MyE28.com

Cut two coils off the rear springs this morning.

Do you need a roll bar at the track you run at? Some of the tracks in NY flag the car once it gets into the 11's and don't allow it back until it gets a roll bar.

That's funny, I always thought it was once your car dipped into the 10s that every drag strip made you get a cage.

M635CSi wrote:Do you need a roll bar at the track you run at? At least some of the tracks in NY flag the car once it gets into the 11's and don't allow it back until it gets a roll bar.

NHRA rules require a roll bar at 11.50. The track i went to all of last year has NO rules. That said, I intend to have a roll bar fabricated for my own protection.

Todd

T_C_D wrote:Cut two coils off the rear springs this morning.

Nice looking car Todd 8)

Can't wait to see this come together!

What mods can be done so you get a good weight transfer when ya launch?

rundatrack wrote:What mods can be done so you get a good weight transfer when ya launch?

I am fighting weight transfer. I only want minimal weight transfer. Too much causes negative camber and less tire on the pavement. I'll have 9" slicks so traction shouldn't be a problem.

Todd

T_C_D wrote:

rundatrack wrote:What mods can be done so you get a good weight transfer when ya launch?

I am fighting weight transfer. I only want minimal weight transfer. Too much causes negative camber and less tire on the pavement. I'll have 9" slicks so traction shouldn't be a problem.

Todd

your using stock shocks I assume...

rundatrack wrote:
your using stock shocks I assume...

Bilstein Sports in front, Bilstein HDs in rear.

T_C_D wrote:

rundatrack wrote:
your using stock shocks I assume...

Bilstein Sports in front, Bilstein HDs in rear.

All the drag cars that I have seen..well on the american side...have used stock suspension...

Is there a reason that you wanted the car to be stiffer than stock...


Good launch with all the weight transfering to the rear....would be lovely....

I know that they dont have drag suspension yet for bmws...but why not stock?

Read my last response ref independent rear suspension.

T_C_D wrote:Read my last response ref independent rear suspension.

Well good luck...nothing dont internally to your motor...?

rundatrack wrote:

T_C_D wrote:Read my last response ref independent rear suspension.

Well good luck...nothing dont internally to your motor...?

MLS, studs and crank scraper.

rundatrack wrote:

T_C_D wrote:

rundatrack wrote:
your using stock shocks I assume...

Bilstein Sports in front, Bilstein HDs in rear.

All the drag cars that I have seen..well on the american side...have used stock suspension...

Is there a reason that you wanted the car to be stiffer than stock...


Good launch with all the weight transfering to the rear....would be lovely....

I know that they dont have drag suspension yet for bmws...but why not stock?

Downward weight transfer at the rear slows the car and unloads the rear tires. Ideally, we want the car to remain level under acceleration. When the rear of the car "dips", it takes weight off the rear tires. If the front of the car is lifting, it is not moving forward and forward is the direction we want to go.

T_C_D wrote:NHRA rules require a roll bar at 11.50. The track i went to all of last year has NO rules. That said, I intend to have a roll bar fabricated for my own protection.

Todd, you may want to consider consulting with Mark McMahon in Akron on this. Excellent quality work and very reasonable cost. Unless you have someone in mind already.

McMahon's doing the 8-pt cage in my E36 in January.
-tammer

T_C_D wrote:Bilstein Sports in front, Bilstein HDs in rear.

In the past I've sent my shocks to the Bilstein operation in San Diego to have them re-valved. What about increasing the rears to the max jounce setting and the fronts to the max rebound setting. This would probably help the car remain level under hard acceleration.

http://www.bilstein.com/services.php?PH ... 60afae3387

Belisarius wrote:
In the past I've sent my shocks to the Bilstein operation in San Diego to have them re-valved. What about increasing the rears to the max jounce setting and the fronts to the max rebound setting. This would probably help the car remain level under hard acceleration.

http://www.bilstein.com/services.php?PH ... 60afae3387

I agree this would be ideal but this project is going to be as simple as possible. I want to see how it performs with this setup first.

I've never seen side skirts like that before. What kind are they?

Thanks, and good luck with the car. Are you selling any interior parts, or were they trashed?

M635CSi wrote: Downward weight transfer at the rear slows the car and unloads the rear tires. Ideally, we want the car to remain level under acceleration. When the rear of the car "dips", it takes weight off the rear tires. If the front of the car is lifting, it is not moving forward and forward is the direction we want to go.

