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Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 07:02AM
I'm feeling really dumb. A bit ago I posted on the Triumph Experience forum about my engine revving too high as my '74 turns about 3800 rpm at 55 mph. Someone there posted that with a stock rear end I should be turning about 2800 rpm.

I'm adding the issue to this august forum to help with a solution...

My car gets loud even when doing 40 mph and the fuel economy, even accounting for my three Weber 40DCOEs, is abysmal...

So I wanted to find out what I have in the differential without tearing it all apart and was told by a shop to get the car up on a hoist and count how many times the drive shaft turns compared to how many times a wheel turns and there's your ratio. Simple!

Well, too simple for me, as with the help of a friend, we jacked up (no hoist readily available) the rear of the car but when we turned the wheel, the driveshaft turned once while the wheel made only half a revolution! And when the driveshaft rotated two turns, the wheel went around once plus a little more-say about 1.2 times.

What the heck? That makes no sense! If I had a stock rear end (3.70:1), the drive shaft should turn almost 4 times for the wheel turning once right?

Where am I making the mistake?





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Re: Trying to figure out my rear end-on my TR6..
Posted by: Dik2
Date: October 20, 2021 07:21AM
Make sure only one wheel is off the ground, then turn driveshaft 10 turns while counting wheel turns.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Dik2
Date: October 20, 2021 07:24AM
Afterthought - It’s easier to turn the wheel and count driveshaft rotations.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 07:48AM
Make sure only one wheel is off the ground, then turn driveshaft 10 turns while counting wheel turns.

Afterthought - It’s easier to turn the wheel and count driveshaft rotations.


That's what we did-both wheels off and one wheel off with the same results.





A throwback image to celebrate Autumn.

Disclaimer: This post is checked for correct spelling, punctuation, and grammar. Any attempts at humor are solely the responsibility of the author and bear no claim that any and all readers will approve or appreciate said attempt at humor.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: NewtonMP2100
Date: October 20, 2021 08:08AM
....celebrate that you have that.....junk.....in the trunk and celebrate your rear-end.....and tap 'dat.....'thang.....



_____________________________________

I reject your reality and substitute my own!



Edited 1 time(s). Last edit at 10/20/2021 08:53AM by NewtonMP2100.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: C(-)ris
Date: October 20, 2021 08:42AM
[shiftsst.com]

Turn the wheel one full rotation and count the number of times the driveshaft rotates.



C(-)ris
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Racer X
Date: October 20, 2021 09:25AM
you can fudge things a bit with taller tires.

My Prius is so low to the ground, I want to go up 2 sizes to get 3/4" or so more ground clearance. Also the speedo is about 9% fast, so that will help get the speedo closer to accurate as well.



********************************************
The police have no duty to respond. See Castle Rock v. Gonzales, 545 U.S. 748 (2005) or Warren v. District of Columbia[1] (444 A.2d. 1, D.C. Ct. of Ap. 1981)
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Re: Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 09:26AM
Turn the wheel one full rotation and count the number of times the driveshaft rotates.

Did that. Drive shaft turns not quite twice. BUT! and thanx for this link because I saw this:

Determining a Rear Axle Ratio of a Non-Posi Rear

Jack one wheel off the ground, put the transmission in neutral and take the parking break off. ***Be sure to use jack stands under the axle for safety precaution***
Mark the wheel and the drive shaft with chalk or masking tape.
Turn the wheel TWO revolutions and count the number of driveshaft turns.
The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio.
For better accuracy, turn the wheel 20 times and divide by 2 for your rear axle ratio.


I apparently have a non-posi rear end.

I didn't turn the wheel twice but noticed that when the driveshaft turned twice the wheel turned about 1.2 times. So by extension, even if the wheel wasn't turned two times, I can presume that if I turn the wheel once more by 1.2 times, then the driveshaft will turn another two times, OK?

So it seems that if I got a little more than 1 turn of the wheel (approx 1.2 turns) to get 2 turns of the driveshaft, then 1 turn exactly of the wheel would be a little less than 2 turns of the driveshaft. and if, by extension again, 2 full turns of the wheel would result in a little bit less than 4 turns of the driveshaft, leaving with the conclusion that I probably have a stock-3.70:1-rear drive...

In which case I either have add overdrive to lower the revs or just simply put up with it...





A throwback image to celebrate Autumn.

Disclaimer: This post is checked for correct spelling, punctuation, and grammar. Any attempts at humor are solely the responsibility of the author and bear no claim that any and all readers will approve or appreciate said attempt at humor.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 09:29AM
....celebrate that you have that.....junk

You know, I don't know what idiot came up with describing their privates as "junk" but I don't consider mine "junk". Maybe his are junk...





A throwback image to celebrate Autumn.

Disclaimer: This post is checked for correct spelling, punctuation, and grammar. Any attempts at humor are solely the responsibility of the author and bear no claim that any and all readers will approve or appreciate said attempt at humor.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Racer X
Date: October 20, 2021 09:47AM
or taller tires



********************************************
The police have no duty to respond. See Castle Rock v. Gonzales, 545 U.S. 748 (2005) or Warren v. District of Columbia[1] (444 A.2d. 1, D.C. Ct. of Ap. 1981)
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Racer X
Date: October 20, 2021 09:56AM
225/70/15s would make your 3.7 rear end act like a 3.5 and give you .75" more ground clearance, and a wider tire too. Next time you need new tires, just go bigger. Should drop down to @3600 rpm too.

There might be a later model manual trans with an overdrive gear you could swap in.

I'm assuming you have the stock 185/75/15 on there.

You also should verify that your tach is correct. They can be calibrated. Is yours cable driven off the distributor, or electrical off the coil?



