mistaken, sorry.

With no gearing at all, at 6000rpm on Model S sized wheels you'd be doing about 500mph.

All cars have reduction gears. ICE cars just have really complicated compound gearsets that these days mean up to 9 different amounts of reduction.

Since we're talking about Porsches look at http://press.porsche.com/vehicles/2013/2013_911_Technical_Specifications... which says that 1st gear is 13.4:1 overall and 7th is 2.44:1.

2050project | August 25, 2014

Steve Saleen was interviewed on Bloomberg providing some more info on the "Foursixteen" today:
http://www.bloomberg.com/video/the-tricked-out-150k-tesla-kg58Hyw9RT~qJQ...

Red Sage ca us | August 25, 2014

Cool. Good interview!

tencoaster | November 23, 2019

TOURQUE! 100% torque instantly at 0 to the first moment of electric motor rotation....That is good, BUT. Let me make this easy. POWER gets work done. Quick acceleration takes a lot of POWER, or HP (horse power). The hp formula is torque times distance divided by time. The distance thing is what few people understand. We use 1 foot in the formula. Foot pound, remember. That foot of distance is the distance the electric motor rotor is moving at its outer edge, or largest diameter. At 1 rev per minute or rpm, you would have almost zero hp. So Tesla engineers set the ratio at 9.73 revolutions of the motor to 1 rpm of the axle shaft. That gives the motor more distance in the formula. Therefore more HP at low RPMs. An electric motor produces power the same way an ICE engine does. No magic juice here!