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E30 understeer after fuel cell


theweis
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Hey guys,

Any pro-tips on getting rid of understeer in our 325is E30 appreciated.

Adding the fuel cell made 2 hour stints possible but has made the car downright horrible to drive.

 

the thing that seemed to help the most was disconnecting the front swaybar...

 

Next we are going to try to increase rear ride height, and use stiffer springs in rear than front.

 

Shift battery forward?

 

Thanks guys.

 

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12 minutes ago, theweis said:

Hey guys,

Any pro-tips on getting rid of understeer in our 325is E30 appreciated.

Adding the fuel cell made 2 hour stints possible but has made the car downright horrible to drive.

 

the thing that seemed to help the most was disconnecting the front swaybar...

 

Next we are going to try to increase rear ride height, and use stiffer springs in rear than front.

 

Shift battery forward?

 

Thanks guys.

I would first try to stiffen up the rear.., the fact that disconnecting the front bar helped indicates that stiffening the rear will help. Thicker rear bar or springs. I know Tyler was playing with different spring combinations last season in his e30 (which has a fuel cell), message him and he might have some wisdom for you.

 

Do you know your F/R weight balance? I would expect that you are pretty close to 50/50 now, so weight balance probably isn't the issue (it's not "bad") so much as your setup needs to be recalibrated/tweaked after the changes ... e30s are a touch nose-heavy with stock tank location.

Edited by enginerd
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Does the handling get better as the fuel load goes down?

 

I will add in - where in the turn are you getting understeer?  Need details like: entry, mid, exit, power-on, power off, high-speed, low-speed, etc.  

 

Did you change anything else when you put in the cell?

Edited by NigelStu
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1 hour ago, theweis said:

and use stiffer springs in rear than front

Because of the different motion ratios of the F/R suspensions, the rear spring is always (well, should be) of a higher spring rate than the front. For example the H&R Race springs: 315lbs/in front, 570 rear.

 

With 5 gallons in my 22 gallon cell, I'm still 55% front weight bias. My car's not a very good representation of the typical E30 though, so you may be closer to 50/50. Reconnect your front sway bar and add roll resistance to the rear.

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16 minutes ago, ross2004 said:

Because of the different motion ratios of the F/R suspensions, the rear spring is always (well, should be) of a higher spring rate than the front. For example the H&R Race springs: 315lbs/in front, 570 rear.

 

With 5 gallons in my 22 gallon cell, I'm still 55% front weight bias. My car's not a very good representation of the typical E30 though, so you may be closer to 50/50. Reconnect your front sway bar and add roll resistance to the rear.

The motion ratios are important to understand, and you are right about alot of the kits out there. It doesn't always mean the rear should be much stiffer though. It just depends.

 

My street M Roadster has a mix of suspensions (BMW used E36 front, and E30 rear). The front motion ratio is .96 and the rear is 0.67, with weights being dead on 50/50 with me in the driver seat. Many of the kit springs out there are much stiffer in the rear. I started with a 500/500 setup with stiffer bars from and back. It was pretty good, but I wanted to be able to put power down faster (300hp). In the end, I unhooked the rear bar and went for a 500/600 setup.

 

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It sounds like something is binding or blown.

 

Disconnecting the front swaybar is kinda a nuclear option to get rid of understeer IMHO (in a chassis like the E30).

 

I am guessing blown front strut or something binding.

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7 minutes ago, wvumtnbkr said:

Disconnecting the front swaybar is kinda a nuclear option to get rid of understeer IMHO (in a chassis like the E30).

 

What do you mean by "nuclear"?  I've debated giving it a shot in our car (FWD Mazda 626) but with a stiff rear sway bar we are already lifting the inside rear on a lot of corners so thinking it wont gain much.

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7 minutes ago, thewheelerZ said:

 

What do you mean by "nuclear"?  I've debated giving it a shot in our car (FWD Mazda 626) but with a stiff rear sway bar we are already lifting the inside rear on a lot of corners so thinking it wont gain much.

Well, on a front engine, rear wheel drive car, it usually creates a bunch more problems than it solves.

 

It might work okay on a FWD car.  I have never raced or even driven a FWD in anger.

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9 minutes ago, thewheelerZ said:

What do you mean by "nuclear"?  I've debated giving it a shot in our car (FWD Mazda 626) but with a stiff rear sway bar we are already lifting the inside rear on a lot of corners so thinking it wont gain much.

It's like you have a gopher problem (understeer), and you use TNT to blow up their holes on the golf course (disconnect sway bar), and your gopher problem has now been solved, yay!! (understeer went away), but what unintended consequences have you now created? (destruction of scenery, body roll, PETA now protesting golf course, less grip everywhere, golf course downtime while repairs are made to the greens, etc. etc.). Like, perhaps you no longer have understeer, so you think all is well, but the car is now turning slower lap times than it did before :o.

Edited by enginerd
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3 minutes ago, thewheelerZ said:

Ok, gotcha!  Makes sense.

 

@wvumtnbkr You never rent cars?!

Not front drivers.  No bias against them, I just haven't had the option to turn some laps in a front driver.

 

I have driven front wheel drive cars, just not in "anger" or in a spirited manner (my wife has!)

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1 minute ago, wvumtnbkr said:

Not front drivers.  No bias against them, I just haven't had the option to turn some laps in a front driver.

 

I have driven front wheel drive cars, just not in "anger" or in a spirited manner (my wife has!)

