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Old Sat, Sep-28-2013, 08:10:49 PM   #13
terraphantm
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Default Re: Z4 M Coupe Steering Rack

Quote:
Originally Posted by GnarMach View Post
I agree with you, not necessarily a case of super tight being a good thing. A little tighter wouldn't hurt (like the ZCP rack). But aren't the Z3 racks something very tight like 11.9:1? That's a pretty massive difference considering the stock M3 is 15.4:1.

With a super tight ratio obviously the car will be a bit more jittery/darty and you run into the issue of tramlining becoming more severe. I'd also worry about the effective turning radius with some of these rack swaps, do these other racks have sufficient travel/extension for the E46 M3 chassis?

Either way, a 13.7:1 ratio rack like the ZHP isn't that tight, still totally drivable. I think there is probably a point of diminishing returns for a street car once you start to get into the 12.x:1 ratio range though.
It's hard to compare ratios directly as the suspension geometry differs from car to car. What matters for direct comparisons is the distance the rack moves for every turn of the pinion, and whether or not the ratio is linear or variable.

The M3 rack is a variable ratio rack and moves ~46.5mm/revolution at center. That number increases the farther from the center you get
ZHP rack moves 50mm/rev at center, and also increases away from center.
Z3 non-M rack moves 53mm/rev and is linear, so it's 53mm/rev throughout the entire range of motion

A shorter rack will make the car twitchier at speed, but it's not like the E46 non-Ms are super twitchy in the first place. I suspect BMW's motivation for the long ratio has more to do with the M3's caster angle; it results in more steering effort. That said, I don't find the steering effort with the ZHP rack to be unreasonable
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Last edited by terraphantm; Sat, Sep-28-2013 at 08:15:08 PM.
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