4-link IC location?

The new blower motor is starting to make my 72 Comet
wheel stand a little too much. I can lower the wheelie
bars a little more yet, but I'm wondering if there is a
less violent 4-link Instant Center location I can try?
Will moving the IC back and/or down, help calm this
down?

Back or shorter will hit the tire harder.

A longer and/or lower instant center theoretically hits the tire with less force/leverage at the initial hit. But hitting the tire harder (or softer) doesn't always mean more (or less) hook. Sometimes reducing the initial hit can make a wheel stand even worse depending on how overworked the tire/sidewall was before with more initial hit

Before making a 4-link change (a major adjustment) it might be a better idea to first try adjusting in some (or some more) wheelspeed to help reduce the wheel stand by making the rear shock extension valving tighter, or adding more air pressure, or lowering the wheelie bars more. Tighter front shock/strut extension valving might also help.

Got any good launch video you can post?

Also I'm guessing it's got decent power numbers since it's a blown combo. How much power are we talking about? And what does the front/rear weight bias percentages look like? If it has monster power numbers to work with, and has a lot of engine set back and/or a lot of rear weight bias, it could make it a lot harder to calm the wheelstand down without adding a bunch of wheel speed.

Engine is set way back in the 110" WB 72 Comet chassis. Although
heavy, distribution is really good at 1665 front & 1625 rear. Not bad
considering the blower added 130 lbs to the motor! The car made
about 1200 HP last year on NOS (w/o the blower). It initially was
wheelstanding really hard last year and I kept moving the IC back
(shorter) and it calmed it down a bunch. Theory being shorter
IC may hit the tire harder but has less lifting leverage? Seemed
to work. So I'm not sure "tire hit" and "chassis rotation leverage"
go hand in hand?

I dumped the NOS & converted to the 1071 blower for this year
(still on alcohol). Very very conservative blower setup with only
about 12 psi boost, but still probably making 1300-1400 HP? Have
only made two passes so far working out other bugs and issues.
But the car is getting way up on the wheelie bars hard & carrying
the front wheels way out 200' or more. It is nice and straight
and not spinning the tires.

I can set the wheelie bars lower. Just curious about if the 4-link
adjustment down or back more might help?

Don't want to hijack the thread, but dilligasdave--- could you elaborate on the wheel speed term? Are you talking about increasing the role out of the tire w/ more air pressure or changing gear ratios, or something different all together?

Thanks,
Jon

460bronco wrote:......could you elaborate on the wheel speed term......

Wheelspeed is just a given amount of controlled (hopefully controlled) traction slippage happening for a given amount of time/distance during the launch to avoid a dead-hook. Some cars work better with a dead-hook, and some work better with X amount of wheelspeed. Just depends on any number of factors (power numbers, tire size/type, weight, suspension, etc, etc). And it can be created/adjusted in a ton of different ways.

AlkyPinto wrote:......Theory being shorter IC may hit the tire harder but has less lifting leverage?.........

That has often been a point of confusion when talking (or arguing about) IC in/out = more/less or less/more leverage because it depends on which end of the "see-saw" (actions ahead of IC vs actions behind the IC) a person is talking about.


To the rear of the IC placement......

The total leveraging effect of the rear suspension's un-restrained (taking the shocks out of the picture) energy/force available to hit the sidewalls at the initial hit is partly created by the amount of leverage from the IC placement, and partly by the engine torque numbers at the launch driving that leverage. The IC placement is the mechanical leverage "hammer" waiting to hit the tire, and the engine torque is the force swinging the hammer. They both work together, one part has no effect on the rear tire without the other part. Engine torque + longer IC leverage point initially hits the sidewalls with a softer/slower/smoother/longer acting amount of force. That same engine torque + shorter IC leverage point initially hits the sidewalls with a harder/faster/shorter lasting generated amount of force. All this does make a lot of people think a harder hit always = a harder hook, but that's not always the case. Your car wanting to wheelstanding more with a longer IC can be as mentioned from the "softer-smoother hit" giving the tire/sidewall a better chance at a dead-hook & "climbing the ring gear". And the shorter IC could be hitting the tire/sidwall so hard it can't dead-hook & creates some wheelspeed lowering the wheelstand X amount.      


In front of the IC placement...............

The leveraging effect ahead of the IC is trying to lift the nose of the car X amount after the initial hit has happened & initial traction has been established. And while it's true the longer IC's mechanical leverage want's to lift the nose a higher total distance, it might not be able to since it needs more generating energy (engine torque) "way out there" to do the work. The shorter IC's mechanical leverage want's to lift a lower total distance, and is easily able to because it has more generated energy available to do the work. It takes more effort/energy/grunt to lift the car at a given point the further out you go with that point. But if you have enough "grunt" to work with way out there, the longer IC will lift a greater distance. It's the ancient example of grabbing a long piece of tubing at it's end & trying to rotate it up vs doing this with a shorter piece of tubing. It's easier to lift the shorter piece of tubing & it happens at a faster rate of speed. It takes more effort to lift the longer tube & it happens at a slower & smoother rate of speed.    