Well, because this has already been resurrected, I thought I'd ask WTF you were smoking when you typed this. How does weight transfer to the rear of the car (pushing the back end down) take weight off the rear tires?

I agree that you don't want too much squat with the IRS on our cars, but your explanation of why is completely bullshit. Weight transfer is good (because it puts more normal force on the tire and increases the area of the contact patch), but bad on the E28s because you increase negative camber (which decreases the size of the contact patch and changes the shape of said patch).

One solution for this would be running adjustable trailing arm bushings to dial out some camber. Ideally, you'd adapt a dual a-arm setup, which I've been considering for the future.

Skeen wrote:

M635CSi wrote: Downward weight transfer at the rear slows the car and unloads the rear tires. Ideally, we want the car to remain level under acceleration. When the rear of the car "dips", it takes weight off the rear tires. If the front of the car is lifting, it is not moving forward and forward is the direction we want to go.

Skeen wrote:Well, because this has already been resurrected, I thought I'd ask WTF you were smoking when you typed this. How does weight transfer to the rear of the car (pushing the back end down) take weight off the rear tires?

This concept is somewhat counterintuitive so I’ll do my best to explain. I don't smoke, so the answer to your first question is "nothing".
As to "How does weight transfer to the rear of the car (pushing the back end down) take weight off the rear tires?", the answer is that during the time the rear of the car is traveling toward the ground (squatting), the rear tires are being (momentarily) unloaded. For maximum traction, as the car launches and weight shifts towards the rear, an equal force needs to countervail this and push up against the body keeping the rear of the car level and pushing the rear tires down onto the ground. When done properly, the effective weight increase on the rear tires can be 50% or more. In fact, it is possible to create a downward force on the rear tires at launch in excess of the total weight of the vehicle.

Skeen wrote:I agree that you don't want too much squat with the IRS on our cars, but your explanation of why is completely bullshit.

A car squatting at all under acceleration will slow the forward movement and reduce traction.

Skeen wrote:Weight transfer is good (because it puts more normal force on the tire and increases the area of the contact patch), but bad on the E28s because you increase negative camber (which decreases the size of the contact patch and changes the shape of said patch).

Weight transfer pushing down the rear of the car is not how maximum traction under acceleration is achieved. Maximum rear tire traction under acceleration is achieved when rear tires are pushed against the ground by pushing the body of the car away from the ground. Thus, the rear of the car will either remain level or rise as the rear tires load during acceleration. This is easier to achieve on a solid rear axle car than one with independent rear suspension.

M635CSi wrote: As to "How does weight transfer to the rear of the car (pushing the back end down) take weight off the rear tires?", the answer is that during the time the rear of the car is traveling toward the ground (squatting), the rear tires are being (momentarily) unloaded.

How?

For maximum traction, as the car launches and weight shifts towards the rear, an equal force needs to countervail this and push up against the body keeping the rear of the car level and pushing the rear tires down onto the ground.

What would cause an upward force against the body? The car body does not stay level, on any car, ever. Not even a go kart without suspension. Wait, I thought of one: a dragster with tires soft enough they expand due to centrifugal force, thus increasing ride height.

When done properly, the effective weight increase on the rear tires can be 50% or more. In fact, it is possible to create a downward force on the rear tires at launch in excess of the total weight of the vehicle.

Source? I could maybe see this, but I'm curious to see real data.

A car squatting at all under acceleration will slow the forward movement and reduce traction.

I never said anything to dispute the first part, but I don't see how increasing the rear contact patch area would decrease traction (all things being equal).

Weight transfer pushing down the rear of the car is not how maximum traction under acceleration is achieved. Maximum rear tire traction under acceleration is achieved when rear tires are pushed against the ground by pushing the body of the car away from the ground.

That is a distinction without a difference.

Thus, the rear of the car will either remain level or rise as the rear tires load during acceleration.

The force of the tire on the body = the force of the body on the tire (both indirectly). That force will not be so great that it pushes the body up. When's the last time you saw the back end of a car rise up when leaving a stoplight?

This is easier to achieve on a solid rear axle car than one with independent rear suspension.

Usually yes, but not always. It depends on the IRS.

Anyone else want to help me out here? If I'm wrong, I'm wrong, but I don't see this argument.

Think of it this way.

On a car with an automatic, the rear end will squat ever so slightly when you put it in gear. Even more if you load the torque converter with your foot on the brake. This is not due to weight transfer, as the car is not moving or accelerating.

