Wheel offset and handling

[Vodbod1]

The susceptibility of the Z3 to tramlining and bump-induced suspension deflection is well documented, as are methods of improving the handling (mainly focussed on replacing the suspension bushes with solid rubber or poly).

As I understand it 17*" wheels are more susceptible than other sizes, and in my case a 1997 1.9 with 7.5x17" standard BMW wheels with 225/45 R17 tyres and standard suspension handling was considerably improved after fitting poly bushes, and raising tyre pressures to 2.5 bar resulted in further improvement - albeit at the expense of ride quality and probably tyre wear.

However, the relative susceptibility of the 17" wheels versus the rest intrigues me, and I note that the wheel offsets are as follows irrespective of rim width - 15" = 47mm, 16" = 46mm, 17" = 41mm. I have searched the threads for discussions on offsets and wheel spacers but they all seem to be more concerned with car/wheel appearance than handling. I assume offset is related to suspension geometry so my question is, would adding a 5mm spacer to bring the 17" offset to more closely match the others result in any noticeable difference to handling? - has anyone tried such (relatively) thin spacers? How do the larger offsets/thicker spacers discussed in many of the forums affect handling/steering effort etc.?

 

[littlefeller]

yes it would, i had a set that had offsets different to the originals, it felt dangerous, in fact i got so concerned by this i re-fitted the original 15 inch( wow what a difference, since then i had swapped the 15 for 18 but kept the offset close (exactly the same at the front, with a difference of just 3 at the back) now it drives fine, still not as sure footed as the 15 inch though. personal i believe this chassis was designed with the 15 inch in mind along with the 15 inch steering geometry. when i first brought my zed it had the original 15 inch which i kept for a couple of years, it wasn't until i swapped them for 16 inch i noticed how bad the bushes were, even with sha88ed bushes it still didn't tramline on the 15 inch wheels.

 

[littlefeller]

oh yeh, welcome to the forum oh and eeerrr pics of your zed are mandatory

 

[Vodbod1]

Thanks littlefella. So you reckon adding a thin spacer (ideally 6mm) to bring the 17" offset up to that of the 15" would provide a worthwhile handling improvement - and for a relatively small investment?

I agree with your comment about the design of the suspension which, I think, was based on the E46 compact which had 15" wheels as standard.

I just find it strange that BMW which prides itself on good handling cars, would fit a wheel which gave such inferior handling (and charged the purchaser an extra grand for the privilege) - but that's marketing II guess!

 

[littlefeller]

at first they were fitted with the 15 inch, i think the 16 inch appeared later, prob due to the changing market. others on here have used spacers with good effect. there is also the idea that the taller side wall of the 15 inch absorbs alot while the narrower 16 and upwards wall tends to transmit into the chassis, this i cant agree with as my side walls are 40 and 35, much narrower than std.

 

[Redline]

Am no mechanical engineer, but I suspect there are a number of things at play here. One of those will be the weight of the wheels/tyres (as part of the unsprung weight) and the distance of that mass on the suspension arms carrying that weight. Increase (or decrease) the mass a little and move it one way or the other is likely to affect the suspension response and therefore affecting the dynamics. I would be surprised if the effect was dramatic but you are talking about quite a few Kgs which are acting as gyroscopes on the end of a metal bar so you never know. You might scoff but put a bike wheel on a metal bar, spin it and then try and move it. You'll get a big surprise!
The original design based on 15" wheels might have been an optimal solution.

 

[jakechan]

yes it would, i had a set that had offsets different to the originals, it felt dangerous, in fact i got so concerned by this i re-fitted the original 15 inch( wow what a difference, since then i had swapped the 15 for 18 but kept the offset close (exactly the same at the front, with a difference of just 3 at the back) now it drives fine, still not as sure footed as the 15 inch though. personal i believe this chassis was designed with the 15 inch in mind along with the 15 inch steering geometry. when i first brought my zed it had the original 15 inch which i kept for a couple of years, it wasn't until i swapped them for 16 inch i noticed how bad the bushes were, even with sha88ed bushes it still didn't tramline on the 15 inch wheels.

Actually it wouldn't. Adding a 5mm spacer to 41mm offset wheels would REDUCE the offset to 36mm, and not increase it to 46mm like the smaller diameter wheels.

 

[GazHyde]

So many things at play here, and in general I think the biggest thing that affects handling is the state to the suspension components rather than the size/offsets of the wheels. What I've learnt over the many years of driving and tinkering with them is that generally you can do what you want size/offset wise if your suspension is sorted. Everything comes from the design of that classic E30 rear sub frame - it just works so well. That said, these cars are never going to drive like a more modern car, but that's where the charm comes from. My Z3 is just more fun to drive than my Z4, which is pretty bland in comparison because the chassis is just so much better.

I've had a set of 18" wheels on both my Z3's for over 6 years since I bought some. I've driven the same set on both, both with and without spacers. I preferred them with 20mm spacers at the rear to line up with the arches, not only from a visual point of view but also that I felt it made the rear end more stable. Front end didn't need spacers as the offset and wheel width were perfect to align on the arches. Yes the ride was a bit firmer, but I didn't suffer from tramlining unless the tyres were getting towards low tread or components needed replacing.

