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 rim backspacing
sdn
Posted: Mar 31 2012, 08:31 AM


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Group: Members
Posts: 26
Member No.: 1,425
Joined: 11-March 12



Gidday fellas,

Can anyone tell me the backspacing needed for a 6" x 16" rim so it keeps the tyre's rubber inside the fenders and bodywork of a CJ3B?

I intend to run 7.50" x 16" NDT tyres- but dont want the rubber hanging outside the original bodywork if at all possible.

thanks,

Stephen.


--------------------
Stephen,

Perth, Western Australia

Way too slow rebuild of a 1990 Mahindra CJ340DP.
Top
oldtime
Posted: Mar 31 2012, 09:59 AM


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Posts: 4,191
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Joined: 12-July 08



The wheel is composed of two parts. The rim and the disk.
The standard disk for the various CJ wheels are almost always inset 3/4" from true rim centerline.
This 3/4" inset locates the wheel 3/8" away from true spindle bearing centerline.

A 6" wide wheel measures 7" OAW. (outermost rim flange to outermost rim flange)
This implies that the Jeep 6" wide wheel has a 4-1/4" backset from the rim flange.
4-1/4" backset will provide the standard Jeep wheelset. (3/4" inset)

The 3B body tub is 57" OAW.
I will estimate that you have a D25 front axle measuring 50-3/4" drum to drum OAW.
*Note not all axles are exactly the same width.
In my opinion the standard wheel inset is not exactly optimum.
The optimum for CJ-3B with D25 is a wheel having 3/8" inset.
That puts the track centerline at 50".
50" track allows a maximum tire tread width of 6.75" to remain just barely under the front fenders.
And it will allow for the minimal amount of spring rubbing at full turn..

My figures indicate an optimum backset of 3-7/8" for a 6 x 16" wheel.
And yes this becomes critical if you try to stuff a 7.50 x 16 tire onto the early CJ.

As always thanks for posting.


--------------------
1953 TRANS-VINTAGE CJ-3B / AC 4693 fuel pump / YF 938 SD / Hurricane / 9-1/4" Auburn clutch / T90-C / 2.46 ratio D-18 / Warn O.D. / 5.375 final drive / Powr Lok Front + Rear / Dualmatic drive flanges / deluxe Koenig half cab / 12 volt generator
2nd full re-build using the best from all vintages of CJ-3B

1962 OPTIONAL-STOCK CJ-3B / Warn O.D. / Tigertop / Transport yellow (orange)
Currently serving as my one and only DAILY DRIVER

St Louis
Top
sdn
Posted: Mar 31 2012, 02:40 PM


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Joined: 11-March 12



Gidday Oldtime, thanks for the quick reply!

I didnt think to mention,

My CJ is an Indian manufactured CJ340DP (CJ3B license built copy) made in 1991, both axles are DANA 44's

The Mahindra workshop manual says it originally came with :
6" x 16" rims with 7.50" x 16" 6 ply NDT tyres, or
7" x 15" rims with P235/75 R15 tyres

The manual states front track should be 51.5" or 51.8"

Rear track is listed at either 49.6" or 50".

I dont have a set of either size original issue rims and tyres to measure- and the body, frame and axles are not together, so unfortunately- its a bit difficult to measure up at the moment. its going to be a few days before I can get to the axles to measure proper overall width.


--------------------
Stephen,

Perth, Western Australia

Way too slow rebuild of a 1990 Mahindra CJ340DP.
Top
captjoe
Posted: Mar 31 2012, 11:00 PM


Capt. Joe


Group: Members
Posts: 159
Member No.: 996
Joined: 26-December 10



Hat's off to Ken once again for amazingly detailed data! I would have to take a stick, tape measure and a pencil to get that straight in my mind. (Which I will have to do later!) The result of those numbers that I experienced with my build using a 7.50 x 16 tire on standard Kelsey-Hayes Jeep wheels (see link below) is that I had to put 1/4" spacers behind the wheel to keep the front tire comfortably off of the springs at full turn, and that has the tire just pushing at coming out from under the fenders. As best as I understand Ken's numbers, my experience and his numbers seem consistent. I believe a 7.00 was the largest tire that was supplied stock on the CJ3B in the US. Can someone verify that?
I hope to have the body (painted) sitting on the frame with those tires by late May. OK, there, I said a date. I must get to work!
Hope this is of some help.
Regards,
Capt. Joe

Tire and wheel picture:
http://z4.invisionfree.com/CJ3B_Bulletin_B...wtopic=3619&hl=


--------------------
One piece, one part, one project at a time in the shop, but their all going in my head!
1964 CJ3B, MD Juan body, pretty darn close to stock, under restoration, gettin' close to done, I think!
Top
sdn
Posted: Apr 1 2012, 01:43 AM


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Group: Members
Posts: 26
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Joined: 11-March 12



These Mahindra (Indian) manufactured CJ3B clones were brought into Western Australia in 1991 or 2- only about 300 of them, and to reduce import duty they were fitted with some Australian content,
heater demister assembly,
rollcage,
retracting seatbelts,
hood and bows,
brake booster,
toolkit and toolbox,
15" rims and tyres and a radio for the upmarket "bushranger" model.
Aussie made alternator, and soe other bits and pieces.