********************************************
The police have no duty to respond. See Castle Rock v. Gonzales, 545 U.S. 748 (2005) or Warren v. District of Columbia[1] (444 A.2d. 1, D.C. Ct. of Ap. 1981)



Edited 2 time(s). Last edit at 10/20/2021 10:02AM by Racer X.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: cbelt3
Date: October 20, 2021 10:00AM
.. but can you get tires with the cool red sidewall stripe ?
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Racer X
Date: October 20, 2021 10:03AM
Quote
cbelt3
.. but can you get tires with the cool red sidewall stripe ?

Red crayon paint markers.



********************************************
The police have no duty to respond. See Castle Rock v. Gonzales, 545 U.S. 748 (2005) or Warren v. District of Columbia[1] (444 A.2d. 1, D.C. Ct. of Ap. 1981)
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Re: Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 10:17AM
I'm assuming you have the stock 185/75/15 on there.



Nope, I have Yokohama Avid 205/70x15s on Panasport rims. I saw a graph once somewhere that showed the Yokos had the same circumference as the stock 185/15s!





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Re: Trying to figure out my rear end-on my TR6..
Posted by: DP
Date: October 20, 2021 10:20AM
but can you get tires with the cool red sidewall stripe

Yes-they're out there! My brother got red stripe Tiger Paws or Wide Oval (don't remember which) radials for his '64 GTO.





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Re: Trying to figure out my rear end-on my TR6..
Posted by: Thrift Store Scott
Date: October 20, 2021 10:50AM
Redline tires are available from Coker Tire ($$$)-

[www.cokertire.com]

If I'm not mistaken, overdrive in those cars was a factory option, a separate unit that bolted on to the rear of the transmission and was electrically engaged in top gear only via a button. My uncle had a TR6 for a while and that's how he described it, anyway. If that's the case, maybe find a rebuilt overdrive unit and add it on?



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Re: Trying to figure out my rear end-on my TR6..
Posted by: lost in space
Date: October 20, 2021 12:17PM
3800 @ 55 sounds steeper than 3.70, more like over 4:1. My old Volvo 122 wagon came with about a 4.5:1 rear end and the engine ran at RPMs similar to yours. An overdrive fixed it right up.

I think some of those TRs had the Laycock de Normanville in them, as did Jaguars, MGs, later Austin Healey 3000s and some Volvos. Aside from obtaining the OD, installation isn't major.

If having one wheel off the ground or having both (and both turn) with the same result you have some sort of posi rear end.







Edited 2 time(s). Last edit at 10/20/2021 12:20PM by lost in space.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Thrift Store Scott
Date: October 20, 2021 04:22PM
Y'all check my math on this:

205/70R15= (205 divided by 25.4 = 8.07 inches wide, x .70 aspect ratio = 5.65 inch sidewall height, x 2 + 15 = 26.30 overall tire diameter, x 3.14* =) 82.58 inches circumference.

5280 feet per mile x 12 = 63360 inches per mile, divided by 82.58 = 767.26 tire revolutions per mile.

60 mph = 1 mile per minute, so 55 mph = .9167 mile per minute.

3800 rpm divided by 767.26 = 4.95:1 rpm reduction ratio per mile between engine and tires if @ 60 mph in top gear (usually 1:1), x .9167 (55 mph) = 4.54 rear axle ratio in the TR6.


* The rolling diameter of a tire is slightly less than it's overall diameter, so using Pi to only two spaces should about even out the difference.



WARNING- If I posted a link in the above message, assume it is at minimum "NSFW- Language". Occasionally you'll be wrong.

Lie to me if you must, but don't lie to me and insult my intelligence in the same sentence.

Resist the Thought Police: George Orwell's book 1984 was meant as a warning, not an instruction manual.

"Political correctness is just intellectual colonialism and psychological fascism for the creation of thought crime" - Steve Hughes

What's my "Super Power"? I can make active threads go stone-dead with a single post. I try to use this power only for good and not for evil but, you know... stuff happens.



Edited 1 time(s). Last edit at 10/20/2021 05:29PM by Thrift Store Scott.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Dik2
Date: October 20, 2021 05:44PM
I bet that whoever hung those Webers on there swapped rears to get more acceleration.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Dik2
Date: October 20, 2021 06:02PM
Here's some good info on what might be mods made to a TR6. Note the wide range of gearbox ratios.
How much do you know about the history of your car?
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Re: Trying to figure out my rear end-on my TR6..
Posted by: JoeH
Date: October 21, 2021 12:51AM
Quote
Dik2
I bet that whoever hung those Webers on there swapped rears to get more acceleration.

Quite possible. Also looking up T6 axle ratios I found mention of three standard ones having been used - 3.45:1, 3.7:1 and 4.1:1

Edit: Also found parts listed for axle ratios of 4.3:1 and 4.875:1.



Edited 1 time(s). Last edit at 10/21/2021 01:13AM by JoeH.
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Re: Trying to figure out my rear end-on my TR6..
Posted by: NewtonMP2100
Date: October 21, 2021 09:39AM
....tap....'dat.....thang.....



_____________________________________

I reject your reality and substitute my own!
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Re: Trying to figure out my rear end-on my TR6..
Posted by: Z
Date: October 21, 2021 03:57PM
We showing off our panasport knockoffs?

Looks like you've got the means to get to an answer above and or at the TREXP thread that I dug up. A tach/dwell meter is probably worth grabbing for general diagnostic purposes.

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Re: Trying to figure out my rear end-on my TR6..
Posted by: lost in space
Date: October 21, 2021 07:08PM
That's a sweet 122. 67? Mine was a '68 with a later B20 +OD + Bilsteins.



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