droop load

[racer-15]

why do guys take droop load out of left rear for slick track? I would think it would take bite out and traction away?

[MasterSbilt_Racer]

why do guys take droop load out of left rear for slick track? I would think it would take bite out and traction away?

The ultimate load up the straight comes from how high that corner is lifted. Initial throttle up from antisquat, if it has sat down some. Droop load rotates the car on entry when the car is slowing.

[racer-15]

So does the bar angle create more traction or the downforce of the body? thanks

[MasterSbilt_Racer]

So does the bar angle create more traction or the downforce of the body? thanks

Other than raising the car from ride height to race height, bar thrust does very little and decreases quickly once the car leaves the corner.

[95shaw]

How is weight derived from aero downforcce applied to LR tire if not through the bars?

[MasterSbilt_Racer]

How is weight derived from aero downforcce applied to LR tire if not through the bars?

You have 2 lr shocks, a spring, a 5th coil, rr suspension. Lots of force paths. The top bar is actually put in tension from the force of the spring behind. It spends a lot of time with little to no compression on it.

[95shaw]

You have 2 lr shocks, a spring, a 5th coil, rr suspension. Lots of force paths. The top bar is actually put in tension from the force of the spring behind. It spends a lot of time with little to no compression on it.
I guess I am not really feeling it.

The lr spring compresses as load is applied. We have used the bars to get to race height, with a zero index birdcage, so the droop load is less than at static ride height.

The shocks still compress, regardless of valving, or we would not get back to static ride height.

5th coil picks up its load in the middle of the car, near static cg. and distributes it to both rear tires. biased by position of lift arm on rearend.

The RR tire cannot be loaded excessively more than LR tire, or car will be verry loose.

Ultimately, need to load the LR somehow.

[Kromulous]

Interesting conversation, i do know the Spoiler on a SLM is wicked on down force. I rode one of my Motorcycles behind a SLM on an open trailer one day, and wow ! The air flow off that spoiler was insane.

Anyway, how many lbs force do you think is being applied MBR? rough cut i know, ball park guess? 400 lbs or better?

How much force does the lower rod feel? I know we tried a set of the "special rods" once, and he said he could feel the difference, so it must be applying a bit of load, because on the smasher they we're set to move at 1000 lbs

[MasterSbilt_Racer]

I guess I am not really feeling it.

The lr spring compresses as load is applied. We have used the bars to get to race height, with a zero index birdcage, so the droop load is less than at static ride height.

The shocks still compress, regardless of valving, or we would not get back to static ride height.

5th coil picks up its load in the middle of the car, near static cg. and distributes it to both rear tires. biased by position of lift arm on rearend.

The RR tire cannot be loaded excessively more than LR tire, or car will be verry loose.

Ultimately, need to load the LR somehow.

The 5th coil picks up both sides of the car. Where do you think that 1200# comes from? The j bar becomes a load carrying device too. I think you'd be shocked what is left on the lr tire at maximum lateral weight transfer, at least through the load paths you are thinking about.

[MasterSbilt_Racer]

Interesting conversation, i do know the Spoiler on a SLM is wicked on down force. I rode one of my Motorcycles behind a SLM on an open trailer one day, and wow ! The air flow off that spoiler was insane.

Anyway, how many lbs force do you think is being applied MBR? rough cut i know, ball park guess? 400 lbs or better?

How much force does the lower rod feel? I know we tried a set of the "special rods" once, and he said he could feel the difference, so it must be applying a bit of load, because on the smasher they we're set to move at 1000 lbs

Thrust load comes from relative acceleration. When you are gaining speed off the corner, the bars see a high load. It drops off quickly. How high does compression go on the lr top? I'm not entirely sure.

[Kromulous]

I guess you could find out how much the aero load could be, if you decreased your droop load a bit at a time and see if the car begins to lower on the straights.

Compression in the shocks will slow this, but high gas pressure could provide some lift thou via rod pressure.

How fun would it be to test all this stuff with the proper equipment !

[95shaw]

Thrust load continues all the way down the straight, even though acceleration drops off.

That aero load comes at a cost. No free lunch.
If not applying enough thrust to at least overcome drag, you are decellerating.

The steep bar angles make it easier to stay up against the chain so less thrust force is needed to maintain than was required to get on the bars initially. This allows the car to hold the added aero weight on the LR at race height.

[MasterSbilt_Racer]

Thrust load continues all the way down the straight, even though acceleration drops off.

