[wakaday]

Next week I'm planning on updating my skinner rear ARB from 12mm to the 15mm 140i ARB. I think the conversion has been completed a few times on the forum successfully. I'm now thinking may be I should also do the front ARB to the 140i update to 25mm (OEM part no. 31356792120) at the same time. I note this ARB is used on 1, 2 and 3 series , but only specified on RWD versions. Firstly does this ARB fit the XDrive derivative and will I really notice the advantage ? I already have the Birds custom springs and dampers fitted.

[Kal101]

Tengocity is your man for this. He's changed various bits inc anti roll bars on his 335xd.

[Tengocity]

The bar will certainly fit. I've had a 28mm front bar and a 20mm rear one at one point.
I'd thoroughly recommend the 15mm rear bar and that's what I have now. The bigger front could be good, but the ratio of rear stiffness to front is what will determine the handling balance. So you're car may corner flatter overall but it will have a smudge more understeer at limit than without it, but the limit could well be higher.

As I was fitting much stiffer suspension with the ACS RS coilovers I wanted to take out some of the stiffness that the really big ARBs gave, to preserve the ride comfort. So it all depends on what you're aiming to achieve.

[rossm]

If you already have stiffer springs I would advise against increasing the thickness of the front ARB unless you want more understeer. Increasing the thickness of the rear one will make the chassis a bit more neutral on fast sweeping bends such as motorway sliproads but you may find the reduction in torsional deflection can also unsettle the car in bumpy corners a bit as well so it's a bit of a trade off. For track use it works very well, but for road use I'd advise against going too big

[Bee Pee]

Originally Posted by wakaday

Next week I'm planning on updating my skinner rear ARB from 12mm to the 15mm 140i ARB. I think the conversion has been completed a few times on the forum successfully. I'm now thinking may be I should also do the front ARB to the 140i update to 25mm (OEM part no. 31356792120) at the same time. I note this ARB is used on 1, 2 and 3 series , but only specified on RWD versions. Firstly does this ARB fit the XDrive derivative and will I really notice the advantage ? I already have the Birds custom springs and dampers fitted.

I sold Tengocity the rear ARB off my M135i and have the front ARB [part no as you quote above] available. If interested, PM an offer.

[Smee]

agfter speaking to tengo i bought the same rear ARB as him can say that the stiffer rear ARB and my lowering springs has sorted the handling out 100%. its perfect now

[wakaday]

I checked some part numbers and noted that the Xdrive version normally gets a thinner ARB on the front than the equivalent RWD model. 135i RWD gets 15/25mm vs. 135i XD gets 15/23.6. So it looks like BMW trying to set-up some more oversteer on the XD version. My current ARBs are 12/22.5mm. Now I’ve heard your advice, seen the RWD vs. XD specs, and experience a little too mush understeer with mine on the limit, I’m sure the 1st step is just to update the rear ARB. I would feel a little more comfortable with the 23.6mm ARB on the front, but let see how it goes.

[Tengocity]

Originally Posted by wakaday

I checked some part numbers and noted that the Xdrive version normally gets a thinner ARB on the front than the equivalent RWD model. 135i RWD gets 15/25mm vs. 135i XD gets 15/23.6. So it looks like BMW trying to set-up some more oversteer on the XD version. My current ARBs are 12/22.5mm. Now I’ve heard your advice, seen the RWD vs. XD specs, and experience a little too mush understeer with mine on the limit, I’m sure the 1st step is just to update the rear ARB. I would feel a little more comfortable with the 23.6mm ARB on the front, but let see how it goes.

I'm not sure BMW is trying to put in oversteer as such, rather that xdrive will lead to more understeer than sdrive, all else being equal, so they're trying to neutralise that to some extent.

[wakaday]

I realise it's been a while since I fitted the 15mm rear ARB to my F31 335d, but I would like to report that I'm very pleased with results. I'm looking into some work on the front end which requires dropping the front sub frame, so at the same time I was considering fitting the a larger BMW front ARB either 23.6 or 25mm. I currently have the 22mm ARB. What do you think would be the best choice?

[Tengocity]

Originally Posted by wakaday

I realise it's been a while since I fitted the 15mm rear ARB to my F31 335d, but I would like to report that I'm very pleased with results. I'm looking into some work on the front end which requires dropping the front sub frame, so at the same time I was considering fitting the a larger BMW front ARB either 23.6 or 25mm. I currently have the 22mm ARB. What do you think would be the best choice?

You might find this calculator useful to help understand what the relative changes are for each option.

http://users.erols.com/dmapes/SWBRCLC.HTM

Looks like the 23.6mm one would be about 32% stiffer, and the 25mm one 66% stiffer.

The change on the rear when going 12mm to 15mm increased stiffness by 144%.

So I'd suggest that going to the 25mm bar would reduce overall roll slightly further and still leave quite neutral handling, but as you get to the limit, the characteristic will be slightly more understeer than now, but probably at a higher speed.

[Rudz]

on the E92 it was always recommended that the front arbs be upgraded, I had Hotchkis in the fronts and it reduced roll significantly

on the F30 the recommendation is quite the opposite ? i.e to upgrade rear arbs ? why is that the case ?

you had to drop the subframe to upgrade Rear arbs on the E92, suspect that may be the reason why most people didn't bother ? Is it the same on F30 as well ?

[Watsey]

Originally Posted by Rudz

on the E92 it was always recommended that the front arbs be upgraded, I had Hotchkis in the fronts and it reduced roll significantly

on the F30 the recommendation is quite the opposite ? i.e to upgrade rear arbs ? why is that the case ?

you had to drop the subframe to upgrade Rear arbs on the E92, suspect that may be the reason why most people didn't bother ? Is it the same on F30 as well ?

Yes, the subframes have to be dropped when upgrading the ARBs. That applies to F & R installations.

I upgraded to Birds (H&R) ARBS front & rear when I had the ACS springs installed and I left them in place when I upgraded again to Ohlins Road & Track. The (far) stiffer ARBs work well with the R&T : car corners very flat but primary & secondary ride are unaffected in the straight ahead.

[wakaday]

I'll most likely go for the 25mm front ARB, it's decent improvement on my current ARB, but not as aggressive as the aftermarket ones. There's some pretty cheap ones on ebay from people upgrading too.

[Kal101]

Originally Posted by Smee

agfter speaking to tengo i bought the same rear ARB as him can say that the stiffer rear ARB and my lowering springs has sorted the handling out 100%. its perfect now

Can I be cheeky and ask what sort of costs are involved.

Thanks in advance

[Smee]

£150 for parts labour about 2 hours

[Kal101]

Originally Posted by Smee

£150 for parts labour about 2 hours

Thanks for that.

Appreciated.

Sounds like money well spent.

[wakaday]

I recently updated the front ARB to 23.6mm (31356792138), the 25.2mm (31356792120) didn’t fit. I've already done the rear to 15mm. The 25mm ARB fouls the drive shafts, that’s ones for the RWD only. Anyway, it’s a good conversion, car feels balanced , corners flatters and most of all the steering feel is more responsive in Comfort mode. I do really like the steering improvement, and I didn’t notice any loss in ride comfort. Good conversion, just a lot work to fit.