Sway bar/suspension advice

[Lamar Vannoy]

When I my 135i it was lowered with stretched tires on 19" VMR wheels (staggered). I was quick to put factory springs back on an replace the 225/245 tires with 235/265 and have since added M3 control arms, sub frame bushings, and M Factory LSD. With the control arms and centering pins removed I only got to -1° camber and still have under steer.

I know lowering the car would get me more camber and help but with the 19s, I rub the fender liners pretty easy on good bumps so I don't see that as an option w/o 1M body. I was reading that a front sway would help a lot and was looking for E93 M3 with thought that I'd do rear later. Lately I've read that rear might be the way to go and did help to reduce some under steer in my FWD Jetta.

[1and1]

Dinan fixed camber plates are an easy way to pick up from 0.7 to 1* of negative camber inexpensively and easily, after that there are adjustable camber plates for the front you can consider.

Lots more skilled suspension folks than me here, so I'm sure others will chime in while you're googling those...

[BimmerAg]

Stiffer rear bar will definitely bias your car more towards oversteer. It won't eliminate understeer but it'll help. Bigger front bar will usually make understeer worse

[John_01]

In the case of the 1 series you get a lot of camber loss due to body roll. Most of the roll stiffness in the car comes from the front sway bar, so upgrading it will reduce the loss of camber in hard cornering. Unless you have something else significantly modified in your suspension setup, I suggest that a stiffer-than-stock front sway bar will help to reduce the understeer and make the steering feel more responsive in a road car situation. If you want really neutral handling in a track day situation, then you will want to upgrade the rear sway bar as well. You just have to be sure that is what you really want as you can more easily provoke lift-off oversteer in a car with neutral handling.

[tracer bullet]

Originally Posted by BimmerAg

Stiffer rear bar will definitely bias your car more towards oversteer. It won't eliminate understeer but it'll help. Bigger front bar will usually make understeer worse

Typically this is true but not 100%, and with our cars it seems to be the reverse. Suspensions are complex... See comment between ours, this is what tends to be true from racer's actual experiences with our cars.

[Lamar Vannoy]

Thanks for the responses. I don't think I even knew about the Dinan camber plates and had only seen the $300+ ones. I just ordered them and am going to hold off on the front bar for now but will likely end up with it was well. Hoping to get 1M body done someday but $$$$

I had my guy use 1M alignment specs after I put control arms in and camber was just shy of the range which starts at -1.1°

[BimmerAg]

Originally Posted by John_01

In the case of the 1 series you get a lot of camber loss due to body roll. Most of the roll stiffness in the car comes from the front sway bar, so upgrading it will reduce the loss of camber in hard cornering. Unless you have something else significantly modified in your suspension setup, I suggest that a stiffer-than-stock front sway bar will help to reduce the understeer and make the steering feel more responsive in a road car situation. If you want really neutral handling in a track day situation, then you will want to upgrade the rear sway bar as well. You just have to be sure that is what you really want as you can more easily provoke lift-off oversteer in a car with neutral handling.

Originally Posted by tracer bullet

Typically this is true but not 100%, and with our cars it seems to be the reverse. Suspensions are complex... See comment between ours, this is what tends to be true from racer's actual experiences with our cars.

I think this can be true, but only up to a point. After installing my H&R solid front bar I've definitely noticed more understeer on track and during autocross. On corner exit specifically, I used to be able to get the back end to step out just a bit as I got on throttle, now the front just pushes wide under the same circumstances. Maybe the hollow M3 bar would be a better compromise and reduce roll enough to prevent camber loss, but not be too stiff to induce additional understeer.

[tracer bullet]

Originally Posted by BimmerAg

Maybe the hollow M3 bar would be a better compromise and reduce roll enough to prevent camber loss, but not be too stiff to induce additional understeer.

Do your remember the diameters, I wonder how they compare? If they are similar then the solid should be much stiffer, if the new bar is significantly smaller then they might be pretty close to each other. Solid bars seem a little uncommon, the extra weight vs. strength ratio tends to have manufacturers upping tube diameters and wall thicknesses instead.

Originally Posted by BimmerAg

on track and during autocross

BTW

[BimmerAg]

Originally Posted by tracer bullet

Do your remember the diameters, I wonder how they compare? If they are similar then the solid should be much stiffer, if the new bar is significantly smaller then they might be pretty close to each other. Solid bars seem a little uncommon, the extra weight vs. strength ratio tends to have manufacturers upping tube diameters and wall thicknesses instead.

I don't remember the diameter off the top of my head, I think it was 28mm? I know it's the same as the stock M-sport bar, but solid instead of hollow. Also, the stock bar has an area in the center where the diameter necks down to ~26mm, whereas the H&R doesn't. Both of these things make the H&R bar significantly stiffer than the stock bar (and probably too stiff for otherwise stock suspension).

Here's a good rundown of some E9X sway bars that shows that the H&R is ~40-50% stiffer than stock, while the M3 bar is ~5-15% stiffer
https://www.turnermotorsport.com/t-t...-e92-sway-bars

[MightyMouseTech]

Originally Posted by BimmerAg

I don't remember the diameter off the top of my head, I think it was 28mm? I know it's the same as the stock M-sport bar, but solid instead of hollow. Also, the stock bar has an area in the center where the diameter necks down to ~26mm, whereas the H&R doesn't. Both of these things make the H&R bar significantly stiffer than the stock bar (and probably too stiff for otherwise stock suspension).