That aero load comes at a cost. No free lunch.
If not applying enough thrust to at least overcome drag, you are decellerating.

The steep bar angles make it easier to stay up against the chain so less thrust force is needed to maintain than was required to get on the bars initially. This allows the car to hold the added aero weight on the LR at race height.

Thrust is directly proportional to acceleration. The car is held up by the need to overcome the loads on the birdcage to rotate it back clockwise.

[Jking24]

I was not their so i can't give all the details but a freind raced two weeks ago and after the heat they noticed the lr tire had gotten into the door. Their not the most observant racers. They pulled the door out and proceeded to eat another hotdog out whatever it is they usually do between races i got a phone call after the heat starting that the car was decent but about half way through the heat the car suddenly got realy tight in. Not knowing anything about the door and tire at the time i just advised him what i would do moving forward given the track conditions he had described. He ran the whole feature. They got back to the trailer after fading from the pole to fifth or sixth and they noticed the tire sitting funny in the wheel well. Finally they looked under the car and the lf top rod had ripped the threads out of a half nut and come off the bird cage. I can't say for sure when it actually came off but the car was raced for at least some amount of laps with no top bar. I would not even thought it could be possible. I can't imagine that ten years ago that would have been possible on say a straight 200#lr spring

[95shaw]

So you are saying that the droop load on the LR spring provides some of the force needed to turn the birdcage clockwise.

Doesn't that mean that the loads are being carried by the bars until that happens?

[MasterSbilt_Racer]

So you are saying that the droop load on the LR spring provides some of the force needed to turn the birdcage clockwise.

Doesn't that mean that the loads are being carried by the bars until that happens?

The top bar has tension from the spring. The lower bar has compression from the spring.

[95shaw]

Just to be clear for the original poster,
The droop load overcomes the force holding the birdcage locked, and lets the chassis lower slightly, to help the rear rotate the chassis.
Less force holding the birdcage locked on a slick track, so lower droop load numbers.

I think I put that right. lol

[Austin34471]

The top bar has tension from the spring. The lower bar has compression from the spring.
The question I would ask would be, does this matter?
Lets say you are running an actual 0 index setup with the shock and spring attached to birdcage, and no indexing through travel.
You have some element of tension in the top bar and compression in the bottom preloaded into the system by the spring

Now lets take the exact same setup, remove the shock mount off the birdcage and put a clamp bracket in the exact same location as the birdcage mount. Now the shock is attached to the axle tube but in the same location. (Also disregard axle wrap in this hypothetical scenario)
Now, at no point will either bar be under tension under acceleration (assuming perfectly smooth track). There are no forces added to the bar loads by the spring. Someone used my favorite phrase earlier: "no free lunch". I am not smart enough to do the math and calculate all of the load paths to verify what I am saying, but I have a hard time believing these 2 setups wouldn't act exactly the same.

[MasterSbilt_Racer]

The question I would ask would be, does this matter?
Lets say you are running an actual 0 index setup with the shock and spring attached to birdcage, and no indexing through travel.
You have some element of tension in the top bar and compression in the bottom preloaded into the system by the spring

Now lets take the exact same setup, remove the shock mount off the birdcage and put a clamp bracket in the exact same location as the birdcage mount. Now the shock is attached to the axle tube but in the same location. (Also disregard axle wrap in this hypothetical scenario)
Now, at no point will either bar be under tension under acceleration (assuming perfectly smooth track). There are no forces added to the bar loads by the spring. Someone used my favorite phrase earlier: "no free lunch". I am not smart enough to do the math and calculate all of the load paths to verify what I am saying, but I have a hard time believing these 2 setups wouldn't act exactly the same.

With all those assumptions, it would be the same. If upper and lower bar angles are the same, spring load washes out. It also doesn't matter once you reach full hike.

[billetbirdcage]

With all those assumptions, it would be the same. If upper and lower bar angles are the same, spring load washes out. It also doesn't matter once you reach full hike.

I think this is key (bold), which is rare unless both bars are equal length and same angle. Since most aren't, the spring load may cause load on bars to vary thus affect the true thrust angle the cage see's and effects car. Since the upper bar typically has more angle then the upper, removing thrust/load from the dominate bar, should change the actual thrust not the calculated thrust.

Again this may be so slight or minimal and vary with bar load vs spring load, that it may make no to little difference.

My favorite phrase is: Everything in racing is a compromise. There generally isn't a perfect solution to anything as you are going to compromise on something for cost, ease, or to lean one direction on something.