The standard "stockman" model kept the original military type low back seats, the 6 x 16 rims with 7.50 x 16 tyres- and had no radio or brake booster.

Unfortunately, our taxman reckoned there wasnt enough Aussie content- so to reduce the tax payable, the "stockman" models had thier tyres and rims destroyed, replaced with the 15" versions, and a brake booster added.

In the end- a lot of the mahindra jeeps were destroyed as the dealer couldnt sell them- and was going to have to pay a huge tax bill.
no one knows how many survived.

I am told that Mahindra built thier version of the CJ3B using the stongest options- dana 44's front and rear, biggest clutch, biggest brakes, transfercase with the 1 1/4" shaft, peugeot diesel, and a 4 speed T90 gearbox.

not too sure how true it is though!

Stephen.


--------------------
Stephen,

Perth, Western Australia

Way too slow rebuild of a 1990 Mahindra CJ340DP.
Top
oldtime
Posted: Apr 2 2012, 09:24 AM


MODERATOR


Group: Co-Admin
Posts: 4,191
Member No.: 3
Joined: 12-July 08



Stephen,
I can figure and explain the optimum "wheelset" for your
Mahindra if you can supply....

The exact front axle width (measure drum to drum)
The exact front fender width. (OAW)
The acual tire tread width will also help.


--------------------
1953 TRANS-VINTAGE CJ-3B / AC 4693 fuel pump / YF 938 SD / Hurricane / 9-1/4" Auburn clutch / T90-C / 2.46 ratio D-18 / Warn O.D. / 5.375 final drive / Powr Lok Front + Rear / Dualmatic drive flanges / deluxe Koenig half cab / 12 volt generator
2nd full re-build using the best from all vintages of CJ-3B

1962 OPTIONAL-STOCK CJ-3B / Warn O.D. / Tigertop / Transport yellow (orange)
Currently serving as my one and only DAILY DRIVER

St Louis
Top
sdn
Posted: Apr 18 2012, 05:11 AM


Member


Group: Members
Posts: 26
Member No.: 1,425
Joined: 11-March 12



sorry fellas - it took a while to get back,

front axle from drum to drum is 50"

rear axle from drum to drum is 48.4" (as near as I can tell)

rear tub is 57.5" across

58.25" across for the front fenders (as near as I can tell) both fenders are being used as plugs for fibreglass moulds at the moment and are a bit messy.

tyre tread dimensions I dont have yet- but I am intending to buy 7.50 x 16 NDT tyres.

All the best,

Stephen.


--------------------
Stephen,

Perth, Western Australia

Way too slow rebuild of a 1990 Mahindra CJ340DP.
Top
oldtime
Posted: Apr 18 2012, 11:23 AM


MODERATOR


Group: Co-Admin
Posts: 4,191
Member No.: 3
Joined: 12-July 08



All Jeep rear axles are slightly narrow compared to the front axles.
For now I will focus your concern toward the front axle only.

O.K....Here are some facts and theory to comprehend.

Concerning tire dimensions..
Sadly the actual measured figures are often different from the manufactures claimed figures.
I usually prefer to use the actual measured tread width figure.
If you want the complete overall tire width inside the fenderwell then use the manufactures claimed overall tire width figure.

Let's say most likely the proposed tire will have a 7" tread face or possibly a little less.
Here, I specifically mean the actual tread face and not the overall tire width.
Let's also say that you choose a wheel with zero set.
Take 50" and add 7" that implies the outermost tire track will be exactly 57" wide.
The tire tread will then be well inside of your Mahindra 58-1/4" fenderwell dimension.
You could potentially outset the wheels about 1/2 " each resulting in a 58" outermost track width.
Or you could install up to 8" wide tread pattern with a zero set (true centerline) wheel.

As always thanks for your questions and comments.


--------------------
1953 TRANS-VINTAGE CJ-3B / AC 4693 fuel pump / YF 938 SD / Hurricane / 9-1/4" Auburn clutch / T90-C / 2.46 ratio D-18 / Warn O.D. / 5.375 final drive / Powr Lok Front + Rear / Dualmatic drive flanges / deluxe Koenig half cab / 12 volt generator
2nd full re-build using the best from all vintages of CJ-3B

1962 OPTIONAL-STOCK CJ-3B / Warn O.D. / Tigertop / Transport yellow (orange)
Currently serving as my one and only DAILY DRIVER

St Louis
Top
sdn
Posted: Apr 19 2012, 09:24 AM


Member


Group: Members
Posts: 26
Member No.: 1,425
Joined: 11-March 12



Thanks for that Oldtimer- all makes sense now!

stephen


--------------------
Stephen,

Perth, Western Australia

Way too slow rebuild of a 1990 Mahindra CJ340DP.
Top
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