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lmaoufle
Samba Member
Joined: June 02, 2015
Posts: 63
Location: France
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 Posted: Wed Jun 05, 2024 1:51 am Post subject: Custom gearbox gone wrong |
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Hi guys!
5 years ago I ordered a custom gearbox for highway drive on my 71 westy.
7/29 R&P (4.142) and 0.82 for the 4th gear.
At this time I had my trusty 1776 and it was pushing hard doing its best but speed and hill was not the best 
This year I invest in a nice custom build 2.1L design with this gearbox, so high torque on low rpm
Now my first tests are a bit deceiving, the figure expected on the gear/ratio calculator https://www.blocklayer.com/rpm-gear are really not on sight.
So my tyre 195/65/15, my 0.82 gear and 4.412 r&p should bring me to 88mph at 4000 (flying to the moon  ) but guess what? Using 2 gps and my smiths tacho I am only at 73mph! That is a real difference there?
I am not an expert but, it is pure math, and theoretical speed and real speed should be very close no?
I have the feeling that my 4th is in fact a 0.89 as at 4000rpm it gives a theoretical speed of 81, much closer to the my real speed.
What are your opinions on that? Do you have some figure to compare? is there a way to check what ratio is my 4th without opening the gearbox?
Thanks for all comments! |
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Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7822
Location: Videbaek Denmark
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| Posted: Wed Jun 05, 2024 3:29 am Post subject: Re: Custom gearbox gone wrong |
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Hello.
To my knowledge there are no 7/29 ratio´s in the ACVW gearbox range. From your numbers I am assuming that your transmission is really a 8/35 (4,375) with a 0,93 4rth. That fits perfectly with your speed at 4000.
This gearing is the stock 1200 (sparkäfer) and 1303/1300.
In a split bus(?) you can easily go with a 4,125/89 4rth. I will not recommend more than that unless you have A lot of lower rpm torque and GOOD cooling.
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lmaoufle
Samba Member
Joined: June 02, 2015
Posts: 63
Location: France
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| Posted: Wed Jun 05, 2024 3:37 am Post subject: Re: Custom gearbox gone wrong |
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Hi
Thanks for your answer, here (in France) we have brand new custom made 7/29 R&p design for split and baywindow.
That is why my builder choose to set up a 2.1 motor to have a lot lower rpm torque and an external oil cooler to keep things fresh.
My questions are:
- should real numbers equals to theoretical figure?
- is there a way to calculate gear ratio without opening the gearbox? |
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b-man
Samba Member
Joined: May 20, 2008
Posts: 636
Location: So Cal
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| Posted: Wed Jun 05, 2024 11:07 am Post subject: Re: Custom gearbox gone wrong |
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| lmaoufle wrote: |
Hi
Thanks for your answer, here (in France) we have brand new custom made 7/29 R&p design for split and baywindow.
That is why my builder choose to set up a 2.1 motor to have a lot lower rpm torque and an external oil cooler to keep things fresh.
My questions are:
- should real numbers equals to theoretical figure?
- is there a way to calculate gear ratio without opening the gearbox? |
Super simple to figure out using simple multiplication.
Determine the ring and pinion using a known constant second gear ratio. Put the transaxle in second and then turn the mainshaft using an old clutch disc or turn the crankshaft pulley) mark the top of the bellhousing (or use the split in the engine case) and the disc (or pulley) using masking tape and a sharpie so you have a solid reference point.
8 turns is a 3.875, 8.5 turns is a 4.125 and 9 turns is a 4.375 assuming the stock 2.06 second gear ratio.
Next put it in 4th and there are really only 3 factory overdrive ratios .82 bus and .89 most common and finally .93 used in late transaxles.
Example of probably the most common so-called freeway flyer gearing combination would be 4.125 x .82 which multiples to 3.3825.
Simply turn the input shaft and count how many turns it takes to make the axle or CV flange rotate exactly one turn. If the diff is worn and both flanges aren’t turning in unison then hold one axle or flange from turning and turn the mainshaft until the axle/flange that’s free goes exactly 2 full turns (2:1 ratio inside the differential is why).
So if you do have the 4.125/.82 combo you should see what looks like 3.38 turns of the mainshaft which is easy to see as a little less than 3-1/2 turns.
Of course this works for any gear you’d like to check, all the factory ratios as well as aftermarket are easy to research so you can then do this test. Once you know your ring and pinion it’s easy to work backwards through the math.
This is easy and works every time. Now you can buy an unknown transaxle and easily determine what’s inside. Simple math and logic, it’s not a mystery just because you can’t see inside the transaxle.
These examples apply to common VW ring and pinion sets, plug in your ring and pinion and do the simple math.