[pa_dirt]

Why we are on this topic...When you increase the droop the bars become steeper and obviously moves the rearend forward ...which also increase roll steer and makes our car way to loose... if we limit the droop (limit the roll steer) the car seems a better ... I’d love to be able to increase the droop... here is my question... should I try to maintain the same bar angles and the wheel base or just focus on the wheel base and keep the bars in the same holes ( with much more angle) with the extra droop?

Hope I explained that ok

[billetbirdcage]

pa_dirt

Your correct increasing the drop with no other changes will add rollsteer and can just loosen the car. If this happens is dependent on where you was on the steer curve vs Lift, meaning if you are already at the point the LR moves forward about as much as it lifts then lengthening the chain will likely add enough steer (loosen) overcomes the extra wedge/anti squat (tightens car) and just end up with a looser car.

You have 2 main choices to keep the lift and reduce the dynamic steer:

1. You move the LR back or the RR forward Statically to end up with less dynamic overall steer. This can be either adjusting bar lengths (alters dynamic path of LR also though and alters the thrust angles and likely index amount) or you can move the 4 link plate on the frame and keep the bars the same (newer cars with movable plates)

2. You can adjust the bar angles so you have less steer. Say you lengthen chain 1/2", maybe you need to lower both bars 1 hole to end up with the same dynamic steer. Obviously this effects the static bar angles and thus thrust and could make the car lazy if the car sets all the way down or on starts when the LR isn't rolled up already from cornering.

Like I said everything is a compromise. Moving the plates on the frame to adjust steer has the least side effects but not every car has these.

[pa_dirt]

I think I understand what you are saying ...So basically if it were you you’d move the plates back and leave the bars in the same holes first .... which would keep the bars the same angle at ride height and make them even steeper at race height... and then recheck the lr trail? What would be your next step? Or are you saying that the bar angle at race height isn’t nearly as important as what they are at ride height and in motion?

[billetbirdcage]

I'm going to leave out thrust angles and indexing, thrust will effect the car as it effects how easy the bars can lift the car (thrust angle) and index effects the spring rate the LR sees and well as spring loads (static or extended loads) so it does effect the car but for simplicity I'm going to leave these out.

So what we are basically left with is LIFT and ROLLSTEER.

Using your car as an example, when you lengthened the chain you add say a 1/2" of lift. Lets say this tightens the car up a 4 (on a 10 scale). However at the same time this allowed the LR to pull forward more, lets say it's a 1/2" more forward thus loosening the car at the same time. We will say this loosened the car a 6 on that 10 scale, so you have a net loosening effect of 2 (6 looser - 4 tighter = net gain of 2 looser).

Making up numbers here: So with the shorter chain length you have a lift of say 5" and LR pulls forward 3". When you lengthen the chain this goes to 5.5" of lift and 3.5" of steer. If you move the LR 4 link plate back 1/2" on the frame the static wheel base moves back 1/2" but with the longer chain the total dynamic steer is the same as the shorter chain and the plate in STD position.

STD shorter chain = 5" lift and 3.0" steer
Longer chain = 5.5" lift and 3.5" steer (.5 more dynamic steer but same static steer)
Longer chain with LR plate back 1/2 also = 5.5" lift and 3.0 steer (actually 3.5" of steer from suspension movement but since the LR was moved back 1/2" the LR tire is in the same location as the shorter chain dynamically)

Ultimately your looking at the ending amount of steer you're getting (actual location of the LR at full hike) because where it starts (static steer) isn't where the car is raced. However you have to keep in mind that when the car is down/flat like on a start the LR is back a 1/2" and can effect the car on starts or when the LR settles a lot on entry.

Hope that is a little clearer, but like I said everything has an effect and the index and thrust can come into play and you have to think about that stuff. Many times it won't overly effect the car at full dynamic hike but can at times when the car isn't at full hike (starts and times on entry if the car settles)

Yes the bars end up with more angle in them at full hike moving the plates and increasing the chain, but your mostly worried about the lift and steer aspects of it. You also need to keep in mind that over indexing of the cage from lengthening the chain can also cause the LR to become more rigid if you get too close to over centering. Many things are going on at the same time, but trying to list and explain all of this at one time can get confusing so I skipped those to just concentrate on the lift and steer aspect.

Side note: Lengthening the chain will change your extended length on the shock, so it will change your extended load on the LR spring and you will likely want to compensate for this.

[pa_dirt]

Thanks for the information billet