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Philosophy of Gearing

Posted by dunhamr1 
dunhamr1
Ryan Dunham
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Philosophy of Gearing
June 16, 2017 10:11AM
Wading into the world of gear ratios and final drives, and curious what makes the difference and why certain ratios (or final drives) are 'better' for rally?

Talk seems to center around a lower ratio (above 4.0) being better. How low is too low? (usually limited by the car's manufacturer, or funds for "Motorsports" diffs)

As for gearboxes, what makes one different than the other?
e.g. for a T5 box the 2.95 first gear (V8 Camaro & Ford Motorsport) gearboxes are held on a pedestal (and the price reflects that) Vs std mustang V8, or V6 (and 2.3 4cyl) boxes. (3.35-4.03 first gear ratios).

From my understanding the lower first gear (eg T-bird 2.3 T5's 4.03 1st gear ratio) gives more grunt off the line (because torque multiplication and maths), but you run out of it faster. A higher gear (such as the 2.95) would allow you to stay in 1st longer (or downshift to first without having to be too slow to do so).

But in the middle it gets a bit muddy. If you're trying to "pick" ratios, what would one be looking for?
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Aaron Luptak
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Re: Philosophy of Gearing
June 16, 2017 11:56AM
I'm no expert, but it's the internet so I'll chime in anyways....

FD is only too short if you run out of gear - ie. you spend lots of time bouncing off the rev limiter in your top race gear. (I say 'race gear' as I've heard folks advocate for having a tight 1-4 with a tall 5th gear for transit).

Running out of gear probably the biggest consideration on 1st. Looking at 2.95 vs 4.03, w/ a 4.1 FD, 205/65-15 tires, and 7k rev limit - the 2.95 will go nearly to 44 mph, vs barely 32 on the 4.03.

The whole grunt-off-the-line thing seems a little less critical if you're grip limited. Acceleration is good, digging holes and extra tire wear from launches not so much.

In between? I imagine, if you were running a $$$$$$ gearbox that gave you a range of ratios for each gear, you'd be looking at dyno curves of the engine, trying to optimize RPM drop between gears, etc. For the rest of us, it seems like the guiding principle is just get as close of gears as you can afford - with some consideration for how you're shifting - a super-tight box might be awesome with a sequential, but obnoxious with a H-pattern and syncros.

I think there's some consideration on strength of the gears as well - the further you get from 1:1 on a gear, the smaller (weaker) one of the gears has to be, as well as more torque multiplication between them.



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Josh Wimpey
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Re: Philosophy of Gearing
June 16, 2017 12:10PM
There are a whole lot of variables involved in figuring out 'ideal' gear ratios and final drive combinations. For gravel rally with a 2wd machine, I think you could start with the following

BASIC APPROACH

First gear & FD combo
Probably good to target a Final Drive & First gear combo that results in a roughly 40mph top speed in gear. A first gear that tops out at 25mph with your engine redlined is not very useful for rally. A low first gear is not necessary but, too high and you may have trouble starting on hills or loading the trailer.

Top gear and FD combo
The other bookend is top speed in top gear. Many US rally events are faster than European events because of wider straighter roads. Gearing for a terminal speed of 95mph at redline in 5th (or 6th) may not be ideal for US roads but it sure will be fun in the tight stuff and accelerating out of corners. Likewise, terminal speed in excess of 125mph in top gear at redline is likely not useful and ultimately hurts performance.

Gears in between 1st and top gear
So, a box with 1st gear top speed around 40mph and top gear top speed somewhere north of 100mph but probably less than 125mph. From there, figure out what combination of ratios gives you the most evenly spaced gear ratios in between these two bookends.


COMPLICATED PART

What is available that fits in your box (and/or rear diff case)?
What fits in your budget?
Are any of the options for your box known to be particularly fragile? (say, that 5.9:1 R&P that likes to shatter)



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Robert Culbertson
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Re: Philosophy of Gearing
June 16, 2017 12:59PM
On the RWD cars that I have played with, the 1st-2nd gap is huge, so I've focused on 2-5th with a short final drive. The low 1st is nice for loading it on the trailer and cruising the service park at a reasonable speed.
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Re: Philosophy of Gearing
June 16, 2017 01:16PM
related: why the hell did toyota decide the 7.8" axle needed to have ANY difference between the V6 and the I4..... grrr.



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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 01:20PM
Quote
Aaron Luptak
I'm no expert, but it's the internet so I'll chime in anyways....

FD is only too short if you run out of gear - ie. you spend lots of time bouncing off the rev limiter in your top race gear. (I say 'race gear' as I've heard folks advocate for having a tight 1-4 with a tall 5th gear for transit).

The whole grunt-off-the-line thing seems a little less critical if you're grip limited. Acceleration is good, digging holes and extra tire wear from launches not so much.

In between? I imagine, if you were running a $$$$$$ gearbox that gave you a range of ratios for each gear, you'd be looking at dyno curves of the engine, trying to optimize RPM drop between gears, etc. For the rest of us, it seems like the guiding principle is just get as close of gears as you can afford - with some consideration for how you're shifting - a super-tight box might be awesome with a sequential, but obnoxious with a H-pattern and syncros.

Agreed on grunt in 1st not particularly the issue on loose surface driving. The problem with thinking about all of these things is that I personally have no experience with how the different ratios feel or drive. Maybe I should start asking my drivers what the setup on the car is and listen and watch on stage...


Quote
Josh Wimpey
So, a box with 1st gear top speed around 40mph and top gear top speed somewhere north of 100mph but probably less than 125mph. From there, figure out what combination of ratios gives you the most evenly spaced gear ratios in between these two bookends.

