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Discussion Starter #1
Hi everyone. Recently got a Sportage 1.6t awd gt line and it's great. We got the car then next day took it to kia for it's service. They picked up that there is a 3mm difference between front and rear tyres and the wheels need rebalancing. They have said that for the AWD system to work properly and safely that the tyres need to be within 2mm difference. Would this be cause enough for the car to vibrate doing speeds of over 60mph? Thanks.
 

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Most likely the balance problem causing the vibration but the difference could put stress on the drive components and cause an expensive repair. If it was mine I'd just put a new set of tyres on and change them around at the specified interval to keep them all the same diameter.
 

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From my previous experience on other vehicles, vibrations such as you describe are most likely your wheels out of balance. I would recommend getting two new tyres and have all your wheels rebalanced, this will allow you to rotate the wheels to allow a more even wear and reduce the risk of damage to the drive system.
 

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I’m still a bit confused with the tread depths. Would a front and rear tread difference of 3mm really cause issues? When I contacted Kia, they suggested that although there is a 2mm tolerance that it seems it’s more important that tyres on same axle are same tread depth and that from how I interpreted it that it’s not quite as important for fronts and rears to be as strict but I really don’t know. Seem to get mixed details between dealership and Kia etc.
when I contacted my local dealership and asked them to rotate tyres at service they suggested that I didn’t need to and wasn’t something they would usually do.

so any advice is appreciated. Shall I just ask them to rotate anyway? The tyres have done almost 6k miles and there is just over 1mm tread depth difference between fronts and rears so my guess would be that it would be better to swap fronts with rears to ensure the tread evens itself out or by rotating tyres, could this cause any other issues that I currently don’t have?
 

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Not a bad idea to swap front to rear. These cars are FWD 99% of the time, the only time the rear wheels are driven is when there is a slip at the front. This is why front-rear tread depth is important, the difference in rotational speed is how it tells whether the fronts are slipping. If the overall diameter of the fronts is more than a certain amount smaller than the rears, then it will start engaging the AWD system much more often, which will wear it out as it’s meant for occasional use only.

I do agree with the comments about about the vibration being far more likely a balance issue. If you feel this vibration through the steering wheel, I’d start with swapping both fronts to the rear and retesting. You probably won’t feel it any more. Depending how worn the rear tyres are, you could swap just those although you don’t want a situation where the rear wheels are suddenly bigger than the fronts again, or you’ll be back where you started in terms of risking the AWD being engaged much more often.
 

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Discussion Starter #6
Thanks everyone. I'll get some new front tyres and all balanced. Annoying because we've had the car less than a week. Happy with the honesty and service from Kia though. It's a great car to drive.
 

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Discussion Starter #7
Got the front tyres changed and wheels balanced. Found Mercedes only tyres on the front and the wheels needed balancing badly. Still got the noise however. Hopefully just the rear wheels need balancing. What you all think?
 

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Got the front tyres changed and wheels balanced. Found Mercedes only tyres on the front and the wheels needed balancing badly. Still got the noise however. Hopefully just the rear wheels need balancing. What you all think?
I would have got all 4 balanced at minimum, because otherwise you'd never really know!
If front wheels needed "balancing badly" that should be taken as a sign to check out the rear wheels too.
When i had a similar problem (not in a Sporty through), it turned out the back tyres had deteriorated in some way but looked perfectly fine with reasonable thread etc. Changed all 4 tyres, noise disappeared.
 

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It's called castling - means there's flat spots on the tyre. My Optima had it, resulted in a noise like a wheel bearing going, not vibration

Mine was gradually getting worse and hadn't really noticed - colleague got in and asked what the hell that noise was!
 

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On the AWD, just make sure the front tyres have more tread than the rears and you'll be fine.
I've just had new front tyres fitted to mine (7mm). My rears are down to around 4mm, and no problems.

The castling issue, will result in a whine, sounds like a wheel bearing failing.
 

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On the AWD, just make sure the front tyres have more tread than the rears and you'll be fine.
I've just had new front tyres fitted to mine (7mm). My rears are down to around 4mm, and no problems.