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1966 Velvet Green Single Cab
1957 L41 Black Oval Window Beetle |
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Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7822
Location: Videbaek Denmark
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| Posted: Wed Jun 05, 2024 1:26 pm Post subject: Re: Custom gearbox gone wrong |
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OK. Did a little more math.
Your speed versus gearing matches a 4,57 R/P and 0,89 4rth too.
I have also NEVER heard of this 7/29 R/P. I am pretty sure somebody is taking advantage of lesser informed people.
If you had a 4,57 R/P and a 0,82 4rth your speed at 4000 rpm would be 127-128 kmh (78 mph)
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lmaoufle
Samba Member
Joined: June 02, 2015
Posts: 63
Location: France
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| Posted: Thu Jun 06, 2024 2:23 am Post subject: Re: Custom gearbox gone wrong |
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| b-man wrote: |
| lmaoufle wrote: |
Hi
Thanks for your answer, here (in France) we have brand new custom made 7/29 R&p design for split and baywindow.
That is why my builder choose to set up a 2.1 motor to have a lot lower rpm torque and an external oil cooler to keep things fresh.
My questions are:
- should real numbers equals to theoretical figure?
- is there a way to calculate gear ratio without opening the gearbox? |
Super simple to figure out using simple multiplication.
Determine the ring and pinion using a known constant second gear ratio. Put the transaxle in second and then turn the mainshaft using an old clutch disc or turn the crankshaft pulley) mark the top of the bellhousing (or use the split in the engine case) and the disc (or pulley) using masking tape and a sharpie so you have a solid reference point.
8 turns is a 3.875, 8.5 turns is a 4.125 and 9 turns is a 4.375 assuming the stock 2.06 second gear ratio.
Next put it in 4th and there are really only 3 factory overdrive ratios .82 bus and .89 most common and finally .93 used in late transaxles.
Example of probably the most common so-called freeway flyer gearing combination would be 4.125 x .82 which multiples to 3.3825.
Simply turn the input shaft and count how many turns it takes to make the axle or CV flange rotate exactly one turn. If the diff is worn and both flanges aren’t turning in unison then hold one axle or flange from turning and turn the mainshaft until the axle/flange that’s free goes exactly 2 full turns (2:1 ratio inside the differential is why).
So if you do have the 4.125/.82 combo you should see what looks like 3.38 turns of the mainshaft which is easy to see as a little less than 3-1/2 turns.
Of course this works for any gear you’d like to check, all the factory ratios as well as aftermarket are easy to research so you can then do this test. Once you know your ring and pinion it’s easy to work backwards through the math.
This is easy and works every time. Now you can buy an unknown transaxle and easily determine what’s inside. Simple math and logic, it’s not a mystery just because you can’t see inside the transaxle.
These examples apply to common VW ring and pinion sets, plug in your ring and pinion and do the simple math. |
Thanks so much it makes perfect sense and that was exactly what I needed! |
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lmaoufle
Samba Member
Joined: June 02, 2015
Posts: 63
Location: France
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| Posted: Thu Jun 06, 2024 2:27 am Post subject: Re: Custom gearbox gone wrong |
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| Alstrup wrote: |
OK. Did a little more math.
Your speed versus gearing matches a 4,57 R/P and 0,89 4rth too.
I have also NEVER heard of this 7/29 R/P. I am pretty sure somebody is taking advantage of lesser informed people.
If you had a 4,57 R/P and a 0,82 4rth your speed at 4000 rpm would be 127-128 kmh (78 mph) |
Question is: what difference with real and theoretical figure are considered normal? 5% %10%? There is room for error, with tyre diameter, transmission loss, and tachometer readings.
Concerning 7/29 r&p, this is an available product here in France made by reputable shops.
https://vintageautoparts.fr/produit/boite-ag-syncr...9-ou-8x34/
I spent half of this price 5 years ago  |
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Root_Werks
Samba Member
Joined: December 31, 2007
Posts: 1050
Location: San Juan Islands
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| Posted: Thu Jun 06, 2024 2:23 pm Post subject: Re: Custom gearbox gone wrong |
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When I did the gear calc for my Berg5, I measured the rolling radius of my rear tires. It was a bit less than any stated diameter stat from tire websites. That gave me a true calculation and accurate graph.
Put a mark on the ground and tire (match them up of course). Roll until mark on tire is back on the ground, measure distance.
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Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7822
Location: Videbaek Denmark
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| Posted: Sat Jun 08, 2024 9:40 am Post subject: Re: Custom gearbox gone wrong |
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| lmaoufle wrote: |
| Alstrup wrote: |
OK. Did a little more math.
Your speed versus gearing matches a 4,57 R/P and 0,89 4rth too.
I have also NEVER heard of this 7/29 R/P. I am pretty sure somebody is taking advantage of lesser informed people.
