[geordiepaul]

According to BMW fitting a tow bar isn't allowed on F31 and F33 models when 20" wheels are fitted. One would assume the wheels meet the same requirements regardless of size.

I only want to fit a bike rack for two bikes

[sahajesh]

It's a rarity - someone with 20" wheels who wants to put their car to a practical use ;-)

BMW are flummoxed!

[MWA]

Could it be the assumption of carrying load which usually adds a load to the rear of the car (Suspension) causing the car to sit lower and scraping against those rather large wheels with less room to play?

[RalphyBMW]

If its only for a bike rack then I wouldn't worry anyway.

Which towbar are you going for? They aren't tricky to fit yourself.

[xenon]

No it's not that - 16" wheels & tyres have exactly the same overall diameter as 20" wheels because the 20" wheels have an extremely low profile and 16" have tall sidewalls, but the overall is exactly the same (otherwise speedo etc would be compromised)

The issue is that the suspension is a combination of the spring and the tyre - on 20" wheels the tyre provides little, if any, compliance and therefore all the work, combined with heavy wheels, is done by the springs. Add the nose weight of a trailer and you're over the limit.

[Tengocity]

It's because the 405m wheels are not rated for the rest axle load that could occur when towing.
It's nothing to do with size of overall wheel and tyre as this unchanged whatever wheels you have, assuming you've got the correct tyres on!

[Ti Rich]

Originally Posted by xenon

The issue is that the suspension is a combination of the spring and the tyre - on 20" wheels the tyre provides little, if any, compliance and therefore all the work, combined with heavy wheels, is done by the springs. Add the nose weight of a trailer and you're over the limit.

But the forged 20" wheels are lighter than both the 19" and 18" wheels so I doubt it is that.

Weights shown here http://felgenkatalog.auto-treff.com/

[xenon]

Ok not the weight then but the compliance of the whole package.

[Tengocity]

I'm pretty sure it's not 20inch wheels per se but the 405M and 664M wheels.
Tyre compliance is equally crap across all the runflats!

[geordiepaul]

I have a Thule bike rack from my previous E90 which had a detachable tow bar, I had planned just to shove it in the boot but the car is too nice to shove a filthy MTB in the back.

Was just looking into at this stage. Will never fit anything more than a bike rack.

I thought all BMW wheels would have had to have the same TUV approval but I take it that's not the case.

I almost specced the factory Towbar when ordering the car but it's was a pricey option.

[geordiepaul]

Originally Posted by Tengocity

I'm pretty sure it's not 20inch wheels per se but the 405M and 664M wheels.
Tyre compliance is equally crap across all the runflats!

Bummer....

incidentally I find the ride to be perfectly fine. Always had firm riding cars so maybe I'm just used to it.

[Tengocity]

Originally Posted by geordiepaul

I have a Thule bike rack from my previous E90 which had a detachable tow bar, I had planned just to shove it in the boot but the car is too nice to shove a filthy MTB in the back.

Was just looking into at this stage. Will never fit anything more than a bike rack.

I thought all BMW wheels would have had to have the same TUV approval but I take it that's not the case.

I almost specced the factory Towbar when ordering the car but it's was a pricey option.

I did as couldn't be arsed with hassle of a retrofit. Works great! Pretty much just for bikes too.

[RalphyBMW]

I've only fitted mine for the same reason - http://f30.bimmerpost.com/forums/sho....php?t=1037668

[Lorcan]

Are your wheels forged?

[geordiepaul]

Originally Posted by Lorcan

Are your wheels forged?

They are the 405m's

[Lorcan]

So yes then, according to BMW: http://www.bmw.co.uk/en_GB/topics/ow...ryId=8612.html

Although the alloy in a forged wheel is formed under pressure, unlike a regular cast wheel, and this makes the alloy itself "stronger" it is often the case that forged wheels are then machined to be lighter, with slimmer spokes etc. This is partly to get a reduction in gyroscopic effect and inertia which can result in quicker steering and improved acceleration but is also sometimes just to achieve a slimmer design.

Unfortunately this also means that the resulting slimline wheel can end up with a lower load rating (in kg) than a regular cast wheel.

[HighlandPete]

Excluding the bike rake application in this topic, I think we all know we can spec' up a car and build in fragility.

BMW have had enough trouble with wheels over the past few years. Putting limits on application makes a lot of sense, to protect themselves from warranty issues, if nothing else.

HighlandPete

[geordiepaul]

Originally Posted by HighlandPete

Excluding the bike rake application in this topic, I think we all know we can spec' up a car and build in fragility.

BMW have had enough trouble with wheels over the past few years. Putting limits on application makes a lot of sense, to protect themselves from warranty issues, if nothing else.

HighlandPete

I agree. I was just wondering as I had no idea what the reason was