Tires: chipping & chunking on tyres, wheel base model, wheel allignment

Question
QUESTION: I have a long wheel base VW Kombi TDi front wheel drive 2005 model. I am experiecing chipping and chunking on the front wheel tyres. I swapped the front tyres to the back and vice versa on 14 000 km and then had chipping & chunking (c&c) on the then front tyres after 27 000 km (ie tyres were 13 000 km on the front). I replaced them and put the new tyres at the back and the rear tyres (which already had a 14 000km stint at the front) at the front again. After 37 000 km (additional 10 000km) they also c&c.

It is the first time in my whole driving career of 38 years that I am experiencing c&c on my tyres. My driving habits have not changed, the roads I travel on have not changed and the way I look after tyre maintenance ie checking wheel allignment, balancing, tyre pressure etc is done at the same devel of vigilance.

I suspect that VW's tyre and related specs for the long wheel base is the same as for their short wheel base model. If so, this could possibly explain the c&c as the LWB is heavier and the required Ackerman Angle dimensions for a longer vehicle should be different.

What is your view, comments and solution to the above?

Many thanks for your assistance.

ANSWER: Wouter,

Did you use EXACTLY the same tire on your Kombi when you replaced the first set?

With the increased emphasis on fuel economy, many vehicle manufacturers are specifying tires with lower rolling resistance - and with that comes poorer traction, wear and / or chip and chunk properties.  This is normally not a problem except for certain locales where the road surface is ... well.... how do I say this?  .... ah ...... different.  There's a whole science on pavement, but it not only is mostly ignored (because of costs!), but some locales would have to import stone and sand - and that is usually not the most politically feasible thing to do.

While I agree that changing the wheelbase should result in a change in Akerman, changing Akerman is not easy. (Plus we need to know that they didn't do that, which is going to require some detective work.)

So I would go after a different tire.  

I'm a little vague on this, but I think there is something unusual about the tire size for the Kombi.  Something along the lines that the vehicle specifies a "C" type tire - meaning a "Commercial" tire in Euro-speak.  This is equivalent to the US "LT" type tire.  You have to be very careful about this.  Load carrying capacity is the backbone for avoiding tire failures. So be sure to match the load
index and maximum pressure capability.




---------- FOLLOW-UP ----------

QUESTION: Initially I replaced only two tyres (on 27000km where they have done 14000 km at the back and 13 000 km in the front). The current tyres that c&c'd on 38 000 km spent 14 000 km in the front, 13 000 km in the back and now 11 000 km in the front again. The current back tyres has only done 11 000 km at the back, and when replaced I had to make them the same as what I had previously ie being Michelins. Michelin is offering to replace all tyres with a new model tyre which is eventually going to replace the current tyre model that I have now on the car.

I have the folloing concern: Logic tells me that there is a mismatch between tyre and VW spec of what the character of the tyres should be. According to Michelin they are within the parameters set by VW and VW maintain that it is mainly a road/driver issue. Obviously if I have not changed my driving habits/roads that I travel on I can discount the latter.

The cagey and scary part of all this is that VW is not prepared to divulge the specs in terms of tyre capabilities required, measurements on toe in, camber, ackerman angle etc etc. Why?

Surely I am not the first one with this problem. I came accross at least 8 VW owners of LWB vehicles in our National Park when on holiday, all with c&c problems. For some of them an uphill battle is not worth the candle and they surrendered to the idea of replacing tyres much sooner. I calculated that this will result in replacing effectively 5 tyres where in normal use only 4 would be effectivly replaced. This cost ca R 12 500 in additional tyre cost over the period that one will normally own the vehicle.

How can one start the detective work and come to a worthwhile conclusion?

I have a set of detailed measurements on the vehicle done by a sophisticated wheel allignment and tyre dealer which I wanted to attach for your info. Unfortunately I do not have access to .gif, jpg,jpeg. I can only forwrd it in pdf format, most probably by normal e-mail if required. Please let me know.

Many thanks for your troubles and effort so far.

Kind regards,

Wouter van Zijl

Answer
Wouter,

First, some of the specs that a vehicle manufacturer specifies are indeed a trade secret that their competitors would love of know - and tire specs are one of them.  Besides, tire specs are highly technical and usually involve a series of tests (generally specified by a test number) and a level of acceptable (usually expressed as a percent of a specified control tire).  This would not only make the specs incomprehensible to the average person, but even tire engineers would need to know the capability of the control tire and have a good feel for how tires perform on the tests specified - and then have to prove it by conducting the tests.

Having said that, the real problem is the rolling resistance (which relates to fuel economy) of the tire spec'd.  That is causing a decrease in the chip / chunk resistance.  VW, of course, is currently specifying tires with different performance parameters than previously (Car manufacturers are notorious for changing specs and only partially because of changes they perceive in the market) and, of course, the current tire meets their CURRENT acceptance criteria.  It's also possible they do not even test for chip / chunk resistance.

Again, I do not think the alignment is the problem - but because the alignment settings are published, there is no secret there - it's just that you'll have to do the engineering work to figure out what the akerman is.  But don't bother - I don't think there is any gold to be mined there.

I would suggest that you either:

1) Accept the offer that has been made on replacing your tires.  The problem here would be that the new tires might have the same chip / chink resistance as the current tires or maybe even worse depending on what VW is currently specifying.

2)  Find a different tire that is not an OE spec tire.  Replacement tires typically are not as good for rolling resistance (fuel economy) but should have better levels of chip / chunk resistance.  Be careful to get tires that are propely sized and with enough load carrying capacity (load index)