Lower gear ratio increase or decrease stress on driveshaft?

[BIG98XJ]

My dad and a buddy of his have different opinions on this, what do yall think?

 

[M. Lake]

Lowering the gears will take the stress off the driveline and put it in the axles. Raising the gears will put the stress on transmition and t-case.

 

[dave]

numerically increasing the gearsets int he rears will cause more stress on the driveshaft due to the fact that it will spin faster.. thus unless it is not balanced perfectly or a ujoint needs grease etc etc etc it will show wear more quickly being that it is spinning faster and undertaking more stress...

 

[GI-John]

numerically increasing the gearsets int he rears will cause more stress on the driveshaft due to the fact that it will spin faster.. thus unless it is not balanced perfectly or a ujoint needs grease etc etc etc it will show wear more quickly being that it is spinning faster and undertaking more stress...

Not flaming, but how in the heck would your driveshaft spin faster with new gears in the axles?

Unless you change your transmission or transfer case final ratio, the DS speed should be the same as before.

Or maybe it's me putting out bad info....

Someone school me if I'm wrong.

John

 

[ChuckD]

It depends, when you regear you axles to match your tire size. Are you going for more of the OEM ratio or a lower one.

Right now I'm at 4.10 with 33's, originally I had 3.07 with roughly 27" tires. If you do the math, I am slightly lower than the original stock ratio and therefore my DS spins a bit faster. Do this create any extra stress or strain, probably not, it can accelerate an existing problem with improper driveline angles.

 

[BrettM]

less stress, higher speed

simple;

less stress because lower gearing places more stress on everything downstream in the system and the stuff upstream has no idea what is going on. same reason it is a bad idea to put a Klune in front of an NP231, you will increase the stress to the 231.

higher speed because the driveshaft will need to spin the pinion a greater number of times to get the same amount of rotation at the axle shafts. this is assuming you only change gears and not tires. if you get tires that are the same percentage larger as the percentage that your gears are lower, than the driveshaft will spin at the same speed.

 

[XJsuperman]

I still don't see how you get the higher speed.....if you are running larger tires....with the proper gear ratio your drive shaft RPM's.....relative to the UPSTREAM gearing of your t-case and tranny ...will remain the same......in other words your driveshaft runs at a constant speed BASED ON your t-case and tranny gearing, and ENGINE RPMs which is IN FRONT of the driveshaft. So your engine RPM's go up...the tranny RPm's go up thus causing the DS to speed up........it all goes in line and cannot run backwards..if regearing your axles made your DS spin faster then the rest of your drivetrain would have to also spin faster....again....it's all relative to what comes BEFORE it.

Your drive shaft RPM's with stock tires and 3.07 diff gears while the engine is at 2500 RPM's is going to be the exact same as your driveshaft RPM's with 4.56 diff gears and stock tires and engine RPM's @ 2500.....no if's and's or but's.....the ONLY thing that is different with this scenario is that with the first case your vehicle will be going MUCH faster at 2500 RPM's than with the second ....but again the DS RPM's remain the SAME because the enigne RPM's are the SAME.

 

[BIG98XJ]

damn this is a tough one. anuone else got opinions

 

[raceready]

CRAP, I was just writing something and i hit the RESET FORM button by mistake.........................................

 

[Eagle]

This is not a tough one, it's an easy one. It's all about mechanical advantage.

Start by defining parameters. Your question asked about lower gear ratios, so to respond to that question we ignore tires. Assume the tires don't change, only the axle ratio.

The actual work to be done is done at the rear wheels. The power to do that work is generated at the engine. Anywhere in between that you introduce gears, you introduce "mechanical advantage." More gear in the diffential (higher numerical, meaning a change from 3.07 to 4.10, for example) means that the drive train now has more mechanical advantage (think "leverage") in transmistting power from the drive shaft to the wheels. More mechanical advantage = less stress.

That's all there is to it. Don't make a simple concept difficult.

 

[raceready]

You are overlooking one important thing.

The reason WHY you want to run bigger gears. By themselves, numerically higher gears do not cause more stress. .


BUT trying to pull a 5000 lb boat and trailer up the mountains is going to cause more driveline stress by default, So will the 38inch swampers because of the weight and added grip. or climbing up rocks.

So, depending on what your application is, yes you will probably be putting more stress on the vehicle by regearing. You will have to deal with the fact that your engine is probably going to be higher up in the power curve when you need it most, and that means you have more power and torque available to break things with.

You don't gain more HP or TORQUE from your engine by installing them. The difference is that you get that HP and Torque much earlier.

 

[Slip Kid]

lower gears make less stress. the tire and the ground is like another gear though, so it raises the ratio. the lower the overall ratio, the more ds speed, and less ds stress.