COMPLICATED PART

What is available that fits in your box (and/or rear diff case)?
What fits in your budget?
Are any of the options for your box known to be particularly fragile? (say, that 5.9:1 R&P that likes to shatter)

Agreed, but how many stages have 100mph+ sections? It'd be interesting to know. All of the Jemba-noted rallies have a speed profile for each stage, would that be useful to help gauge top speeds (it's usually based on a group N (production) Subaru)?

Doubly agreed on budget/ available ratios / failure points.
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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 01:26PM
Quote
Robert Culbertson
On the RWD cars that I have played with, the 1st-2nd gap is huge, so I've focused on 2-5th with a short final drive. The low 1st is nice for loading it on the trailer and cruising the service park at a reasonable speed.

How low is too low to be useful for racing, in your experience? Vice versa, how high is too high to be easy enough to drive at slower speeds?
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Josh Wimpey
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Re: Philosophy of Gearing
June 16, 2017 01:42PM
Quote
dunhamr1
Agreed, but how many stages have 100mph+ sections? .

Depends on how fast your car is, how fast you were out of the previous corner, and how brave you are.

In the some 30 events I have raced, I think the only event that I have not used 5th gear on was my very first and the twistiest in the US __Cherokee Trails. AND, I have never had a box with good gear ratios. Shifting to 5th gear in both of my cars means you have already exceeded 100mph 109mph in the 16v and 111 in the TDI. Never topped out 5th in either as I don't think there are straights long enough even on flat pavement 139mph for the 16v and 147mph for the TDI are the redline speeds in top gear.



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Re: Philosophy of Gearing
June 16, 2017 01:57PM
Quote
Robert Culbertson
On the RWD cars that I have played with, the 1st-2nd gap is huge, so I've focused on 2-5th with a short final drive. The low 1st is nice for loading it on the trailer and cruising the service park at a reasonable speed.

And that's the way a lot of people do it. The problem is that you end up driving on a teeny fifth gear that was designed to be an overdrive, and a ring&pinion with really small teeth.



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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 02:01PM
Quote
hoche
Quote
Robert Culbertson
On the RWD cars that I have played with, the 1st-2nd gap is huge, so I've focused on 2-5th with a short final drive. The low 1st is nice for loading it on the trailer and cruising the service park at a reasonable speed.

And that's the way a lot of people do it. The problem is that you end up driving on a teeny fifth gear that was designed to be an overdrive, and a ring&pinion with really small teeth.

Is running the 5th gear and final drive in that combo not good (i.e. Are really small teeth prone to breaking?)
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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 02:06PM
Quote
Josh Wimpey
Quote
dunhamr1
Agreed, but how many stages have 100mph+ sections? .

Depends on how fast your car is, how fast you were out of the previous corner, and how brave you are.

In the some 30 events I have raced, I think the only event that I have not used 5th gear on was my very first and the twistiest in the US __Cherokee Trails. AND, I have never had a box with good gear ratios. Shifting to 5th gear in both of my cars means you have already exceeded 100mph 109mph in the 16v and 111 in the TDI. Never topped out 5th in either as I don't think there are straights long enough even on flat pavement 139mph for the 16v and 147mph for the TDI are the redline speeds in top gear.

So, to that point you'd be better suited (jonesing for the "ideal" gearbox) with a lower final drive where you could be using (and topping 5th) on the longest straight your car would theoretically see?
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Josh Wimpey
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Re: Philosophy of Gearing
June 16, 2017 02:24PM
Quote
dunhamr1

So, to that point you'd be better suited (jonesing for the "ideal" gearbox) with a lower final drive where you could be using (and topping 5th) on the longest straight your car would theoretically see?

That depends. The 16v with a lower FD would end up with a useless 1st gear. And the FD does nothing to improve the rpm drop between gears.

The tdi with a lower FD might be an improvement but i have a quick amd free solution to swap out 5th gear with one that tops out at 123mph



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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 02:32PM
Quote
Josh Wimpey
Quote
dunhamr1

So, to that point you'd be better suited (jonesing for the "ideal" gearbox) with a lower final drive where you could be using (and topping 5th) on the longest straight your car would theoretically see?

That depends. The 16v with a lower FD would end up with a useless 1st gear. And the FD does nothing to improve the rpm drop between gears.

The tdi with a lower FD might be an improvement but i have a quick amd free solution to swap out 5th gear with one that tops out at 123mph

For reference, what are the gearings and the final drives in your 16v and the TDI?
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hoche
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Re: Philosophy of Gearing
June 16, 2017 02:58PM
Unless you have a substantial amount of power, you're never going to be able to get to some of the high speeds. Both my Golfs redline around 100 and neither of them will get there in dirt unless it's a really long straightaway on hardpack or we're going downhill. That being said, at Prescott I'm usually banging off the revlimiter on part of Perkinsville and First View.

And that reminds me; if you have a substantial amount of low-end torque, you can get away with a higher first gear than otherwise.



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dunhamr1
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Re: Philosophy of Gearing
June 16, 2017 03:33PM
Quote
hoche
Unless you have a substantial amount of power, you're never going to be able to get to some of the high speeds. Both my Golfs redline around 100 and neither of them will get there in dirt unless it's a really long straightaway on hardpack or we're going downhill. That being said, at Prescott I'm usually banging off the revlimiter on part of Perkinsville and First View.

And that reminds me; if you have a substantial amount of low-end torque, you can get away with a higher first gear than otherwise.

We were well over 100 on First View in the Jetta on my first rally... such a fast rally.

Hmm, low end torque... like a turbo motor?
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