Changing to tires with different radius front and back

[BoMadsen]

Since my self-built camper ended up being a "bit" too heavy for its 3,5t sticker, I am in the process of upgrading it to a 4,6t. One of the things I need to change is the tires. They need to be changed from the standard 235/65R16 to a 205/75R16 front and 285/65R16 back

I got a new set of wheels and had them fitted yesterday, and they fit really well and all was good...until I actually tried to drive the thing...then ALL lamps went off and the car went into limb mode. Apparently the standard software does not like wheels with different radius front to back...

The mechanic thinks we should just upgrade the fronts to a radius that matches the rears, but since the once I got is the standard size I already have enough challenges getting the car certified as a 4.6t as it is, I would much rather use the correct wheels.

So does anyone have any experience upgrading/changing the software to accept different size wheels?

 

[Eric Experience]

Bo.
Over weight campers are common. Running odd wheels is a problem for the electronic stability system, over weight causes a lot of other problems with the drive train. What is installed that is so heavy and why can't you remove the offending items? Eric.

 

[BoMadsen]

Hej Eric,
Wheels are not the only things that are going to be changed, also springs, shocks, brakes and some other bits and pierces resulting in a van that for all intents and purposes are a 4,6t ?

The van is a high xlwb that we use for racing motocross, it has water tanks, shower, generator, large awning etc. Getting it to 3,5t loaded with passengers is not possible.

Odd wheels are standard on the 4,6t model, so I know it can be done. Just looking for advice on what is required ?

 

[Kajtek1]

You need to compare rpm of the tires. Per experience, ESP can accept up to 5% difference, but above that the 1st thing to go is CC and in drastic case - a discotheque and limp like you experienced.
If your comparison comes close to 5% - sometimes changing tire inflation can give you 1%, but if you have more - you need to change the tire.
Any knowledgeable tire man would know that, but when you talk to mechanic, or barber, you are not going to go very far.

 

[autostaretx]

You could also inquire of folks who are adept at modifying the ECM programming to see if it's possible to tell it about different radii (or diameters, if you must).
The T1N's programming only allowed one entry, which was expected to be correct for all 4 (or 6) wheels.

--dick

 

[Kajtek1]

You could also inquire of folks who are adept at modifying the ECM programming to see if it's possible to tell it about different radii (or diameters, if you must).
The T1N's programming only allowed one entry, which was expected to be correct for all 4 (or 6) wheels.

--dick

When I am not much of programmer, I don't think that will be possible. Some forum wisdom say that modern MB use any of 4 wheel speed sensor for speedometer reading.
So would program allow driving on different rpm wheels, the speedometer would jump back and forth with steady driving, depends what sensor it takes at the time.
The best comparable here are AMG vehicles, who come with staggered wheels.
When wheels have different width, they still have to have matching rpm

 

[autostaretx]

If (when originally written) they wanted to include staggered wheel sizes, it's trivial to do.
Provides flexibility (instead of having to pay Bosch (again) to implement the function later).
They're already doing the diameter-to-circumference-to-revs-of-wheel for road speed ... it becomes only one extra step in the logic to pull one of 4 wheel sizes from a 2- or 4-entry table instead of from a single common "one size".

But "simple logic" does seem to escape MB at times...
--dick

 

[BoMadsen]

Seeing as the different wheel sized I have mounted are standard on the 4,7t I am sure the ECU can handle it. I was just hoping that someone had tried it, and could shed some light on what was required to make it work

 

[Cheyenne]

Seeing as the different wheel sized I have mounted are standard on the 4,7t I am sure the ECU can handle it. I was just hoping that someone had tried it, and could shed some light on what was required to make it work

Was the 4.7t a 'Super Single' (or SuSi) by any chance?

If so the change would have to be made by 'Variant Coding' the ECU to let it know it is a different option now. This would likely have to be done by an MB dealer. Have you approached an MB truck and van (not car) dealer yet?

Keith.

 

[Eric Experience]

Bo.
If you are determined to go ahead you can use a device made for digital speedo correction, you would need two of them. If you choose which wheel gives you closest speedo reading then then insert the speedo corrector on the other two wheels. Be prepared for major repairs down the track. Eric.

 

[BoMadsen]

Was the 4.7t a 'Super Single' (or SuSi) by any chance?
If so the change would have to be made by 'Variant Coding' the ECU to let it know it is a different option now. This would likely have to be done by an MB dealer. Have you approached an MB truck and van (not car) dealer yet?

I have just contacted my local MB dealer, and he said that they could reconfigure the software to have the Super-Single option (code RH9 apparently) and it should work. I will try that thanks!