drive train noises

[Andrew b Lurker Find all posts View Profile Close Andrew b]

Hi I get some rather unplesant drivetrain noises clonking not too dismiilar to tyres rubbing on the chassis (checked not the issue) when unloaded driving fowards turning very low speed or reversing, backing down a boat ramp with my 2 ton Alloy crusier... normal driving no noise had several auto mechanics have a look but we couldn't simulate the noise for an opinion and they couldn't offer an explination...

I think the noises came shortly after I swaped the tyres around was running 265/70/16 in front and 265/75/16 rear about a 1 inch differance... now the other way round..

thought is that when the smaller tyres were on the front the load was delivered to the rear wheels no noise

with the smaller tyres on the back the load is now bing delivered to the front wheels.. The load is forcing the chain to dislocating in the transfer case trying to jump the gears...

Remedy replace smaller tyres with 265/75/16... (make 4 same size tyres) even up the load across the driveline

Have a mix of tyres as want BF goodrich AT, they don't come in 70 series... and two of the four orignials were in good condition... figured that with a centre diff size differential didn't matter... and I guess it didn't untill I changed tiryes around..

Will replacing the smaller tyres with new 265/75/16 balance up the load... thoughs anyone?

[Corigator Avid PP Poster! Find all posts View Profile Close Corigator]

Re: drive train noises

Hi Andrew,

Good to hear from a Kiwi.

My understanding is that you should always match the size of the tyres on a 4WD, as unmatched tyres will cause undue wear on the centre diff. (And the issue you have).
Running unmatched tyres means the transfer case will be slipping all the time, which it is not really intended to do. It will cause even more issues when you lock the transfer case as the tyre circumference difference means a tension will develop in the drive chain, known as wind up. While you are on gravel this is not a major problem, but rocky terrain will be less forgiving. It will also mean that in locked mode two wheels will always be slipping - defeating the purpose.

Bottom line - Always match your tyre sizes all round, and really your spare as well.
Cheers

[Andrew b Lurker Find all posts View Profile Close Andrew b]

Re: drive train noises

Thank you Corigator for your response much appreciated...

Understand tyres should be the same size... And that miss sizing them causes increase wear on centre diff... My estimation was that as the diff was designed to manage mismatches of tyre speed in the normal course of use (even with same size tyres...) my intent was relitively short term use till the smaller tyres were worn sufficently to be changed....

What I wasn't expecting was the missmatch of tyres (assuming that is the cause) would create this unhealthy chatter noise when drive train load moves from the being engine driven (normal driving) to being weight driven off load driving (backing down the boat ramp)

I think my question is has anyone else had simular noises. Was the noise the transfer case chain chattering from missalignment ..

if so did they have miss-sized tyres

If so did changing the tyres to same size same type (pretty much accepting I will have to do this...) remedy the chatter or am I up for a chain replacement because it is worn (note milage is 120,000kms) out as tyre size doesn't matter..

Is that type of noise from some other issue... have checked the park brakes for binding, universal joints have all been replaced and shafts ballanced... don't think it is diff gearbox or wheel bearing related.. no nises when on hosts or general driving... just off load when moving very slowly and most noticable when backing down the ramp in reverse gear again no load... (from engine)

I understand that without hearing the noise advice is at best a guess I am looking for some insight as to my thought process. Does it makes sense... and other ideas on what might cause this issue... would prefer not to spend the sub $1k on new 265/75/16 just yet if real issue is mechanical fault...

please offer thoughts all contibutions welcome...

[Andrew b Lurker Find all posts View Profile Close Andrew b]

Re: drive train noises

I bit the bullet and spent up on the needed new tires so now have the same size allround... and to date the noise has gone... so clearly the center diff doesn't tolerate the mismatch of larger tires at the front smaller at the rear.. does this indicate that the center diff isn't a true differential? Or have I just got a worn trasfer case chain that jumps the gears off load....

Car is on a 1500km trip south and back for the girls and the minime... so will see how it goes... thanks for your reads and ideas...