Cleanest Design Produced for the Class

[problemchild]

Can you expand on your thought.

Why would the FV, zero roll, rear wheels weights not be equal in a corner? Use a corner like the RA Carousel where there can be a long steady state period, no longitudinal accelerations or decelerations.

Brian

I do not have the ease in "engineer talk" to build this simplified model you seek. "Steady state" is a misnomer, as any moving vehicle has loads and forces, on every part of that vehicle once in motion, that is transferring weight between the wheels by the milli-second. There is always deceleration and acceleration. All I have is common sense and logic, having worked on these cars for decades, as you have.

If you need engineering mumble jumble to study, I believe the most similar application is a trike with the single wheel on the rear. Perhaps there are papers studying the Morgan or Can Am Ryker. I believe most or much of the theory would transfer.

[Ed Womer]

Here is a picture of the chassis when I checked to see if my ride height worked out as planned in the design and it did.
I copied the D13 style of attaching the tranny to the chassis with that aluminum tab bolted to the chassis and tranny. You can also see the tab for the front motor mount that uses the lowest motor bolt under the oil pump that is the most common front motor attachment using VW engines.
Ed

[Daryl DeArman]

All I have is common sense and logic, having worked on these cars for decades, as you have.

If you need engineering mumble jumble to study, I believe the most similar application is a trike with the single wheel on the rear. Perhaps there are papers studying the Morgan or Can Am Ryker. I believe most or much of the theory would transfer.

Common sense, logic and experience will get/has been very good for you.

At the risk of this turning into another falling arrow thread, I'm going to toss out another over simplification/generalization, perhaps somebody more educated than I will delve down into the 3rd and 4th approximation level to disprove my statement. A zero roll FV will transfer almost zero weight between inside and outside rear tires during lateral acceleration. This is the primary reason I believe they handle as well as they do despite all their other design compromises. They are allowing the inside rear tire to do more of its share of the work.

Now, that would be true if the roll center was exactly centered left to right and did not move laterally with suspension travel, a pinpoint fulcrum on a beam if you will. I haven't modeled it, but I imagine there is some lateral movement of the rear roll center so there will be some weight transfer across the rear but very, very little compared to more traditional designs.

Put a zero roll FV on a set of scales. Increase left front tire pressure by 15psi, what happened to the weight across the rear tires? Now do that on a FC car and see the difference.

[swiftdrivr]

Okay, I understand the picture with the lines-of-force arrows shown earlier in this thread, and kind of "get it," to a very simplistic level, but if true, why does every other racecar rely on torsional stiffness, and not zero-role? It seems simple and light, but I don't think F1 has car with a similar design. [could be wrong about that, I guess]

[problemchild]

Common sense, logic and experience will get/has been very good for you.

At the risk of this turning into another falling arrow thread, I'm going to toss out another over simplification/generalization, perhaps somebody more educated than I will delve down into the 3rd and 4th approximation level to disprove my statement. A zero roll FV will transfer almost zero weight between inside and outside rear tires during lateral acceleration. This is the primary reason I believe they handle as well as they do despite all their other design compromises. They are allowing the inside rear tire to do more of its share of the work.

Now, that would be true if the roll center was exactly centered left to right and did not move laterally with suspension travel, a pinpoint fulcrum on a beam if you will. I haven't modeled it, but I imagine there is some lateral movement of the rear roll center so there will be some weight transfer across the rear but very, very little compared to more traditional designs.

Put a zero roll FV on a set of scales. Increase left front tire pressure by 15psi, what happened to the weight across the rear tires? Now do that on a FC car and see the difference.

Because I do not have the education or interest to know all the proper terminology, I cannot have an engineering discussion on those terms. So my approach in tuning race cars is to try and manage weight transfer at each wheel. At each milli-second of a vehicles movement, weight is transferring between the contact patches. Whether we use proper terminology or not, we are all managing that weight transfer.

Yes, to simplify the operational model, I treat the rear on a zero-roll car as a single contact patch and the overall vehicle as a triangle. But at the same time you have to isolate the rear end as a separate model ..... with weight oscillating back and forth through the spring and droop limiter. Remember that a swing axle rear suspension is jacking from corner entry through maximum g-load to corner exit. I have no idea whether the weight transferring back and forth is 5% or 30% of the total, but weight has to be transferring as the rear suspension is bouncing off the limiter.

