Zero keel increases pushrod loads?

[ginsu]

Steve Matchett wrote recently in F1Racing regarding the F248:

The zero-keel is an attempt to counter this (lack of twin-keel stiffness), but positioning the wishbones high on the chassis's flanks dramatically increases the pushrod loads, placing the components under increased strain and upping the chances of mechanical failure. If one considers any supporting column (a table leg, for example) the theoretical ideal would be to have that column mounted vertically. Unfortunately, raising the height of the wishbones moves the pushrod closer to the horizontal and further away from it's load-bearing ideal. As a consequence, to make the suspension more durable, the only solution is to manufacture bigger, heavier components.

Personally, I haven't heard of this downside to zero-keel. I always thought the only negative of zero-keel was the suspension droop.

After drawing a diagram, I see what he's talking about, but the angle change looks relatively minor and doesn't seem to outweigh the benefits of cleaning up the airflow. Steve mentions this as the main reason why the F248 still uses a single keel design. Is this really why Ferrari hasn't switched to a zero-keel?

[DaveKillens]

It is logical, as you point out. The force vector through the pushrods would be higher than a single keel design. But remember, this is critical unsprung weight, and each gram counts. And I must assume the cumulative weight gain is relevent. Pushrods, rockers, bearings, torsion bars, and anchors would all have to be slightly increased. And a lot of the front suspension components reside above the driver's feet, well above the center of gravity. To me, it seems resaonable to assume a no keel design would carry about two to five kilos extra as mass, most located very high.
It's cumulative weight gain, in two critical areas, unspring weight and high above the center of gravity.

[joseff]

I don't get it. Zero-keel moves the wishbones, but the pushrods can stay in the same place. All things being equal, the tub is still in the same place, and the hubcarriers are still in the same place.

Even the shocks, torsion bars, everything stay put where they've always been. The only difference between the layouts would be the wishbone mounting points on both the hubs and tub. Am I missing something here?

[ginsu]

2-5 kilos would certainly be alot. And certainly the best compromise would seem to be the V-keel. Is it any wonder the R26 is so fast? It would've made sense for Ferrari to update to the V-keel especially after 2005, but they've remained conservative. Steve goes on to point out how Ferrari exclusively blame the tires for their lack of pace in 05. But, I think they are unwilling to develop the chassis at the pace of their competitors. I wonder how long they are going to hold on to the the single-keel.

[ginsu]

Even the shocks, torsion bars, everything stay put where they've always been. The only difference between the layouts would be the wishbone mounting points on both the hubs and tub. Am I missing something here?

Personally, I didn't really see it until I drew a free body diagram in Microsoft Paint. I wish I could post them, but I'll do my best to explain it to you. I'm looking at the car from the front looking back.

On the single keel the wishbones effectively make a rectangle with the front tub, and the wheel hub. The pushrod almost forms a diagonal on this rectangle (cutting into the upper wishbones, making two right triangles). Now the zero keel doesn't make a rectangle because of the suspension droop, it makes more of a parallelogram. This makes the pushrod have less of an angle with the wishbones (running them more parallel with eachother). Now the pushrod cuts through the upper wishbones forming two obtuse triangles.

Because the triangles are now obtuse instead of right triangles the pushrod is less effective at controlling vertical motion. You can also look at this in the extreme case where the pushrod would be completely horizontal with the pushrod and it wouldn't control vertical motion at all. I hope this helps.

[zac510]

I think you are assuming that the dynamic wheel geometry (I forget the exact technical term) is less desirable than the old single keel geometry and causing the pushrod to be under increase compressive loads, when infact we can't be sure that the teams have not matched the dynamic wheel geometry across the wheel's travel.

Assuming the pushrod mounting points have not changed, I fail to see how the pushrod can not control the wheel rate any more.

The forward wishbones might be under more compressive load but that would subsequently take some load off the pushrod.

Is there more or less leverage effect occuring when the lower wishbones pivot on a single keel? Why should a lower wishbone be acting in tension and compression when its purpose is to hold the car up?

Anyway, I'm no susp engineer so please shoot me down

[scarbs]

I think the point matchett was trying to get to is that the pushrod is usually mounted low down near the lower wishbone. If the wishbone is raised up to put it in better relation to the chassis (as opposed to a lower keel) then the installation angle of the pushrod is more acute.
As the pushrod is sloping at a flatter angle to the chassis and hence rocker\torsion bar will be "pushed" less, than a rod at a steeper angle. As it needs to transfer the same load through a smaller distance it will be stressed more.
Hence the pushrod may need to be stronger and rocker may need to have more travel to operate the dampers.

I do not think the extra stress and its impact on the weight and dynamics of the suspension are a problem.

Teams have got around this installation angle issue in several ways. Toyota mount their pushrod spaced well inboard near the stub axle and hence its close to the orginal angle on the single keel car. Honda have raised the rocker inside the tub,this is why there are two bulges on the topof the chassis to clear the higher rocker.

[zac510]

Thanks scarbs, that's interesting. Sounds like the wheel geometry over its travel is certainly different with the zero keel setup, as you would expect it to be (I just never underestimate the teams!).

I'm working backwards from an assumption that the teams have a desired wheel geometry,ride height and travel for the tyres and will work to retain that known quantity when moving to the zero-keel. If they can match it from one keel to the other, I don't see how the pushrod operation will change. But obviously they cannot match it and a rethink of the stresses is in order.

[Lukin]

Even the shocks, torsion bars, everything stay put where they've always been. The only difference between the layouts would be the wishbone mounting points on both the hubs and tub. Am I missing something here?

Personally, I didn't really see it until I drew a free body diagram in Microsoft Paint. I wish I could post them, but I'll do my best to explain it to you. I'm looking at the car from the front looking back.

Ginsu is on it! If the load through the pushrod is changing (assuming the vertical load from the tyre is constant) it means that the Roll Centre is changing, which is what we all know.

Most books will show you how to find the geometric based roll centre from the pushrods. When you do all those lines, basically your doing a free body diagram. Your finding out the proportion of the load that goes through the pushrod (and into the spring, damper and ARB's) and what goes through the suspension links.

(Probaby revision for everyone, but RC/SprungMassCOG = % of load through suspension arms, and 1- RC/SprungMassCOG = % of load through pushrod. Basically it's anti-roll if you consider anti-squat and dive)

So given twin keel moves the suspension pick up points, thus changing the roll centre height then yeah, there will be different loads through the pushrod.

[RH1300S]

The roll centre does not need to change, it depends if the wishbones converge/diverge inboard (although this will also affect the camber curve).

I think the higher loads are simply because the lower wishbone (inboard is higher than twin keel) will move in a different arc relative to the pushrod. Also, is it not the case that the outer end of the lower wishbones will be mounted higher relative to the wheel centre to try and bring back some of the "lost" geometry, this could cause problems with the pushrod loads if the outboard mounts are lifted with the wishbones - but why should they be?

Like everything in F1 these are probably small difference that add up - explained well by Dave. Find an aero gain, but trade-off with mechanical/structural advantages.