Additional factors that "muddy the water" & help/hurt/mask what might be effecting what's happening are things like how stiff/loose the shock (and/or strut) valving is set at, & having excessively steep down-hill 4-link bottom bar angles. As mentioned earlier calming down a "dead-hook" wheelstand by increasing wheelspeed isn't always done with an IC change. Wheelspeed can at times also be created/increased by stiffer shock extension valving, air pressure changes (sometimes more air, sometimes less air), and of course by lowering the wheele bars. Gear ratio changes can also sometimes have an effect, but that's not something easily adjusted or fine tuned. Using some form of engine management can also be a suspension tuning tool to calm things down if you don't mind spending the coin on one of the adjustable ignition/traction boxes from MSD or Davis.

[Edit] You could also just go old school & hang 5-10 lbs ballast way out on the nose just for the hell of it to see what happens.

AlkyPinto wrote: Theory being shorter
IC may hit the tire harder but has less lifting leverage? Seemed
to work.

I think this logic is a little flawed.  Take a ladder bar for example to simplify the IC to the front bolt mounting hole.  I think we can all agree that when power is applied, there is a force pointing straight up (or tangent to arc of rotation of rear axle) and one pointing straight forward caused by the tires actually moving the vehicle forward.  The sum of these two vector forces will point up and forward at the mounting bolt hole at some angle.

For those lifting forces caused by rotation of the rear axle.... The motor has (X) a set amount of torque to work with.  So, is it easier to tame that force with a very small mechanical advantage (short IC) or large mechanical advantage (longer instant center). Another example might be is it easier to bog down a grinder with a 3" disc or a 10" disc?  Less force is seen way out there at 10 inches than at 3 inches.

A shorter IC will see higher lifting forces at the IC and a longer one will see lower lifting forces....  The relationship of these vectors to the center of gravity of the whole car is a large part of what causes the nose to lift or the rear to separate.

Jon

That short ic he is describing is hitting the tire so hard and fast it bounces it off the track and produces wheel spin . if you get into the math on leverage points , vectors and such just remember the 4 link brackets stay in the same place and ic is the place where the bars would intersect "if" they were actually that long or short .

I am not spinning the tires and I dont have "wheel speed".
The car is hooked up, hits hard and goes straight. It carries
the front wheels and actually climbs to well over 200' out.

AlkyPinto wrote:I am not spinning the tires and I dont have "wheel speed".......

You kinda make it sound like having some wheelspeed (aka "getting up-on the tire") is always unwanted or always a bad thing, it's not depending on the car/combo. It can be a useful tool & in fact sometimes 100% needed to help keep the nose down on some types of drag cars. Generating/increasing wheelspeed is usually exactly what happens when you try to lower a wheelstand by either lowering the wheelie bars more, or stiffening up the shock extension valving more, or increasing/decreasing the air pressure X amount. Changing the IC can also create/change/remove X amount of launch wheelspeed vs launch dead-hook happening. But doing it with the IC isn't as easy or predictable as doing it with the other methods mentioned.


Every car is different. Some work best with a decent amount of wheelstand, and some work best with little or no wheelstand. Some are best with wheelspeed, and some are best with a dead-hook. The fun times    (or churning stomach acid time   ) is track testing & trying to find out what it needs.

DILLIGASDAVE wrote:You kinda make it sound like having some wheelspeed (aka "getting up-on the tire") is always unwanted or always a bad thing,

I said nothing of the sort! I have a data recorder and I'm simply stating the fact that
the car is NOT spinning the tires and doesnt have wheel speed issues (good or bad).
Since it does leave nicely and behave at the hit, I think I'll leave the 4-link alone for
now and just try lowering the wheelie bars more??? See if it bounces off the bars
and does spin? Be interesting to see how it effects the 60' time??

Although, I'm sure my 60' times now are being triggered off the rear tires.....

Keep the frontend down to a reasonable level and you should pick up in your 60'.as some have said wheel speed can be a very good thing.   

Some questions....

What is the current IC height & length?

How big are the slicks?

How much air pressure?

How long are the wheelie bars?

Distance ground to bottom of the w/b wheels?

How many total clicks (or sweeps) do the rear shocks have on both extension & compression valving?

How many clicks/sweeps back from "full tight" are both the rear shock extension & compression valving set?

Same valving questions again, but for the front shocks (or struts)?

How much travel does the front suspension have & are travel limiters used?

At around 1300-1400 hp & around 3200 lbs, what kind of ET & MPH numbers so far?

Good lauch video would help, got any?