What's happening is the force being imparted on the wheel axles is actually drawing the suspension upwards towards the car. The visual effect is squat, but on a sudden launch or application of power, what's really happening is the rear tires are being lifted upwards, thus reducing the contact patch and decreasing traction. Available traction will increase as the weight of the car settles onto the tires, but at that point the wheels are already spinning, particularly on a manual transmission car.

With an automatic, the suspension is already settled at launch due to loading the torque converter, which is one of the reasons that high powered car equipped with automatics are easier to launch hard.

Jeremy

Jeremy wrote:Think of it this way.

On a car with an automatic, the rear end will squat ever so slightly when you put it in gear. Even more if you load the torque converter with your foot on the brake. This is not due to weight transfer, as the car is not moving or accelerating.

Right, though technically changing ride height will transfer weight, but that's negligible.

What's happening is the force being imparted on the wheel axles is actually drawing the suspension upwards towards the car.

I'd say it's really pulling the body down, because the wheel is not lifting off the ground. The body is getting closer to the ground (squatting). What do you think scales would read if you stood on a stall converter indefinitely?

The visual effect is squat, but on a sudden launch or application of power, what's really happening is the rear tires are being lifted upwards, thus reducing the contact patch and decreasing traction. Available traction will increase as the weight of the car settles onto the tires, but at that point the wheels are already spinning, particularly on a manual transmission car.

Alright, let's consider a shifter kart for a second--no suspension and no preload. What causes a force to make the rear end move up? Centripetal force pushes down on the pavement. Of course, the opposite is true too, but it doesn't physically make the ride height increase. At that point, the vehicle is traveling forward, weight has transferred, contact patch has increased.



Bigger tires don't have a bigger contact patch either, doesn't that blow your mind. Just thought I'd blow this can of worms up altogether.

Skeen wrote:

M635CSi wrote: As to "How does weight transfer to the rear of the car (pushing the back end down) take weight off the rear tires?", the answer is that during the time the rear of the car is traveling toward the ground (squatting), the rear tires are being (momentarily) unloaded.

Skeen wrote:How?

As the back of the car is going down, the tire is coming up to meet it. Not inch for inch, but there is movement and a reduction in tire loading.

M635CSi wrote:For maximum traction, as the car launches and weight shifts towards the rear, an equal force needs to countervail this and push up against the body keeping the rear of the car level and pushing the rear tires down onto the ground.

Skeen wrote:What would cause an upward force against the body? The car body does not stay level, on any car, ever. Not even a go kart without suspension. Wait, I thought of one: a dragster with tires soft enough they expand due to centrifugal force, thus increasing ride height.

OK, we're not talking about street cars here. We're talking about dedicated track cars which are modified with the intention of making it down the 1320 ASAP. Having said that, what is done on a solid axle car is to tie the rear axle housing to the chassis - say about 36 inches forward. As the axle housing rotates when the clutch is engaged, the rotation of the axle housing pushes up on the rear of the car. As the housing rotates and the car lifts, the resistance pushes back and drives the axle housing toward the ground thus increasing tire loading.

M635CSi wrote:When done properly, the effective weight increase on the rear tires can be 50% or more. In fact, it is possible to create a downward force on the rear tires at launch in excess of the total weight of the vehicle.

Skeen wrote:Source? I could maybe see this, but I'm curious to see real data.

Good lord man, I learned this shit like 30 years ago. I have no idea what the source is but of this I'm certain, once you get the concept, you will have zero problem understanding why it works the way it does.

M635CSi wrote:A car squatting at all under acceleration will slow the forward movement and reduce traction.

Skeen wrote:I never said anything to dispute the first part, but I don't see how increasing the rear contact patch area would decrease traction (all things being equal).

You're correct, it doesn't. What happens is that momentarily the contact patch is reduced, tires are unloaded and traction is lost. Once traction is lost, tire loading goes down and traction is reduced further. Game over...

M635CSi wrote:Weight transfer pushing down the rear of the car is not how maximum traction under acceleration is achieved. Maximum rear tire traction under acceleration is achieved when rear tires are pushed against the ground by pushing the body of the car away from the ground.

Skeen wrote:That is a distinction without a difference.

Oh no, MAJOR difference. In the first case, the back of the car goes down and weight comes off the tires. Traction is lost and the tires are unloaded further. In the second case, the torque of the engine is used to lift the body of the car away from the rear axle thus pushing the tires against the ground. The more powerful the engine, the more tire loading takes place. The faster you can lift the rear of the car when you drop the clutch, the more force there is pushing the tire against the ground.