If you had a 4,57 R/P and a 0,82 4rth your speed at 4000 rpm would be 127-128 kmh (78 mph) |
Question is: what difference with real and theoretical figure are considered normal? 5% %10%? There is room for error, with tyre diameter, transmission loss, and tachometer readings.
Concerning 7/29 r&p, this is an available product here in France made by reputable shops.
https://vintageautoparts.fr/produit/boite-ag-syncr...9-ou-8x34/
I spent half of this price 5 years ago |
Yes. I did a search on them after my last post and found them too. I have NO idea where they came from or whether they are custom made for a local transmission guy or something from another trtansmission which happens to fit. Bottom line is that no known gear ratios fit your speed versus rpm versus gearing. So something aint right.
The calculated speed versus real speed should be very close when you take the generic tyre size into the calculation too. The variation between different tyre manufacturers should be within 2%
If we take that into consideraion, there is the 4,142 with a 0,93 4rth gear which gives 120 kmh @ 4000. Minus 2% for a small rolling diameter= 117,6. Sooo. 
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lmaoufle
Samba Member
Joined: June 02, 2015
Posts: 63
Location: France
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| Posted: Wed Jul 02, 2025 10:19 am Post subject: Re: Custom gearbox gone wrong |
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Hi guys!
I am back with my funky gearbox
In fact it died a weeks ago and just came back brand new.
So I did the test:
- in first, one full turn of the clutch disk give 17 turns of wheel s
- in fourth, one full turn off the clutch disk give 3.8 turn off wheel
Can someone help me with the math  to check for the r&p and the fourth?
Talking with the transmission guy that rebuild it, it is 7/29 and 0.82.
He mention the r&p comes from USA, it is for a T3 but re machine for a T2 shaft.. |
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Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7822
Location: Videbaek Denmark
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| Posted: Wed Jul 02, 2025 11:12 am Post subject: Re: Custom gearbox gone wrong |
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| lmaoufle wrote: |
- in first, one full turn of the clutch disk give 17 turns of wheel s
- in fourth, one full turn off the clutch disk give 3.8 turn off wheel
Can someone help me with the math to check for the r&p and the fourth?
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This makes no sense at all.
To make sure we are on the same page, lock one wheel. Put it in 4rth gear. Turn the clutch disc 100 turns, and have someone else count the revolutions the free wheel is taking. Then we know where we are.
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oprn
Samba Member
Joined: November 13, 2016
Posts: 14712
Location: Western Canada
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| Posted: Wed Jul 02, 2025 11:17 am Post subject: Re: Custom gearbox gone wrong |
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Here in Canada the 4 speed type 3 and type 1 gearboxes as far as I know are identical and totally interchangeable. Maybe the auto stick or the 3 speed fully automatic had a different ratio? Or maybe the type 4?
7/29 = 4.14/1. Never heard of it, maybe one of our transaxle guys can shed some light on this?
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Our cars get old, we get old but driving an old VW never gets old! |
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jsturtlebuggy
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Joined: August 24, 2005
Posts: 4595
Location: Fair Oaks/Orangevale, CA
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| Posted: Wed Jul 02, 2025 1:14 pm Post subject: Re: Custom gearbox gone wrong |
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I see brickworks.co.uk also carries the 7-29 4.14 ring&pinion for the T3 (Vanagon) 4 and 5 speed.
I can see how you could use it in the earlier 091 transaxle by cutting end off.
I wonder how the quality is compared to the on 4.13 Weddle sells for a 091?
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Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
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raygreenwood
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Joined: November 24, 2008
Posts: 23098
Location: Oklahoma City
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| Posted: Wed Jul 02, 2025 1:33 pm Post subject: Re: Custom gearbox gone wrong |
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| oprn wrote: |
Here in Canada the 4 speed type 3 and type 1 gearboxes as far as I know are identical and totally interchangeable. Maybe the auto stick or the 3 speed fully automatic had a different ratio? Or maybe the type 4?
7/29 = 4.14/1. Never heard of it, maybe one of our transaxle guys can shed some light on this? |
Nope...not the type 4's I can tell you that. The 004 four speed had two final drives 3.91:1 (11/43) and 3.73:1 (cant remember the tooth count n that one).
Ray |
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modok
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Joined: October 30, 2009
Posts: 27655
Location: Colorado Springs
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| Posted: Wed Jul 02, 2025 11:09 pm Post subject: Re: Custom gearbox gone wrong |
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Put it in fourth, jack up one wheel
Mark the wheel
Spin the engine ten revolutions
Count how many times the wheel turns
use math
Figure out your gear ratio |
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Willin
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Joined: October 10, 2019
Posts: 256
Location: Bozeman Mt
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| Posted: Mon Jul 07, 2025 7:33 am Post subject: Re: Custom gearbox gone wrong |
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If you haven't measured the rolling circumference of your rear tires, with proper inflation and on the car, and then convert to diameter.
Then your gear calculations are just a guess.
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