From digest.v7.n1100 Mon Feb 9 15:34:29 1998
From: James Moran <bavarian_at_geocities.com>
Date: Mon, 09 Feb 1998 10:18:03 -0500
Subject: Re: WTB: M5/M6 sway bars
Stock M6 anti-roll bars are 21mm/16mm front/rear. The '88 M5 stock bars are
25mm/18mm front/rear. Dinan and Suspension Techniques make upgrades that are
25mm/19mm front/rear and adjustable. Note that adding stiffer anti-roll bars
will put more stress on their mounting and attachment points:
- The attachement of the front links to the strut tubes (little hat shaped
pieces) have been known to break. Boxing these points on one side helps here.
Another solution is to run a metal strap from the hat to the spring pan.
- The chassis mounting points of the front bar can break. In fact, the stock
mounting point on the '88 M5 (25mm front bar) is strengthened in comparison to
stock E28 and E24 cars (including the M6). Dinan bars come with the
strengthened mount. Use it.
- The subframe mounting points on the rear bar can break. Both Pete Read and
Walt Selva have had this happen. I believe the Dinan bars come with a
reinforcement for these as well.
Now, to do a little analysis:
>From "How to Make Your Car Handle" (Fred Puhn, HP Books), via Pete Read on the
Mformation digest, roll bar stiffness is proportional to diameter to the fourth
power. Therefore, going from 19/16 to 25/18 means the front bar will be 200%
stiffer (three times!) [(25/19)^4 = 2.997] and the rear bar will be 60% stiffer
[(18/16)^4 = 1.601]. If you choose a 19mm rear bar, it will be 98% stiffer (2
times). Either of these options will result in more understeer. Increasing
front stiffness relative to the rear (via springs, bars, or tire pressure)
results in more understeer.
With the adjustable sway bars (25/19) you can soften the front bar (towards the
outermost mouting hole) and/or stiffen the rear bar (towards the innermost hole)
to try and reduce the understeer and tailor your handling to your preference.
Another thing to consider is that anti-roll bars are just springs that come into
play when the ride height between the left and right side wheel on the same axle
is different. Using stiffer than stock springs also reduces roll, so these
could be another option. However, stiffer springs work all the time - like
going over freeway expansion joints or speed bumps where the ride height, left
to right, is the same and bars would have no effect.
> From: Scott Buchanan <87535is_at_bellsouth.net>
[ Help ]
> Date: Sat, 07 Feb 1998 21:41:01 -0500
> Subject: WTB: M5/M6 sway bars
> Hello again everyone. I'm looking for a set of M5 or M6 sway bars to
> put on my '87 535is. I understand the stock setup I have is 19mm/16mm
> front/rear. I'm looking to go to 25mm/18-19mm front/rear. If anybody
> has these or can point me in the right direction I'd appreciate hearing
> from you. Thanks in advance.
> Scott Buchanan
> Charlotte, NC
> '87 535is