Here's a good rundown of some E9X sway bars that shows that the H&R is ~40-50% stiffer than stock, while the M3 bar is ~5-15% stiffer
https://www.turnermotorsport.com/t-t...-e92-sway-bars

A hollow or solid bar makes almost no difference to the torsional stiffness.

[tracer bullet]

Originally Posted by MightyMouseTech

A hollow or solid bar makes almost no difference to the torsional stiffness.

As usual, it depends. What is the wall thickness of these bars? 5mm? 10mm? If it's a thick wall i.e. 10mm then agreed there's practically no change. If it's say 5mm then it could be 10-20% or so. If it's 1mm (obviously it isn't this) then it'd be a huge difference.

I'm basically agreeing w/ you, but there's a chance that if stock bars are thin enough walled you could see a decent bump in stiffness going to solid.

Even if the H&R bar is not as much stiffer as Turner claims, it should still be at least a little stiffer, interesting it showed up giving more understeer.

Anything else change at the same time? Any chance they are not apples / apples comparisons i.e. different tracks at different speeds / turning radiuses? This would change the suspension geometry (simply put how far the car leaned over) and potentially change things like camber, etc. making it a really hard comparison.

[BimmerAg]

Originally Posted by MightyMouseTech

A hollow or solid bar makes almost no difference to the torsional stiffness.

It depends on the wall thickness of course. Material very close to the center of the bar has no effect on stiffness, because the polar moment of inertia is related to the diameter to the 4th power. But if the wall is fairly thin, then there can be large differences in the stiffness of the bar.

For example, a hollow 28mm bar with a 4mm wall thickness has a polar moment of 71.4 cm^4, while a solid bar of the same diameter has a polar moment of 96.5 cm^4, making it 35% stiffer.

[BimmerAg]

Originally Posted by tracer bullet

Anything else change at the same time? Any chance they are not apples / apples comparisons i.e. different tracks at different speeds / turning radiuses? This would change the suspension geometry (simply put how far the car leaned over) and potentially change things like camber, etc. making it a really hard comparison.

Unfortunately I haven't had a chance to go back to the same track since my sway bar install. There is a small local track that does time attack events once a month where each car gets 3 heats of timed laps. There is usually about an hour or so between heats so next time I'd like to try swapping bars at the track to get a back-to-back comparison of how they perform quantitatively and qualitatively.

[MightyMouseTech]

Originally Posted by BimmerAg

It depends on the wall thickness of course. Material very close to the center of the bar has no effect on stiffness, because the polar moment of inertia is related to the diameter to the 4th power. But if the wall is fairly thin, then there can be large differences in the stiffness of the bar.

For example, a hollow 28mm bar with a 4mm wall thickness has a polar moment of 71.4 cm^4, while a solid bar of the same diameter has a polar moment of 96.5 cm^4, making it 35% stiffer.

Hollow sway bars are usually quite thick wall. They do it to reduce weight without really reducing the torsional stiffness.

4mm wall thickness would just buckle. They are more like 9 or 10mm thickness.

[bNks334]

Originally Posted by MightyMouseTech

Hollow sway bars are usually quite thick wall. They do it to reduce weight without really reducing the torsional stiffness.

4mm wall thickness would just buckle. They are more like 9 or 10mm thickness.

You guys need to also consider that the M3 and stock sways have multiple bends in them, the sway bar end tabs are curved, and they taper in the middle. The H&R bar is not just solid, it is more effective in every way while also being 1mm thicker (27mm).

I have yet to see anyone put a bigger front sway on an otherwise stock 135i and not complain about MORE understeer. Yes, on the street it will feel more agile, but everywhere else the car is going to push more. Camber loss is a completely exaggerated phenomenon. Tires don't just magically lose all of their grip when the camber curve goes positive. You can gain a bit of traction by keeping the tires at a slight negative camber (they are designed to grip better at a camber angle), but to claim camber loss is a problem on a 135i is nonsense. The suspension geometry is designed very well and losing 1.5* camber at max compression travel (stock suspension) is not going to make your tires useless especially considering BMW gives these cars some static camber from the factory. The main contributor to understeer is the staggered tire setup and the massively stiffer front sway vs the non-existent rear sway.

These cars come off the factory floor setup to put power down as best as possible to keep the driver from power over-steer. It's more of a drag setup than anything else.

[Lamar Vannoy]

Well, I found someone local parting out a M3 so I'm probably going to pickup the 26.5mm front bar from him and possibly a 15mm (red in color) rear bar. Hopefully that will be a good combo. Ideally I'd like a somewhat neutral setup and am unlikely to be in a lift off oversteer situation. I did notice a bunch of body roll in a good turn over the weekend too...

[chris_flies]

I've got a pretty neutral set-up. It power oversteers happily in the wet (predictably and controllably, but still limited by the eDiff-icult), it's extremely easy to balance it out by using throttle and trail braking, and it'll understeer if you decide on being over-ambitious...

Check my build thread link in my signarture for more details.

The basic rundown is shocks, springs, Dinan plates, M arms, Eibach 28mm front sway, ECS 15mm rear sway, and HD end links...