TheSamba.com :: View topic - Comparison....4.12 to a 3.88 ring and pinion

Sun May 17, 2015 7:15 pm

EVfun wrote: It looks like VW kept the vehicle speed in high gear at 3000 rpm between 58 and 62 mph from the 40 HP era to the the end of U.S. models in 1979.

So here's an easy and safe way* to get 2000RPM on the road. Just cruise at 40mph!

*Probably not safe on 85MPH highways.

Sun May 17, 2015 8:52 pm

EVfun wrote: The 4.125 R&P came with a 0.88 4th, for a 3.63 overall gear ratio in high gear.
The 3.875 R&P came with a 0.93 4th, for a 3.60 overall gear ratio in high gear.

the "AN" trans IS 3.875 with .89(rounded up) 4th..from the factory

Mon May 18, 2015 1:57 am

air-h2o-air wrote: EVfun wrote: The 4.125 R&P came with a 0.88 4th, for a 3.63 overall gear ratio in high gear.
The 3.875 R&P came with a 0.93 4th, for a 3.60 overall gear ratio in high gear.

the "AH" trans IS 3.875 with .89(rounded up) 4th..from the factory
No, An AH has a 4.12 R&P.
Perhaps you were thinking of an AN Karmann Ghia trans. It is 3.875 x .883?

Mon May 18, 2015 6:55 am

Bruce, typo on my part...corrected now

yes Im taking a 4.12 "AH" out and putting a 3.88 "AN" in it's place

Mon May 18, 2015 10:07 am

Bruce wrote: Volks Wagen wrote: EVfun wrote: The 4.125 R&P came with a 0.88 4th, for a 3.63 overall gear ratio in high gear.
The 3.875 R&P came with a 0.93 4th, for a 3.60 overall gear ratio in high gear.

I have slightly different numbers. The 4.125 R&P came with a 0.89 4th, for a 3.67 overall gear ratio in high gear.
68 and earlier gearboxes came with coarse tooth gears. That 4th gear is 0.8888888:1
69 to 72 had fine tooth gears. It's ratio is .88333333:1

So, for the numbers nerds,
4.125 x .88333333 = 3.64375
4.125 x .88888888 = 3.66666

Finally, the 4th gear that a Beetle gets with a 3.88 is .9310345 : 1

3.875 x .9310345 = 3.6077586

The difference is then .9976%. Or, on the freeway, the car with the 4.12 is doing 60mph, the other with the 3.88 at the same rpm is doing 60.59857 mph.

Many years ago, my buddy used to say in a case like this, "it's two-tenths of f@ck-all"

Bruce;
I am trying to calculate my estimated RPM and speed with the transaxle I have in.

I am using some online calculators and they ask for "differential ratio" or "gear ratio". Would that be the R/P times 4th gear, e.g., 4.12 x .82, or just the R/P, 4.12?

I'm not a mechanic so pardon the questions! :-Z

m

Mon May 18, 2015 2:05 pm

You need to be careful with the online tools. Many of them were designed by the Camaro/Mustang/Dodge guys. In their cars, almost exclusively, a 4 speed has a 1:1 ratio, so there's no need to account for the transmission. It's only the R&P ratio that counts to them.

In our case, the R&P in a stock Beetle is usually 4.37, 4.12, or 3.88. If the online calculator doesn't ask for the ratios of the transmission, you will have to multiply the above R&Ps by your 4th gear.

Post links to the online calculators you have found.

Mon May 18, 2015 3:33 pm

Thanks, Bruce. I wasn't sure what the correct number would be.

So, if they are asking for "differential ratio", I would use the R/P x 4th gear numbers (my bus - 4.12 x .82 = 3.3784).