 Haha, Im just kidding around.  The first thing I do in my rental Malibu/Altima/Sentra/etc is an attempted burnout and then drive it in anger until it gets returned!:D

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1 hour ago, red0 said:

 

I ran a rental Nissan Versa at VIR full the day before the Chumpionship this year. The brake pads were smoked, and the rotors warped. 

I may or may not have returned a rental minivan near US129 (the Dragon) with smoke pouring from the brake pads. :) The bikers thought it was hilarious ...allegedly.

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6 hours ago, thewheelerZ said:

 Haha, Im just kidding around.  The first thing I do in my rental Malibu/Altima/Sentra/etc is an attempted burnout and then drive it in anger until it gets returned!:D

I once got 12mpg with a chevy cruze eco...

 

The problem with a fuel cell in a E30 is it moves the weight out behind the rear wheels.  

 

Thats going to change how the car feels.  In addition, your going to feel different dynamics as the fuel burns off.  If the cell is cheap or poorly baffled, the sloshing fuel will make the car drive terribly also.

 

I have never driven a cell'ed E30 - personally, I question the safety of them.

 

 

So I can't comment on how the weight change affects handling, but in theory it should make the car oversteer more.

 

 

 

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11 minutes ago, u-turn said:

I'm not familiar with e30s but I have never cured any handling problems by raising the ride height. 

 

I have.  If the car is sitting too low you lose all handling and grip almost disappears when you hit the bump stops. 

 

My track Mustang has adjustable ride height and was assembled by a local race shop for owner #1 and he ran some fast times at the local track.  Owner #2 thought it looked cool with the ride height dropped down as low as it could go.  I'm owner #3 and going through a turn when the back end step out on me out of nowhere.  I raised up the rear to set the shock travel at 2/3-1/3 and the car was good again.

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@HuggyWith the cell out back, I can imagine it trying to cantilever the front wheels off the ground. So would that mean more understeer in steady state, but with the weight way out back could cause more snap oversteer when the back starts moving around?

 

Would be interested to hear if the handling gets better as fuel gets low. 

 

If raising the rear height is to put additional weight on the front wheels, does anyone have any idea how much weight transfer you can see by doing that?

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1 hour ago, thewheelerZ said:

@HuggyWith the cell out back, I can imagine it trying to cantilever the front wheels off the ground. So would that mean more understeer in steady state, but with the weight way out back could cause more snap oversteer when the back starts moving around?

 

 

This makes sense if you think of the car in a "statics" frame of mind (AKA sitting still)

If the car is a cantilevered beam with the support at the rear axle, moving weight rearward will take weight off the front axle by pivoting about the rear one.

 

When you look at it from a dynamics situation, the opposite is true.

 

The simplest example is comparing a P911 to a Civic.  The civic understeers. The Pcar oversteers.

 

 

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2 hours ago, thewheelerZ said:

If raising the rear height is to put additional weight on the front wheels, does anyone have any idea how much weight transfer you can see by doing that?

Very little weight moves , less than 25# per axle.  @ 2 in of change.  The rear roll center moves up and the roll couple line changes , Rolling onto the outer rear tire  more, reducing understeer. but also reducing inside rear tire load. 

Keep the car as low as you can .

   Double check the alignment values, add rear spring rate   600 spring = about 350 wheel.  I thought that my car was good on 500 rears and 450 fronts. 

Edited by flyinglizard
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9 hours ago, Ron_e said:

 

I have.  If the car is sitting too low you lose all handling and grip almost disappears when you hit the bump stops. 

 

My track Mustang has adjustable ride height and was assembled by a local race shop for owner #1 and he ran some fast times at the local track.  Owner #2 thought it looked cool with the ride height dropped down as low as it could go.  I'm owner #3 and going through a turn when the back end step out on me out of nowhere.  I raised up the rear to set the shock travel at 2/3-1/3 and the car was good again.

If he is raising the rear to get off the bump stops, I agree. 

I thought he was raising the rear thinking he was going to transfer weight.

 

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2 hours ago, flyinglizard said:

Very little weight moves , less than 25# per axle.  @ 2 in of change.  The rear roll center moves up and the roll couple line changes , Rolling onto the outer rear tire  more, reducing understeer. but also reducing inside rear tire load. 

Keep the car as low as you can .

   Double check the alignment values, add rear spring rate   600 spring = about 350 wheel.  I thought that my car was good on 500 rears and 450 fronts. 

I'm curious on your conversion of 600# springs to 350 #/in rate at the wheel for the E30. How did you derive it?

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55 minutes ago, JBgotM said:

I'm curious on your conversion of 600# springs to 350 #/in rate at the wheel for the E30. How did you derive it?

The spring perch is located roughly halfway between the trailing arm hinge and the wheel hub (exact must be .58 ish according to his numbers). So for every X displacement of the wheel, the spring compresses ~1/2 X

Thhe front springs have a conversion of closer to 0.9 IIRC (spring rate to wheel rate) because of the geometry of the macphearson struts. 

Edited by enginerd
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1 hour ago, enginerd said:

The spring perch is located roughly halfway between the trailing arm hinge and the wheel hub (exact must be .58 ish according to his numbers). So for every X displacement of the wheel, the spring compresses ~1/2 X

Thhe front springs have a conversion of closer to 0.9 IIRC (spring rate to wheel rate) because of the geometry of the macphearson struts. 

For displacement, yes, but for rates you have to square the motion ratio.

 

https://en.wikipedia.org/wiki/Motion_ratio

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