The castling issue, will result in a whine, sounds like a wheel bearing failing.
What about the potential for dif wind up. All the articles and posts I’ve read recommend changing all 4 tyres at once and all 4 have to be same make and model with minimal tread difference. You say you have added new front tyres but already the tread difference is 3mm between front and rears. Kia informed me that 2mm is the maximum difference tolerated.

what do others think or is it ok to have a bigger tread difference between front and rears than the 2mm?
 

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Provided the best tyres are on the front then this will not falsely trigger AWD so that is fine. When they wear down to less than the rears it is time to swap them around, once again putting the better tyres on the front. I know that some pundits favour best tyres on the rears. This comes from some entirely spurious reasoning regarding under/oversteer. That advice is out of date and does not account for AWD transmission systems and does not account for nearly all modern cars being designed to favour moderate understeer so they do not "snap oversteer" (exceptions apply - most US muscle cars and many Porches do snap oversteer but AFAIK none of them are AWD)
 

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What about the potential for dif wind up. All the articles and posts I’ve read recommend changing all 4 tyres at once and all 4 have to be same make and model with minimal tread difference. You say you have added new front tyres but already the tread difference is 3mm between front and rears. Kia informed me that 2mm is the maximum difference tolerated.

what do others think or is it ok to have a bigger tread difference between front and rears than the 2mm?
Christ not this again....
Diff wind up doesn't happen from tyre depths. Diffs are designed to allow vehicles to travel off road where differences between axles & sides is WAY more than a few mm of tyre depth.
If anyone had created a diff that wasn't able to handle tyre depth differences (and diff manufactures are not that stupid) then you would have warning stickers everywhere, big bold warnings all over your owner manual and they certainly would not provide space saver spare wheels.
 

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What about the potential for dif wind up. All the articles and posts I’ve read recommend changing all 4 tyres at once and all 4 have to be same make and model with minimal tread difference. You say you have added new front tyres but already the tread difference is 3mm between front and rears. Kia informed me that 2mm is the maximum difference tolerated.

what do others think or is it ok to have a bigger tread difference between front and rears than the 2mm?

Never heard of diff wind up. The differential(s) (one for 2WD vehicles and two for AWD Kias) are there to accommodate the difference in wheel rotation speed when cornering and I can't see how one or two would wind up unless it is a severe limited slip diff and on a very tight corner - when all that will happen is some tyre slip.

Or did you mean transmission wind up? See my post #13
 

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Never heard of diff wind up. The differential(s) (one for 2WD vehicles and two for AWD Kias) are there to accommodate the difference in wheel rotation speed when cornering and I can't see how one or two would wind up unless it is a severe limited slip diff and on a very tight corner - when all that will happen is some tyre slip.

Or did you mean transmission wind up? See my post #13
I’m not too sure what I mean now lol. I’ve basically been told that damage will occur and has done in past (honest John website has a couple of stories on there about it) when people change tyres but they aren’t changed together so one tyre might get changed instead of all 4, this has apparently caused something in the AWD system to burn up and cause thousands of pounds of damage. That’s why I’ve been a bit obsessed with this and trying to get all the facts so that it doesn’t happen to me and thanks to this forum I now know that tyre rotation is a must for these cars to ensure even tyre wear and to avoid this issue.
 

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I’m not too sure what I mean now lol. I’ve basically been told that damage will occur and has done in past (honest John website has a couple of stories on there about it) when people change tyres but they aren’t changed together so one tyre might get changed instead of all 4, this has apparently caused something in the AWD system to burn up and cause thousands of pounds of damage. That’s why I’ve been a bit obsessed with this and trying to get all the facts so that it doesn’t happen to me and thanks to this forum I now know that tyre rotation is a must for these cars to ensure even tyre wear and to avoid this issue.
Yes that's transmission wind up. The KIA AWD system will automatically engage rear wheel drive if it detects that the front (driven) wheels are slipping. If the front wheels are worn about 3mm more than the rears then this causes them to turn faster than the rears and this looks like wheel slip to the transmission system and it can falsely engage 4WD. On a sound grippy surface the difference in rotation speed between the wheels (especially when cornering) will strain the entire transmission train and cause noticeable lurching because the only place the difference in speeds can be released is by slipping the tyres. As well as adding to tyre wear and reducing overall grip this puts large loads on the whole drive train.
 
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