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Removing weight == understeer?

So now that my car is back on the road after all of the fixes & upgrades, I've noticed mild understeer on the street. I wasn't expecting this, to say the least.

I haven't weighed the car since I finished the work, but here's what I did and the approximate weight savings:
  • 965 front bumper: approx -5 lbs by eliminating homemade bumper mounts, lighter bumper, shaving pieces of the bumper I didn't need.
  • fiberglass hood: -18lbs (according to my bathroom scale)
  • Oil fitting cleanup ~-1lb - replaced brass adapters with alumiunum, reduced the number of fittings.
  • cut down bumper mounts ~-2lbs
  • RSA door panels - unknown
  • manual aero mirrors - -6lbs

So I removed about 30lbs from the front of the car. I guess it's time to dig out the corner balance scales.

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Old 02-02-2004, 11:37 AM
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Are either one of your sways adjustable. At least you will be able to tune for that.
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Old 02-02-2004, 11:39 AM
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Wow thats great the mirrors are that light. You must have a sensitive but to feel 30 lbs :-)) I suspect you can adjust it out with the weights.
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Old 02-02-2004, 11:44 AM
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Quote:
Originally posted by masraum
Are either one of your sways adjustable. At least you will be able to tune for that.
For the time being, I'm going to soften the front bar and see if that helps.
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Old 02-02-2004, 01:27 PM
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Edit to correct mistype: If you want to reduce understeer, you need to soften the front sway bar or firm the rear bar.
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Last edited by Chuck Moreland; 04-11-2005 at 11:33 PM..
Old 02-02-2004, 03:25 PM
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Re: Removing weight == understeer?

Quote:
Originally posted by widebody911
So I removed about 30lbs from the front of the car. I guess it's time to dig out the corner balance scales.
Something else must be going on. A thirty pound variation of the front end weight happens all the time, based on how much gas is in the tank.

At ~6 lbs/gal, you are looking at the equivalent of a 1/4 tank.

Tom
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Old 02-02-2004, 03:44 PM
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What Chuck said.
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Old 02-02-2004, 03:57 PM
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Quote:
Originally posted by Chuck Moreland
If you want to reduce understeer, you need to firm the front sway bar or soften the rear bar.
i believe you've got it backwards
Old 02-02-2004, 04:02 PM
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Quote:
Originally posted by Phil Gilsdorf
i believe you've got it backwards
That's what I was thinking.
Old 02-02-2004, 04:14 PM
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Whew.....for a minute I thought I was losing my marbles. Stiffening the rear induces oversteer (reduces understeer).
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Old 02-02-2004, 06:15 PM
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Old 02-02-2004, 07:23 PM
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Maybe try a 1 - 2 lbs less air in the front tires.


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Old 02-02-2004, 07:35 PM
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If the weight reduction also resulted in the car riding higher, could have changed your settings a bit too. Check your toe setting in particular.
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Old 02-02-2004, 07:37 PM
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Thom just a thought but your car may have been a tad tight before the weight loss, which would be the reason why it got tighter, most drivers run thier car a tad tight thinking its loose because they can break the car loose,
by the way im just throwing stuff in the air, but these are commin problems,

Car is loose in when you lift the rear grabs causing a push, then you break the tires loose comming out the whole time thinking the car was tight but was actual loose on entry, Kevin
Old 02-02-2004, 08:02 PM
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The symptom I'm having is the front end doesn't steer as well relative to the amount I'm turning the wheel. For example, If I lay the hammer down in a transition rampthe front end wants to keep going to the outside of the turn, so I have to turn in more to keep the car turning.

It makes sense about the alignment settings; I can imagine taking that much weight out of the trunk. 30lbs doesn't seem like a lot, to be effecting these sorts of changes, but I'm just relaying my experience.

I went out to the garage to loosen the front bar; turns out it was already full-loose. As an aside, the removable bumper with the Dzus setup makes playing with the front swaybar a snap! Pop the front bumper off, and I can kneel down and get to the drop links easily! Since the front bar was full-loose, I tightned up the rear one notch. We'll see what happens.
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Last edited by widebody911; 02-02-2004 at 09:27 PM..
Old 02-02-2004, 08:49 PM
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Thom when the car raised from the weight being removed you added more rear percent, a couple percent can mean alot, my car come off the corner better with 52 rear, at 50 it sucks, Kevin
Old 02-02-2004, 10:57 PM
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Quote:
The symptom I'm having is the front end doesn't steer as well relative to the amount I'm turning the wheel. For example, If I lay the hammer down in a transition ramp the front end wants to keep going to the outside of the turn, so I have to turn in more to keep the car turning.
I think that Kevin's on the right track. From your description above, you are not necessarily describing "understeer". Keep in mind that there are different kinds of understeer also; Corner entry, mid-corner and corner exit. Each one has to be addressed differently. Gross understeer is when all 3 are occurring. In your case it sounds like Corner Exit Understeer.

30 lbs can make a difference, but not if your definition of throttle control is to "lay the hammer down". Below the car's limit, 911's tend to understeer. Applying the throttle in a 911 will often make it worse. Conversely breathing the throttle in a corner will often induce some oversteer (aka Trailing Throttle Oversteer) to help rotate the car. Insensitive use of the brakes or throttle will have a drastic affect on a cars handling and cause understeer, oversteer or a combination of them. It's also common for a driver to come in describing that a car is loose if they are unaware that they are trying to drive around a push (or vice versa).

It's happened to me and the description is the exact opposite of what you described. I was racing a rented Spec Racer that was set-up with a pretty strong push. In trying to drive around the problem I was pitching the car into corners and then applying a lot of throttle. The result was that I was often getting sideways on the exit and complaining of corner exit oversteer. So, the driver can often have the biggest impact on the car's handling. Have you tried changing your line so that you are taking a later apex? That alone may fix your problem.

Anyhow, mechanically loosening the front sway bar or tightening the rear sway bar should help some. Reducing the rear toe-in (small changes can make a big difference btw!) or increasing the rear tire pressures can also help. Softer front shocks in low speed droop or stiffer rear shocks in low speed bump can also help. Increasing the rear spring rate (thicker T-Bars) could also help. You might also find that lowering the front a little may help. Finally, if you don't mind moving your suspension mounting points around, raising the rear roll center while adding anti-squat (via Smart Racings slotted trailing arm mounts or a Turbo Trailing Arm update) will address the fundimental problem if your car has really sticky tires and a lot of HP.*

In general, with the exception of the front shocks, I suspect that the "fix" will be either in the driver's seat or in the back since the situation that you are describing sounds like the front end is being picked-up (unweighed) under hard acceleration. The front tires just don't work very well if they are off the ground or lightly loaded.

*Small print: Please note that all of the mechanical changes described WILL affect other portions of your car's handling and as such may have unwanted side affects in some other area of your car's handling!

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Last edited by jluetjen; 02-03-2004 at 06:13 AM..
Old 02-03-2004, 06:09 AM
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