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My BBC progress

General engine tech -- Drag Racing to Circle Track

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prairiehotrodder
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Re: My BBC progress

Post by prairiehotrodder » Sun Jun 09, 2019 4:12 pm

Could any of my speedtalk friends refer me to somebody making big power with a g-body ? I think i need some experienced advice of how to solve this problem. It kinda scared me. I don't want to give away my traction, but i don't want to do another big wheel stand. I'm gonna try and get a video up as well. Not sure how.
Brian
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rustbucket79
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Re: My BBC progress

Post by rustbucket79 » Sun Jun 09, 2019 9:44 pm

Front end travel limiters :wink: preferably adjustable. The less travel before liftoff, the lower the wheelie. (In theory)

F-BIRD'88
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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 11:27 am

If you lower the back of the lower control arm the instant center moves back. This will decrease front end lift and increase body l;ft at the back, on launch.
Add weight to front..
Remove weight from back behind axle.
May now need adjustable upper controlarms and
Mounting points. Yes change front shocks.

You will now still want/need wheelie bars for safety.

prairiehotrodder
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Re: My BBC progress

Post by prairiehotrodder » Mon Jun 10, 2019 11:35 am

UMI is telling me to move the bars up to keep the front end down.

Just to clarify, i'm talking about the back of the lower rear control arms. I have about 4 or 5 bolt holes to move them up or down under the rearend.
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F-BIRD'88
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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 12:06 pm

Draw out the geometry on a sheet of paper. You will see.

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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 12:13 pm

Was any new weight added to the trunk area. Bigger fuel cell? Sway bar
What was the weight reduction at the front?
Tubular control arms etc.
The 90-10's got to go now.
The new engine is making a lot more torque.
May want to control that during initial launch by controling ignition timing during stage/launch.
A dial up mini timer control set the mini time (hundreds of a sec) till full timing is restored after transbrake release. Controls the launch hit.
So much for needing a 3 speed trans eh.
You have exceeded that power/weight ratio threshold now.

You will like the wheelie bars. For the obvious reasons.

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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 12:58 pm

Further to moving the lower contril arm up down you also now need to add the adjustability to the upper control arm up down, and length.
This gives you a lot more control over the for aft position of the instant center. But also the up down height position of the instant center. The upper and lowers combined positions control this.
Right now you only have limited adjustability with the lower control arm positions.

Lowering the back of the lower arm does 2 things.
1. It increases the mechanical leverage ratio of the twist force of the axle on body lift (like adding gear) .

Where, for/aft the location of the instant center is determines if that axle twist force is applied to front end lift or to rear axle/body separation lift on launch.

2. it also moves the instant center rearwards a bit.

You can also raise the front of the lower arm to further move the instant center back. But i/c moves up too.
Rear axle rear body lift increases. May become excessive. Over reacts on launch.
Rear shock adjustment.

Can you create a "weight box" at the front to add front end ballast? (At near front bumper)

Can you reduce weight at near the rear bumper?

Put 6 eggs in a egg carton and move them around to see the tipping effect of moving them around.

I'd really like to suggest that you now upgrade the upper control arm location adjustability and add wheelie bars.

What happens if when footbraking it?

Moving the back of the lower arm up decreases the mechanical leverage (gear ratio) effect but also moves the instant center forward .

If you draw it out on paper you will see the effects of moving any of the control arm locations up down and can make your own best decisions at the track.
At least you have lots of traction.

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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 1:26 pm

Another way to move the instant center back (using what you got) is to instead of moving the back of the lower arm down (which increases the mechanical leverage gear ratio effect) ("the hit") you can add ride height at the back.
Add air to rear air bags. "Drag bags" or increase rear springs stiffness so the car now sits a bit higher in the back. This changes the geometry of all the control arm locations at once. This is why I like the AirLift air bags on coil spring cars like yours.
Low cost. To add airlift air bags easy to adjust ride height and rear spring load bearing capacity (like when in a wheelie on launch)
You could combine this with moving the back of the lower arm up as UMI suggests to kill the hit AND move the i/c back a bit too.
I think you will be your own best expert on dialing in your chassis.

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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 1:46 pm

You are moving away from having as fast, fun pump gas friendly street car (that you take to the track too)
Towards a more track dedicated race car.
Less street friendly, higher operating cost.

prairiehotrodder
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Re: My BBC progress

Post by prairiehotrodder » Mon Jun 10, 2019 2:45 pm

thanks for all that F-bird.

My main question is this : To reduce wheelies do i want to move the IC forward or back ?
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Re: My BBC progress

Post by GRTfast » Mon Jun 10, 2019 2:57 pm

prairiehotrodder wrote:
Mon Jun 10, 2019 2:45 pm
thanks for all that F-bird.

My main question is this : To reduce wheelies do i want to move the IC forward or back ?
From my road racing sport bike experience, I’m gonna say forward. Do not use my answer though, I haven’t done the physics for a launching drag car.

6.50camaro
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Re: My BBC progress

Post by 6.50camaro » Mon Jun 10, 2019 3:00 pm

The higher and farther to the rear of the car the instant center is the quicker and more violent the suspension reacts to the torque application .
The lower and further foward the IC is the smoother and longer the torque is applied to the suspension .
Wheelstands that go high and smash components are a violent reaction. Wheelstands that come up easy and carry down track then set down easy without bouncing are a slower smooth application of torque .
With that said finding the balance on a stock mount loction g- body my be difficult. I would try bmr's advice first if they supplied the control arms and rear suspension components. The only thing is that is also can have a tendency to cause wheel stands depending on upper arm location. The other option would be to lower the front mount of the bottom arm some. I think you need a lower and or slightly further forward IC to slow down the torque application not make it more violent. Dan

prairiehotrodder
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Re: My BBC progress

Post by prairiehotrodder » Mon Jun 10, 2019 3:04 pm

hhmmmmm.....

i would have to say by watching the video it is probably not what you would call a violent wheelstand. Its a far wheelstand and may have come down more softly if i had held on farther.
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6.50camaro
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Re: My BBC progress

Post by 6.50camaro » Mon Jun 10, 2019 3:40 pm

OK , if it was high enough to make you back out it was probably to high (not your first go round) .once the front tires are off the ground 100% percent of the car weight is on the rear tires .anything higher than a foot of air under the tires and your using up power to pivot the car on the rear tires instead of moving it forward . Dan

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Re: My BBC progress

Post by F-BIRD'88 » Mon Jun 10, 2019 3:47 pm

Basicly moving the instant center forward puts leverage in to lift the front of the car.
Moving the instant center back puts leverage in to cause rear axle- body separation (rear lift) as opposed to rear squat, on launch.
Moving the rear point of the lower bar down increases the mechanical leverage "the hit" like as adding more gear...

Don't tell me you backed out of it?
Very bad...
When we did my friends 8 sec nova. (With wheelie bars)
I got my friend totaly used to launching wheels up without fear of the wheelie.
The wheelie bars keep you out of trouble.
Some cars like to be up high on the bars, on launch.
Like a Superstock car launches...
Like my buds nova.. Some want the wheelie bars set low to the track to keep the front low on launch.
With the wheelie bars you don't need to back out of it.
Which is most always expensive. As you found out.
Time for wheelie bars.
Last edited by F-BIRD'88 on Mon Jun 10, 2019 3:55 pm, edited 2 times in total.

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