El Cheapo HalfDork
Nov. 27, 2015 9:20 a.m.

I'm trying to understand how backlash and contact patch seem interrelated when setting up a differential. The picture below is a diff I'm building and the backlash is being adjusted. The next step is to paint the ring gear and check contact patch. My understanding is if the contact patch needs adjusting.... it will effect the backlash. Is this true or am I not understanding the concept?

For the curious, the ring and pinion ratio is 3.90 and were harvested from an early model RX-7. The gear set fits in a '94 up Miata open differential. I love how obscure parts from a different era can be mix-matched with Mazda.

EvanB UltimaDork
Nov. 27, 2015 9:38 a.m.

When I did the same thing I only needed to set the backlash and the contact patch was fine since I didn't touch the pinion depth.

So that totally doesn't answer your question.

Rogue New Reader
Nov. 27, 2015 9:51 a.m.

Interested too……hard to see, so are you building a 3.9 Clutch pack?

El Cheapo HalfDork
Nov. 27, 2015 9:57 a.m.

In reply to EvanB:

Actually that is helpful. I have zero experience and its good to know.

El Cheapo HalfDork
Nov. 27, 2015 9:59 a.m.

In reply to Rogue:

Its just a regular open diff..

Knurled MegaDork
Nov. 27, 2015 10:35 a.m.

Changing backlash affects what the proper pinion depth should be, and changing pinion depth affects backlash.

Basically the gear mesh happens on a plane that isn't remotely close to either of the axes you can adjust, so it is one part voodoo and two parts knack.

Start with the shims that came from the gearset in the differential housing, meaning the Miata gears. Gears are usually machined more precisely than the iron housings, so start with the shims that go with the housing.

Also, measure backlash as close to tangential to the ring gear as possible. That image is exactly how not to do it.

If you're installing used gears, you probably can throw the little guides out the window. Used gears won't really have a pattern anymore since the gears are worn-in. Shoot for the most contact as possible, and bear in mind that the gears WILL make a bunch of noise for the first 10,000mi or so until they wear-in again since it will be impossible to get them the same as the housing they were worn-in with.

On the bright side, this is one of the easier diffs to set up. You only have to juggle shims on the pinion, the differential gears have threaded adjusters. Since you have to remove the differential to change pinion shims, don't worry about backlash alteration since you have to reset it every time you pull the diff out.

The FSM is vague for how much preload to set on the diff bearings. My method for setting backlash is to set it too tight (practically zero lash, like .002-003" backlash) with no preload on the bearings, then tighten the right side bearing 2 1/2 holes. That SHOULD open up backlash to .004-.005" or so. But really for used gears the backlash should be set to what they were in the old housing. If you don't know that spec, set the backlash up a little tight and don't mind the screaming gears as they wear in again...

El Cheapo HalfDork
Nov. 27, 2015 10:57 a.m.

In reply to Knurled:

Awesome! thanks for taking the time to write that. The word voodoo puts it all in prospective.

Also, thanks for the heads up on the dial indicator position!

flatlander937 Reader
Nov. 27, 2015 11:39 a.m.

Here's a how to I did years ago for regearing a Jeep D44. Should answer your questions. I've set up many rear ends, mostly under trucks and Jeeps but its all the same concept. I've fixed a Mercedes R350 front diff too... With unobtanium bearing races from France. Saved $2k though...


In a nut shell: if pattern needs adjusted you change pinion depth in direction needed, reset backlash to spec, THEN recheck pattern. Continue until happy.

Ranger50 PowerDork
Nov. 27, 2015 12:30 p.m.

And sometimes OE's set up gears to a std spec shim and mark the pinion depth from that spec, so you can just grab that shim and torque. Then you play with the backlash to get the desired pattern.....

Knurled MegaDork
Nov. 27, 2015 1:57 p.m.

I don't think I've ever seen a depth on the end of a Mazda pinion. Plenty of them on GM pinions, and it's super nice for those because we have the measuring tools for GM rears so we just measure, install the appropriate shim, and everything's good.

IF you have the measuring tools...

Knurled MegaDork
Dec. 4, 2015 8:08 p.m.

Hey look what I found.

 photo GEDC0767.jpg

A wear pattern on an actual Mazda gearset, Note that the drive side basically has no pattern anymore, while the coast side still looks like something you'd see in a book. This diff had .010 backlash when I got it. I put a limited slip it it and tightened the backlash to .004 because setting up a little tight will compensate for everything spreading apart under load. This was the result - the drive side is a little too close to the root (but it still was over the whole length of the tooth, at least) and the coast side was all up high but who cares about coast.

Well, if it was a customer car, I'd care (and probably just open the backlash up to what it had worn into) but for me, screw it, it'll be noisy, I just want it to not break.

You'll need to log in to post.

Also on Grassroots Motorsports

Tech Tips: 2006–’15 Mazda MX-5

12 hours ago in Articles

We asked a couple experts for the best way to upgrade the NC MX-5.

Project Turbo Miata: Hitting the Dyno With a Fresh Engine

12 hours ago in Project Cars

After getting a new engine, our turbo Miata project rolled onto the dyno.

Reader's Ride of the Week: Proud Prius

16 hours ago in News

Paul Sexton takes the checker while staying green.

Your Projects: Throwback MR2

16 hours ago in News

Take a Toyota MR2 AW11, add mud flaps and a cool livery, and you're ready for rallycross.

You Need This: Wacky Mazda RX-8

16 hours ago in News

Out with the rotary and in with the luxurious Infiniti drivetrain.

From the GRM Vault: Subaru Bounces Back

3 days ago in Articles

Proof that motorsports involvement can lead to corporate success.

Your Projects: SkinnyG's V8 Pontiac Firefly

3 days ago in News

Talk about a sleeper.

Calling All LS-Powered Track Machines to Holley LS Fest

3 days ago in News

Holley LS Fest will once again feature the Grassroots Motorsports Road Course Challenge.

From the GRM Vault: A Dual-Engine Honda CRX and 10 Other Cool Cars Hit the Dyno

4 days ago in Articles

Back in 1997 we put 11 cool cars on the dyno in search of horsepower and truth.

You Need This: Our Mustang GT CAM-C Project Car

4 days ago in News

Soon for sale: Our Mustang GT autocross project.

Deal of the Month: Christmas in July!

4 days ago in News

Get a year of our magazine for just $10.

Your Projects: Smith College's $2017 Challenge Miata

4 days ago in News

Smith College is entering the Grassroots Motorsports $2017 Challenge


Grassroots Motorsports Magazine

Subscribe Today

Also get your instant access to the digital edition of Grassroots Motorsports Magazine!

Learn More