Bongo Fury

Just a thought...

If the transfer box (I've also heard it called a viscous coupling) is there to transfer power to the front wheels if (and only if?) the rear wheels are slipping, then I can't immediately see why matching the tyre circumferences front to rear really matters that much. For the front wheels to be powered, the rear wheels should already be slipping.

I'm not proposing to do anything other than fit a matching set and rotate as necessary, but as a thought exercise, can anyone correct my thinking?

Matt

I don't think the difference in rotational speed of the front & rear propshafts needs to be much at all for the viscous coupling to lock up, thus causing bad tyre wear.

We're still running the factory spec 195/70 front - 215/65 rear setup and have done for a couple of hundred thousand kilometres - but we do get uneven wear on the fronts. Whether four new identical tyres would help I haven't tried yet

Think of it this way Matt, the gearbox turns the rear propshaft at a speed of x. This then turns the rear wheels at an averaged speed of x as well. (If, due to steering, the right is >x, then the left is <x, but the nett result is still a speed of x).

So, the van moves along the road at a speed of x.

The road pushes the front tyres round at a speed of x, and the front axle turns the front propshaft also at a speed of x. The 2 halves of the viscous coupling thus are both rotating at the same speed (x). There is no difference in speed. The VC is doing nothing.

Taking an extreme example, if the rear wheels are on ice and turn at a speed of x and they don't propel the van forward then the front wheels don't turn, and neither does the front propshaft. In this case, the 2 halves of the VC are going at different speeds (x and 0). The fluid in the VC is then subjected to a shear situation. It will then turn from a fluid to a solid (ish), and push the front propshaft round at a speed of x (or very close to it)

In reality, it doesn't take more that a fraction of a rotation for the VC to shear and lock up. Jack up a single front wheel and try turning it to feel the resistance.

So, if you have small front tyres and big rear ones then when you drive along with a rear propshaft speed of x, the motion of the van will push the front axle round faster than x. The VC will feel this difference, and lock up which will try to slow down the front axle. As you turn, the inside wheel with less weight on it will scrub to even out the speeds.

This means that with smaller front tyre radii, the VD is always under strain, and the tyres are always scrubbing.


Note : I know I've over-simplified things by comparing rotational speed and linear speed, but you see what I mean!

Thanks, Dave. You've painted a good picture.

I wasn't taking into account that the forward motion of the vehicle would turn the front propshaft and under normal conditions would set up a balanced 'steady state' in the viscous coupling. I understand that a difference in wheel circumference will set up a bias and 'steady state' now represents a shear situation.

I should probably have googled viscous couplings, but what you say makes sense to me now.

Cheers,
Matt

Muzorewa wrote:I don't think the difference in rotational speed of the front & rear propshafts needs to be much at all for the viscous coupling to lock up, thus causing bad tyre wear.

This also makes more sense to me now I read it again with the understanding that it's the wheels that drive the front propshaft.

Thanks.

I've got a 4wd Volvo XC70 & was reading this post (extract below) from the Volvo Owners Club Forum yesterday in response to a member who has just destroyed his second transfer box in two weeks. To answer the question - yes, it can make a difference.

"Early cars with the viscous coupling are very sensitive to differences.
This is how your AWD works: your viscous coupling has a fluid inside which when both ends are rotating at the same speed remains viscous, however, when there is a difference in the speeds of the input and output shafts, which means the shafts from front to rear of the car, the fluid starts to, lets say harden. Once the fluid has hardened then the two shafts are essentially locked together and rotate at the same speeds. Thus transferring power from the front wheels to the rear as now they are all locked together. (it's not quite as cut 'n dried as that, the fluid "drags" even if it's not fully locked and transfers some power to the rear if there is a very small speed differential)
So, you're in snow, the front wheels start to spin, the viscous coupling "locks up" and you get power to your rear wheels and you drive off. Perfect.

Now imagine a small difference in diameter of the tyres, even 3mm. Drive at 50mph and each end of the car the tyres are rotating at a different speed.....
So your viscous coupling starts to "think" the front wheels are slipping and starts to lock up as described above, and is designed to do. Once it has then there is mechanical wind-up in the prop shaft between the front and rear.

If you're on mud or snow then there will be a slight slippage at the tyres and the force is dissipated, but you are on tarmac so this can't happen.
The next weakest point will break to relieve the strain, which unfortunately, and expensively, is either the angle gear or if you're lucky the splined coupling between angle gear and gearbox.

If you think this isn't a known problem, just google for Range Rover mysterious crashes from the '80's. I'm sure it will bring up many hits. They changed the transfer 'box design to a viscous coupling and then found the vehicles were having crashes on motorways where they would spin for no apparent reason. Turned out to be related to tyre pressures / tyre diameters etc locking up the viscous coupling and causing mechanical wind up in the transmission. They'd made the 'box so strong though that eventually the wheels would slip to unload the stresses and spin the whole car round....

In 2003 (AFAIK) Volvo dropped the viscous and went with a Haldex unit, which works in a totally different way and is computer controlled, so doesn't suffer AS MUCH. Later than 2005 the Haldex was refined and they are much less sensitive to a small difference in tyre sizes, but if you're smart you still play safe and rotate your tyres and change as a set!
I swap mine front to rear every 6k or so, just to be sure

Your problem could very well be tyres. Different brands of the same size do actually vary in size: one makers 215x65x16 won't be exactly the same as another makers, fitting one brand to the front and another to the rear could well be the cause.
If I were you I'd jack the car up, take off a front and rear and put them side by side to compare them. Also bear in mind that if one has a softer tyre wall / different pressure then it will compress more and be a different "rolling radius".