M635CSi wrote:Thus, the rear of the car will either remain level or rise as the rear tires load during acceleration.

Skeen wrote:The force of the tire on the body = the force of the body on the tire (both indirectly). That force will not be so great that it pushes the body up. When's the last time you saw the back end of a car rise up when leaving a stoplight?

You don't see it at stoplights because street cars don't run race suspension and 12" slicks with 10 pounds of air. Back in the day, I saw a picture of a AA/Altered gasser launch so hard all four wheels came off the ground from the momentum. That isn't seen these days simply because the cars are dialed in now and shows like that don't win races so sponsors don't support them.

M635CSi wrote:This is easier to achieve on a solid rear axle car than one with independent rear suspension.

Skeen wrote:Usually yes, but not always. It depends on the IRS.

Actually it's fundamental to the ability of the rear axle housing on a solid rear axle car to be tied to the chassis and its rotation used to load the rear tires.

Skeen wrote:I don't see this argument.

When the student is ready, the teacher will appear...

When the student is ready, the teacher will appear...

I'm listening, am I not?

If your whole argument is based on racecars with 10 pounds of air pressure in the tires, I can see that. The rotational force on the tire makes it expand and increases the ride height. At least that's most of what can visually be seen.

As the axle housing rotates when the clutch is engaged, the rotation of the axle housing pushes up on the rear of the car. As the housing rotates and the car lifts, the resistance pushes back and drives the axle housing toward the ground thus increasing tire loading.

Do you think this is true on the IRS that our cars have? That is where this discussion started. Purpose-built solid rear axle car, yes. But I think the force of weight transfer towards the ground is going to overcome the force upwards on the chassis on almost any standard car.

Skeen wrote:If your whole argument is based on racecars with 10 pounds of air pressure in the tires, I can see that. The rotational force on the tire makes it expand and increases the ride height. At least that's most of what can visually be seen.

As the live axle rotates, the suspension links push up against the body and down against the tire so that actually increases the weight on the rear tires off the line.

M635CSi wrote:As the axle housing rotates when the clutch is engaged, the rotation of the axle housing pushes up on the rear of the car. As the housing rotates and the car lifts, the resistance pushes back and drives the axle housing toward the ground thus increasing tire loading.

Skeen wrote:Do you think this is true on the IRS that our cars have? That is where this discussion started.

I think independent rear suspension has its own unique set of issues which must be addressed to get a car to hook off the line. The front of the differential on the E28/E24 is mounted such a short distance from the centerline of the axle that it won't provide meaningful vertical lift to the car. Also, the E28/E24 differential is essentially fixed to the body at the rear and the axles are independent of it. The rotation of the differential won't plant the rear tires on the ground the way a solid rear axle will. Having said that, the same rules do apply: To the extent the rear of the car body is lifting under acceleration as a function of the rear tires being forced against the ground, traction will improve. And a smaller rear tire will achieve the same level of traction as a larger tire without body lift. We want the narrowest rear tire possible for a given level of traction.

Skeen wrote:Purpose-built solid rear axle car, yes. But I think the force of weight transfer towards the ground is going to overcome the force upwards on the chassis on almost any standard car.

This is correct. And to the extent it does, maximum rear tire traction under acceleration is reduced.

New guss high 10"s
Mabe not on the first run but just give it a few passes

M635CSi wrote:[
I think independent rear suspension has its own unique set of issues which must be addressed to get a car to hook off the line. The front of the differential on the E28/E24 is mounted such a short distance from the centerline of the axle that it won't provide meaningful vertical lift to the car. Also, the E28/E24 differential is essentially fixed to the body at the rear and the axles are independent of it. The rotation of the differential won't plant the rear tires on the ground the way a solid rear axle will. Having said that, the same rules do apply: To the extent the rear of the car body is lifting under acceleration as a function of the rear tires being forced against the ground, traction will improve.

Well, I'd say the length of the trailing arm is more important being that the differential is fixed to the body/subframe.

M635CSi wrote: And a smaller rear tire will achieve the same level of traction as a larger tire without body lift. We want the narrowest rear tire possible for a given level of traction.

Now, why is this? I know a tire's contact patch will not increase with section width given tire pressure and construction are the same with a given normal force, but why would it decrease "body lift."