So yes, in your static model of a zero-roll car on a pad, the rear corner weights remain equal. Presumably, they would remain equal if you jacked one front wheel off the ground. But if you could measure at 1000 Htz as that wheel was jacked off the ground, would they always be equal? If the car had front droop limiters, would your data match? If you jacked the front wheel at different rates, would the data match? So .... if that car was on track, going around corners, over bumps, curbs, with engine applications, gear changes, steering inputs, etc ...... would the rear corner weights remain equal? I do not believe so, but I believe that platform is stabile enough to provide more usable consistent grip than most conventional suspension designs applied to the swing axle application.

I was fortunate enough to be at the same race track as Carrol Smith in the mid-80s and hang out a bit. He refused to even discuss FV suspensions. I don't think that many real smart engineers were ever motivated to spend the time to figure out what is happening with zero-roll. I briefly owned an Adams FV and learned a lot in studying that car and how Mills, Binks, and the others involved in that project dealt with the zero-roll dynamics.

[Daryl DeArman]

So yes, in your static model of a zero-roll car on a pad, the rear corner weights remain equal. Presumably, they would remain equal if you jacked one front wheel off the ground.

But if you could measure at 1000 Htz as that wheel was jacked off the ground, would they always be equal? If the car had front droop limiters, would your data match? If you jacked the front wheel at different rates, would the data match? So .... if that car was on track, going around corners, over bumps, curbs, with engine applications, gear changes, steering inputs, etc ...... would the rear corner weights remain equal? I do not believe so, but I believe that platform is stabile enough to provide more usable consistent grip than most conventional suspension designs applied to the swing axle application.

I don't know, but I suspect they would not. Largely because there is some "physicsy" stuff going on and some resistance to roll. That resistance to roll, no matter how little as long as it is something, will allow some weight transfer across the rear tires. As soon as there is contact with the rear droop limiter there is weight transfer. The shape/progressiveness of that rear droop limiter determines when that weight transfer begins and how abrupt it is.

[problemchild]

I don't know, but I suspect they would not. Largely because there is some "physicsy" stuff going on and some resistance to roll. That resistance to roll, no matter how little as long as it is something, will allow some weight transfer across the rear tires. As soon as there is contact with the rear droop limiter there is weight transfer. The shape/progressiveness of that rear droop limiter determines when that weight transfer begins and how abrupt it is.

Never mind the "phsicsy" stuff. if you are in a corner and the rear is on the limiter ..... and you hit a bump with one rear wheel. That wheel absorbs force and transfers weight through the spring to the other wheel. Then the first wheel tries to drop (off the backside of the bump), hits the limiter, and comes off the ground, presumably transfers all the weight to the other wheel through the limiter ..... which immediately tries to transfer back through the spring .... like an out of control teeter-totter ...... and all undamped forces. How much weight moved in that fraction of a second, ignoring all the other dynamics of the rest of the car?

[Daryl DeArman]

Never mind the "phsicsy" stuff. if you are in a corner and the rear is on the limiter ..... and you hit a bump with one rear wheel. That wheel absorbs force and transfers weight through the spring to the other wheel. Then the first wheel tries to drop (off the backside of the bump), hits the limiter, and comes off the ground, presumably transfers all the weight to the other wheel through the limiter ..... which immediately tries to transfer back through the spring .... like an out of control teeter-totter ...... and all undamped forces. How much weight moved in that fraction of a second, ignoring all the other dynamics of the rest of the car?

Yes, when you are on the limiter all things regarding weight transfer at the rear of a zero-roll car change.

As to hitting a bump with one wheel (on a mono-shock zero roll design) the rear suspension does not know the difference between roll and that bump instance.

[Hardingfv32]

A challenging question?

The consensus from the above statements seems to indicate that the rear wheel weights, for discussion sake, stay about equal side to side in a corner using a swing axle zero roll rear suspension layout. The swing axle has a very high roll center at or near the axle enterline. You can produce this same high roll center using using a double control arm suspension found on most formula cars. A mono shock system, with no sway control, could also be applied to this high CG rear suspension.

