Regear rear diff and no front?

Sorry for my ignorance first of all. Does the front diff need to be regeared if I regear the rear to 4.88? I have a 2018 trd4x4OR. I was at a shop and the guys at the counter started to argue about not having to do the front when only changing the rear gears. I seem to think both the front and rear gears should be matching ratios.

 

Yes, you need to do both. Find a different shop.

 

Yeah. Both need done. If you don't and flick on 4Hi, you're going to grenade something like the front diff, T case, U joint, etc.

 

4X4 needs front and rear to match.

 

run far away from that shop.

 

I figured but I'm no expert. Reason for being at the shop was to buy the seal for my front driver axle. I asked about the arb locker in the front and that's where it went sideways.

 

Holy moly! What shop? People need to avoid that place. Yikes. Tell me you don't know how a transfer case works without telling me you don't know how a transfer case works.As soon as you engage 4WD the different gear ratios caused by not upgrading the front will cause the transfer case to grenade. At any given speed your rear wheels will be spinning more than your front wheels. The transfer case cannot handle this - it is meant to take the power from the motor and transfer it evenly to the front diff and the rear diff. There is no clutch.

Whomever was behind the front counter needs to be given a broom and a piece of duct tape over his mouth. The only way that solution works is if you never engage 4WD until you upgrade the front gears. As soon as you engage 4WD you are hosed.

 

you are correct
avoid that place like the plague!

 

The guy who said the rear only can be done was new. He was in the back and just walked out front. Nice guy but i figured he was wrong. The other guy was trying to explain why but after a few minutes I just wanted the seal and left. I wont throw any shops under the bus like that but maybe the shop really does to keep him busy working in the back restocking like he was.

 

Oof… Double check the part number on the bag. They may have sold you an oil filter gasket for a nissan frontier.
“It’s the same size so it’ll work” -That shop probably

 

Sounds to me like the kid had good intent and wrong information. I obviously wasn't there, but your story to me sounds alot like a probably young/new kid who had bad information (maybe assuming all tacomas were the same from doing a 2wd model job previously), and he probably just wanted to save you money thinking that the other guy was trying to upsell you. This goes 2 ways: the kid has your best interest in mind (with bad info, but he's teachable), or the kids just plain dumb. Use your noodle whether you wanna give the kid a shot at touching your truck. Your judgement is the only one that matters here.

 

I gave them the part number haha and yeah all I did was ask about the front arb locker. I wanted to know how much to do both at the same time.

 

I mean he's not wrong if you're building a big mud bogging truck, some will purposely run ratios front/rear that are largely different from one another. Some crawlers too but we're talking beefier drivetrains/solid axles and its done for a reason.

I'm guessing he wasn't focused in on that little niche though and just didn't know what he was talking about, yes you should make sure they match

 

Also if for some really esoteric project they were to run different tires front and rear. Yeah don’t trust that place. Both diffs need to have the same gears otherwise.

 

Don't take this the wrong way, it is banter for fun. But, if we're going to elevate to a place of authority, let's be correct and cover all the bases. Our assumptions for this discussion will be a part-time transfer case and equal sized tires front and rear.

First, let's agree there is some tolerance when it comes to matching gear ratios, final total ratio (including tires), and binding. Having mismatched ratios and engaging 4wd will not automatically *explode* your tcase or any other drivetrain component. It will depend on the strength of your drivetrain, the amount of mismatch/bind, and the amount of grip available at the tires.

Let's also agree that perfectly matching ratios are perfectly matched when driving straight, but as soon as you turn (done all the time when wheeling, even on higher grip surfaces) you are effectively mismatched for the conditions and binding. This binding while turning is no different than straight line mismatched ratios and neither will automatically explode or break anything.

There is known acceptable ratio mismatch tolerance in 4x4 vehicles. A couple percent one way or the other and you'll never know the difference. Also note, if your axle ratios are exactly matched, but your tire rolling radii are not matched (due to different pressures and/or normal loads) you actually aren't matched when in comes to final total ratios including the tires. This also introduces binding.

It is also known that intentionally mismatching ratios by running a 5-10% higher front axle ratio than rear axle ratio is a performance improvement for certain types of wheeling. First, the mismatch created by the gears is 'unwound' towards making final total ratios match when turning since the front axle wants to travel a wider arc (i.e. longer distance) than the rear axle. So this gear mismatch effectively creates drivetrain bind going straight but can reduce it when turning. Also, it has the effect of jacking the vehicle lower on tractive surfaces under power which aids climbing and is especially effective in rockcrawlers and competition buggies. Running a 7-9% axle ratio mismatch has been found to be very powerful in these types of rigs.

Moral of the story, 4x4 drivetrain bind is inevitable and slightly mismatched ratios work. Also, I'm sure the guy at the shop that this thread is about is a total moron.

Finally, let's describe a part-time 4x4 transfer case correctly. They are not designed to transmit engine power evenly to the front and rear diff. It is designed to transmit equal speeds to the front and rear diff and by its design will bias torque to the axle with traction. Since torque can vary at the outputs but the speeds can't, that means the power (P=T*w) is not required to be equal either.

 

Much more eloquent than my response. My compliments.

 

But u can definitely regear ur rear end, n then not use ur 4wd until u get the front done though????? Is that correct ????? Sorry im new on here jus signed up