I think this is a physics question

[Sonoran_Desert]

I drive up a .25 mile 35% grade paved road to get to my home everyday. I can ascend in S1 at 1.5 - 2K rpms going really slow. No transmissions shifts.

Alternatively, I can drive up in S3, get a running start, get transmissions shifts out of the way before I hit the steep grade at same 2K rpms and go much faster.

Assuming there are no shifts on the transmission during the steep grade climb with both alternatives, which is easier on the vehicle? It "feels" like the velocity at the faster speed in S3 is placing less force on the frame and engine.

 

[ShermanOlsen]

I think you should send a note to Guinness to claim the title of world’s steepest street. World's steepest street: Harlech loses title to New Zealand - BBC News. But I don’t think a running start, with an automatic, is particularly stressful either way.


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[Jetboy]

If you're maintaining the same speed in both cases - meaning you stay at a steady speed all the way up - it takes more power to climb the hill faster. Therefore, the engine will be producing more power and applying more power to the input of the transmission the faster you go.

As far as torque applied to various parts of the transmission - that would take a lot of calculations and you'd have to do that to try to figure out which one is better/worse. For example if the first scenario required 40hp, but was turning a shaft internally at 2000rpm, then the torque on that shaft would be 105 ft-lbs. If the second scenario requires 80hp, but the internal shaft is spinning at 7000rpms, then that shaft will experience a torque of only 60 ft lbs. So it would have less torque applied while higher horse power.

So - the point there is that it's possible to have a transmission apply more power with less force on an internal part. BUT that's pretty rare in most cases. Normally most parts rotate slower than or equal to the input shaft speed (except in overdrive, but that's not really meaningful here). But I think the general rule in most cases the more power transferred is normally going to translate to more heat, more force on the input parts, and more wear. The least wear is probably to speed up on the flat ground slowly, and then use the momentum and drive up as the vehicle slows down.

If your elevation gain is 400 feet in 1/4 mile - ballpark numbers, if you're going to climb the hill at 30mph, it'll require an extra 120hp over flat ground. If you're going to go 10mph, it'll take an extra 40hp. I think you could assume that it's likely that pushing another 120hp through the transmission will probably cause more wear than 40hp. That's not a certainty, but I think in this case without knowing a lot more about the transmission internals to tell me something that would suggest otherwise from what I would normally guess to be the case, I'd say going slower is easier on the transmission than going fast.

 

[Mudhorn]

For example if the first scenario required 40hp, but was turning a shaft internally at 2000rpm, then the torque on that shaft would be 105 ft-lbs. If the second scenario requires 80hp, but the internal shaft is spinning at 7000rpms, then that shaft will experience a torque of only 60 ft lbs.

What he said.

You can start driving up the vertical driveway and then switch over to S to see which gear the 4Runner is in. I haven’t messed with it too much, so not sure if the 4Runner works the same way as other brands.

 

[Sonoran_Desert]

[MENTION=99501]Sherman[/MENTION]Oslen - When they re-paved the street a few years ago, 35% grade was what the asphalt company quoted but not sure how accurate that was. They had to redo the job several times as asphalt kept traveling down the slope.
[MENTION=118321]Jetboy[/MENTION] - thanks for the calculations and a better understanding of how to think about hp vs. torque vs force. I read somewhere that eliminating transmission shifts during a steep grade was worth the effort. With the going faster method, I don't always manage to avoid an auto-shift, so locking in at S1 and going slow might be the way. It will be interesting to see what happens to this vehicle over the years with this daily challenge. All my other driving is easy and low wear and tear.

@OTOYA - I think 4Runner always defaults to S5 when shifting from D but need to test.

 

[epjfsa]

Here's how you get your answer: put it in auto and see what the transmission map decides. This is the best gear to be in, because instead of going by feel which is subjective, they used objective engineering judgement, sensors, math, modeling, etc. to develop transmission maps.

 

[Mudhorn]

I think 4Runner always defaults to S5 when shifting from D but need to test.

I think you are right. Played around with it today and it always showed S5

 

[Sonoran_Desert]

Here's how you get your answer: put it in auto and see what the transmission map decides. This is the best gear to be in, because instead of going by feel which is subjective, they used objective engineering judgement, sensors, math, modeling, etc. to develop transmission maps.

Another good point of info, seeing what the vehicle does on its own. The transmission went to S2, then S3 for a brief relief in the grade and then back to S2 to get to the top.

 

[<<<<]

Rolling start even creeping rolling start is hell a lot easier on your drivetrain than any start from dead stop most races start from rolling mostly for safety reasons but also easier on trans and engine. All dragsters will try to catch green light while inching forward. As for acceleration I keep my brisk to get best gas milege. Ironically with my 4runner I found out it gets worse mpg while granny driving (hypermiling and slow accelerating).

 

[Sonoran_Desert]

Rolling start even creeping rolling start is hell a lot easier on your drivetrain than any start from dead stop most races start from rolling mostly for safety reasons but also easier on trans and engine. All dragsters will try to catch green light while inching forward. As for acceleration I keep my brisk to get best gas milege. Ironically with my 4runner I found out it gets worse mpg while granny driving (hypermiling and slow accelerating).

I guess it makes sense that momentum gained from higher speeds with such a heavy vehicle might be better on gas.

Thanks for the heads-up on avoiding heavier loads/ steep grades from a dead stop. I unfortunately have another very steep grade in front of my house and 4Runner was going from a dead stop to climb a steep hill after reversing out of my driveway. A few months ago I finally started backing into garage so that I get about 25 feet of roll down my level driveway before a hard right turn up the steep hill while never stopping. That it took me two years to figure out the benefits of backing into the garage was totally lame on my part.

 

[MTN4RNR]

My take is that running the motor at higher RPM won't affect the motor really in any noticeable way, provided there are no cooling issues. Starting in 3rd and letting the transmission work its way down in gears as you go slower at the top will make the transmission and the converter work harder and build more heat.

Generally, I'll err towards the side of keeping the transmission cooler. If I were you (and the transmission can't hold 3rd or 2nd gears without downshifting) I would keep it in 1st and keep your revs up so you're not crawling up the hill. This will keep the converter locked and transmission temperatures down.

 

[Kamalani Mahie]

After reading through this thread, I don't need to add any more math or engineering to this, and while it's true that in certain circumstances faster might be better, most of the time slower is better. MPH. And RPM to a certain extent.

 

[Space King]

I drive up a .25 mile 35% grade paved road to get to my home everyday...

Physics question indeed!

How your 4Runner is able to climb the equivalent of a 35+ story building - within the span of a few city blocks - is nothing short of amazing.