Brilliant. Thanks, Simon.

The best debate on the subject that I have seen on here. Well done all. Happily, I have a 2WD, so my nightmares are confined to getting stuck in a remote muddy field. There are none of those on my daily commute to work ha ha

A RWD as always been OK for me ..

I've never had a four wheel drive .. and In fact ... I've only ever owned one front wheel drive motor a Vauxhall Cavalier 16v DOHC ..

Thats a good article from the Volvo mag.

Not so sure about it being Range Rovers though - they have a proper 3rd diff but the Freelander destroys transfer boxes and diffs with notoriety! I've seen a diff casing smashed through at the pinion due to a failed VC.

Most problems come from owners only fitting 2 new tyres at a time and causing different rolling circumferences

I think the later old-shape Range-Rovers had a vc, we had an '89 model without selectable diff-lock so assume it had a vc.

Buy an Alvis Stalwart for some proper axle windup

Most problems come from owners only fitting 2 new tyres at a time and causing different rolling circumferences

This seems to add up to my own exeperience. I was lucky in that my 4WD came over from Japan shod in 215 65R15's all round and when the time came to renew them Wheelquick sold me four tyres of the same size so I have never had a problem.

From posts that I have read it would appear that people running the Mazda specified setup of 195x15 on the front axel and 215x15 on the rear axel often change just two tyres at a time as some say that the tyres on the front wheels tend to wear down faster. If the above is true then this must create more strain on the diff.

This is quite a reminder as most people have a spare wheel with the minimum of tread on it. Therefore it should only be used to get you to the nearest garage as prolonged use may damage the diff.

Tony

Yes, incorrect tyre sizes are notorious for destroying 4x4 units. The Bongo transfer box (the viscous coupling is just a component within the transfer box Matt) is a very beefy unit, but I have seen VC's destroyed by incorrect tyre sizes. I had an X3 in here recently who appeared to have dreadful bearing noises from the rear of the vehicle, on inspection the rear tyres were brand new & the owner hadn't twigged the noise started after they were fitted , sent him off to have 2 new fronts & all was well.

On the same subject but in a different vein, many cars that have odd tyre sizes on the same axle (new spare fitted with the other tyre very worn for example) will often upset the auto gearbox. Many transmissions have what is known as 'cornering strategy' a great idea that stops the transmission changing up halfway round a corner if you have to lift off a little, in the past the 'box would change up & could upset the balance of the car if being driven hard . Most modern car transmission ECU's use the ABS sensors to monitor if a vehicle is cornering or not. So incorrect tyre on the same axle can cause really odd shifting problems as the trans ECU thinks the car is cornering when it isn't & gets itself in a fog, most common vehicle for this was the BMW E39 5 series up to about 2003.

A porche technician told me of a warranty problem with a 911 which wouldn't engage 7th gear - turns out it's similar to your story - one new tyre fitted fooled the 'box into thinking there was a fault and it stayed in 6th "for safety reasons"

Thought you might be interested to read the outcome of the post on the Volvo Owners Club forum I referred to previously (full article here: http://www.volvoforums.org.uk/showthread.php?t=142512)

"I just got my car back with another brand-new Volvo transfer box fitted. The first thing I just did was to measure the tyres. I measured the RADIUS of the FRONT wheels (from the ground to the centre of the wheel) and they're both more or less bang-on 12 inches. Then I did exactly the same measurements on the REAR wheels - both more or less bang-on 12.7 inches. SO... here is a BIG difference between front and rear tyre sizes - The fronts are about 1.4 inches diameter smaller than the rears.

All 4 are the exact same size (215/65 R16 98H). The fronts were fitted around 4 months ago and have covered maybe 1k miles. The rears were fitted around 2 months ago and have covered maybe only around 250 miles (I've been without the car for 4 out of the last 8 weeks).

HERE's THE THING:- The front 2 are Goodyear Eagle and the rear 2 are Yokohama.

Originally there were all 4 Goodyear Eagles fitted. I drove these all to pretty low tread and then put 2 new Goodyear Eagles on the front (September 2011). This was around the time I would have began to notice a whine at over 50 mph - which I (and my mechanic friend) put down to front wheel bearings - but which I now realise could have been the transmission struggling with different size tyres front to rear.

At the beginning of November my mechanic friend fitted 2 new front wheel bearings AND a new pair of Yokohama's on the rear. Then the noises seemed to have changed - and they also seemed to be noticeable at lower speed.

A couple of hundred miles later and on the M25 at 6.30 am on a Sunday morning. No traffic so I put my foot down. 5 miles later... BANG - Transfer box wrecked.

New transfer box fitted and I got the car back just before Christmas. Around 200 miles later New Years Eve and again on the M25 - loads of smoke and horrible noises - Transfer box wrecked AGAIN...!

Because the Yoko's on the rear have only done a few hundred miles I've decided to replace the Goodyear's on the front with identical Yoko's. These should arrive today and will be fitted immediately. Then I'll measure all 4 again and pay particular attention to tyre pressures.

I'm off to Norwich on saturday so please wish me luck.

Having now got a rudimentary understanding of how my viscose coupling works, I certainly won't be doing any mix-n-match with the tyres again."