The poly bush argument is something that gets me a little frustrated. What generally happens is that your suspension is soft/failing so you replace it with poly and WOW, all of a sudden your car is awesome. What is always missed is how good your Zed will be for replacing soft/failing components with fresh standard components. Replacing something that is broken/failing with something that isn't will always make a big difference...

Having had the rear sub frame of my 2.2i entirely rebuilt with OE quality rubber (except for the diff bush for convenience of fitting) I can tell you that for standard/fast road driving poly bushes are not necessary. I've taken people for a passenger ride in my 2.2 who've wanted to poly bush everything, and they have changed their mind pretty quickly. Sure, it's personal preference but I think BMW knew what they were doing back then

 

[GazHyde]

Should have added a couple of other points.

Tyres also play a big part in this. Incorrect pressures, old tyres, budget/cheap tyres, mismatched tyres and so on all play a part. We all seem to have our own favourite brands/tyre patterns - some are more suited to the Z3 than others.

Finally, handling is subjective and everyone will have their own thoughts on what is harsh, or soft and even what tramlining actually means. I've driven cars with sorted suspension on roads and still get pulled all over the place. Some of the ruts and trenches on UK roads aren't that easy to see, and even the best cars struggle with them. Driving an old (or even new) car on UK roads is just generally a bad experience.

 

[Vodbod1]

You are, of course, correct regarding the reduction in the resulting offset (ET) if a spacer is introduced. However, what we are trying to adjust is the position of the centre line of the tyre relative to the hub flange, and to achieve this with the 17" wheel (ET41) to that of, say, the 15" wheel (ET47) we need to move the flange outboard by 6mm - i.e. fit a 6mm spacer (to each wheel).

 

[Vodbod1]

Sorry Gazhyde - we must have been posting at the same time!. My only experience is with the 17" wheels and I thought the handling was wayward even when the car was brand new! I have only fitted the poly bushes to the fronts and they have made a big improvement to handling (admittedly fitted after some years so the original rubbers were getting a bit old).

The other point of interest from your post is your experience of 18" wheels (which others reported were also less susceptible to tramlining etc) - do you know the ET for these?

 

[jakechan]

You are, of course, correct regarding the reduction in the resulting offset (ET) if a spacer is introduced. However, what we are trying to adjust is the position of the centre line of the tyre relative to the hub flange, and to achieve this with the 17" wheel (ET41) to that of, say, the 15" wheel (ET47) we need to move the flange outboard by 6mm - i.e. fit a 6mm spacer (to each wheel).

I will say it again, fitting a spacer to each wheel will NOT increase a 41mm offset to 46mm, it will REDUCE the offset to 36mm. You need to study how wheel offset works. The effect using a spacer will be to push the wheel closer to the wheel arch.

Edit, if you want 17" wheels with 46mm offset, some of the Z4 wheels will work. style 200 on my car, only I'm using spacers and thus have an effective offset of 35mm~ish

 

[GazHyde]

My Breyton wheels are 8.5" x 18 x ET35, shown below without any spacers.

Aylesbury 2014 Meetup

• GazHyde
• Dec 14, 2014

Organised by BMBabe. More details here -...

 

[Vodbod1]

Thank you Jakechan. Yes you are correct and I understand the offset issue much better now!

At least it shows the merit of discussing such issues with other members of zroadster.org!

Thanks again for your time.

 

[Vodbod1]

Thanks for photo gazhyde - those wheels look superb!

How do you rate the handling with these?

 

[Vodbod1]

Sorry gazhyde - I think you have already answered my question regarding handling!

Thanks again

 

[oldcarman]

Just to get a different view I will tell you that I run the same sized rims all around on my car with the same size tires as you have. The tires are a cheaper brand and I experience no tramlining at all and the car handles to my expectations. That said, all my bushes are BMW standard. I have 15" rims but no tires but will run them this winter I think due to the ruts in the snow pack road conditions! HTH. JIM

 

[Antm72]

I find 18's fine have had the car on 17's and two sets of 18's the ride on both great.
The first set of 18's a square set up et38 and 8j all round..the rears i spaced 20mm to make it feel more planted and look right.
I replaced the 18's with a staggered set et38 8.5 front and 9.5 et33 rear this enables me to fill the arch with no spacers just a dished rear wheel.
All wheels as mentioned weigh different and behave different as do tyres.
Its always taste and preference with wheels and again is more down to set up than wheel choice.

 

[jakechan]

I find 18's fine have had the car on 17's and two sets of 18's the ride on both great.
The first set of 18's a square set up et38 and 8j all round..the rears i spaced 20mm to make it feel more planted and look right.
I replaced the 18's with a staggered set et38 8.5 front and 9.5 et33 rear this enables me to fill the arch with no spacers just a dished rear wheel.
All wheels as mentioned weigh different and behave different as do tyres.
Its always taste and preference with wheels and again is more down to set up than wheel choice.
View attachment 40490

As far as looks go, relating to wheel size anyway, that's the dogs nuts right there. That's what a Z3 should look like in my opinion, is it lowered too?
Very nice

 

[Antm72]

Yeah the cars lowered on Apex springs lowered but not harsh.