It seems to me that a rear suspension the maintains equal side to side wheel weights in a corner would be the Holy Grail of rear suspension design.

So why is it we see no high roll center zero roll resistant (mono shock) rear suspension designs?

Brian

[Hardingfv32]

"Steady state" is a misnomer, as any moving vehicle has loads and forces, on every part of that vehicle once in motion, that is transferring weight between the wheels by the milli-second. There is always deceleration and acceleration.

'Steady state' is a very important concept needed to keep a discussion simplified.

That fact is that any moment in time there is no deceleration or acceleration on a chassis. In our example we are talking about wheel weights side to side at the rear axle. It is a specific number at a moment when time has stopped. There is no attempt to show the number changing over a time period. We simply want to know the how many lbs on each rear wheel.

Brian

[Daryl DeArman]

You can produce this same high roll center using using a double control arm suspension found on most formula cars. A mono shock system, with no sway control, could also be applied to this high CG rear suspension.

It seems to me that a rear suspension the maintains equal side to side wheel weights in a corner would be the Holy Grail of rear suspension design.

So why is it we see no high roll center zero roll resistant (mono shock) rear suspension designs?

Perhaps (I don't know) it's because the roll center height and CG height of a FV move very close to in-unison. While in a double A-arm type rear suspension the RC height doesn't necessarily change proportionally to CG height.

We also don't like the camber curves so much.

[problemchild]

Perhaps (I don't know) it's because the roll center height and CG height of a FV move very close to in-unison. While in a double A-arm type rear suspension the RC height doesn't necessarily change proportionally to CG height.

We also don't like the camber curves so much.

Zero-roll on FVs only works if very very soft. It does not translate well to higher wheel rates. It's hard to generate grip when your tires are in the air most of the time, and significant weight is oscillating between the rear tires.

[Daryl DeArman]

It's hard to generate grip when your tires are in the air most of the time, and significant weight is oscillating between the rear tires.

Agreed.

More mechanical grip is found with softer wheel rates, until your rates are so soft that they allow too much travel making geometry/alignment go places we don't want it to go, and/or the requisite ride height puts the CG too much higher than we'd like.

My "engineering" education comes from the great Kent Fisk (RIP) and while he may have not been as well known as Carroll Smith (I did read all his books) I believe he knew his stuff every bit as well. Mr. Fisk also was pretty good with engine theory, working closely with Edelbrock for decades. Anyway, I digress. Mr. Fisk's theories around chassis and suspension design spent considerable time emphasizing that RC's that moved a bunch or transitioned from above to below ground during travel/roll could prove difficult to tune/drive.

[Hardingfv32]

Perhaps (I don't know) it's because

You gut feel on this subject is completely wrong and that, in fact, wheel weights do not stay the same in a corner using a std FV rear. suspension.

Brian

[Hardingfv32]

Zero-roll on FVs only works if very very soft.

I have personally proven that this is incorrect. I have raced with wheel rates and/or spring rates 200-300% stiffer than a std zero roll FV. Always able to set pole times at Nat/Major events. The downside was that the tire temps got too hot on long runs which was a negative for races. Had to give up on the setup. FV tires are developed for soft suspensions and that was not going to change for one stiffy sprung car.

Wheel movement over bumps is the same for all cars. You hit a bump and the wheel changes vertical load. The difference in your zero-roll example is that the wheel on the opposite side of the transaxle see the change in vertical load. So in fact a soft suspension is good for all car when it comes to the 'very narrow' topic of vertical load changes.

Brian

[Daryl DeArman]

You gut feel on this subject is completely wrong and that, in fact, wheel weights do not stay the same in a corner using a std FV rear. suspension.

Okay, if you insist. I didn't say they remained the same. I said "I don't know, but I suspect they would not. Largely because there is some "physicsy" stuff going on and some resistance to roll. That resistance to roll, no matter how little as long as it is something, will allow some weight transfer across the rear tires. As soon as there is contact with the rear droop limiter there is weight transfer."

[rgu]

Ed Womer finally got his car out with a Dean Curtis engine at the FRP event at PIRC.
Result in practice running alone under the SCCA record by 4 tenths. Won the Sunday Race.

PIC