The links:
http://www.advanced-ev.com/Calculators/TireSize/

http://www.sierragear.com/differential-gear-ratio-calculator/
(you have to scroll down to the RPM/MPH calculator)

m

Mon May 18, 2015 7:35 pm

4.125 x .88333333 = 3.64375
4.125 x .88888888 = 3.66666
3.875 x .9310345 = 3.6077586
3.875 x .88333333 = 3.422917
4.125 x .82 = 3.3825
3.875 x .82 = 3.1775
3.44 x .88333333 = 3.038667
3.44 x .82 = 2.8208
3.44 x .75 = 2.58 (about 70% as fast as the first line)

Wed May 20, 2015 1:23 pm

Juanito84 wrote: 4.125 x .88333333 = 3.64375
4.125 x .88888888 = 3.66666
3.875 x .9310345 = 3.6077586
3.875 x .88333333 = 3.422917
4.125 x .82 = 3.3825
3.875 x .82 = 3.1775
3.44 x .88333333 = 3.038667
3.44 x .82 = 2.8208
3.44 x .75 = 2.58 (about 70% as fast as the first line)

2.58:1 means the crank rotates 2.58 times for every 1 rotation of the axle.

Wed May 20, 2015 2:53 pm

bikerbus wrote: So, if they are asking for "differential ratio", I would use the R/P x 4th gear numbers (my bus - 4.12 x .82 = 3.3784).
For both of those calculators, you are correct.
When you multiply the R&P by 4th gear's ratio, we call that the "final drive ratio".

Wed May 20, 2015 2:56 pm

Juanito84 wrote: 4.125 x .88333333 = 3.64375
4.125 x .88888888 = 3.66666
3.875 x .9310345 = 3.6077586
3.875 x .88333333 = 3.422917
4.125 x .82 = 3.3825
3.875 x .82 = 3.1775
3.44 x .88333333 = 3.038667
3.44 x .82 = 2.8208
3.44 x .75 = 2.58 (about 70% as fast as the first line)

Be careful, our resident numbers nerd might get upset with your rounded off ratios.

.82 is actually .8214286 and 3.44 is 3.4444444

Wed May 20, 2015 8:55 pm

Thanks, because I had no idea. I'm just going off of memory from catalogs. I'll correct it when I get a chance.

Wed May 20, 2015 9:06 pm

Bruce wrote:
Be careful, our resident numbers nerd might get upset with your rounded off ratios.

.82 is actually .8214286 and 3.44 is 3.4444444

Good one, Bruce!

By the way, I'm calling up the trailer park boys and they're bringing Bubbles over to your house for a "little chat."

Just kidding, they're not really going to chat.

:)

Sat May 23, 2015 4:27 pm

Bruce wrote: Volks Wagen wrote: Bruce wrote: Volks Wagen wrote: Bruce wrote: Volks Wagen wrote: I still maintain it will run cooler with taller gearing. Not only is this false, the opposite is true. Demonstrated by many in the real world. Not theorizing.
In the example I gave above where I switched to a taller tire, I saw the temps increase by 20ºF. After this, I built a new gearbox where instead of a 3.88x.82 top gear, the new one was 4.12x.82. By making the gears shorter, my oil temps dropped by that same 20ºF
Just stating it is false doesn't make it false. It was under lab conditions?
And stating it is true doesn't make it true. It is the real world.
What real wold testing have you done to verify your claims?

I'm content to accept VW did the testing as they progressed to taller ratios.
You are correct there. They did determine that taller gears caused higher operating temps, so when they changed the R&P from a 4.12 to the taller 3.88, they lowered 4th gear so the final drive ratio was the same as with the 4.12.

Bruce - aren't you just making that up?

Sat May 23, 2015 5:25 pm

Volks Wagen wrote: Bruce - aren't you just making that up?
Well, it seems only the Karmann Ghia got taller gearing in the U.S. As an EV guy, I can say with confidence that the Ghia is a lot more aerodynamic than the Bug. Electric conversions using the same parts show the Ghia to use considerably less power to go 60 mph compared to a Beetle. I can see why it got taller gearing, it needed less power to go 60 mph. I'll gear my Beetles to run somewhere around 3000 rpm at 60 mph and figure those smart German engineers that worked for VW knew how to calculate cooling needs and available cooling.