Zero roll "bump" limit?

problemchild
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Re: Zero roll "bump" limit?

Post by problemchild »

For the purpose of this discussion ...
Any time that I went stiffer than about 130lbs wheel rate on the rear .... on any track thats not perfectly smooth .... which in the NEDIV is every track. The car got darty loose and scarey. My interpretation was that since FVs and FST cars are essentially running on the droop limiter in the corners, I was just lifting the rear off the ground. The softer rate allowed more compliance and kept the tires more on the ground than in the air. Of course, you could dampen that out with shock valving but you would be going away from common FV valving. It is no wonder Brian is confused.
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smsazzy
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Re: Zero roll "bump" limit?

Post by smsazzy »

I think another issue would be that you don't have a lot of shock travel with a really stiff setup. The shock needs to move to do it's job. That's why the "real" formula cars have motion ratio rockers. To allow the shock to work.

Brian may have that figured out though, as I have never seen his rocker arms up close, so this isn't directed towards his car, but rather most vee's. That may be part of the issue with putting such a stiff setup on a common vee rear suspension. Brian's car is a one-off that he designed and built.
Stephen Saslow
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jpetillo
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Re: Zero roll "bump" limit?

Post by jpetillo »

Finally some civility in the new posts by folks. It was getting difficult to read these, and it was obscuring the intent of the thread to learn something, and perhaps keeping people from posting. Let's all keep it up.
smsazzy wrote:
jpetillo wrote:can you explain why you think this is a bad idea?
Results. In order to have the spring that stiff, the back end is so hard that the car is undriveable. One lucky lap without a spin may be fast, but in 4 races, I have seen that car finish only once on the lead lap and that was in 6th place. All three other races it was lapped at least once by at least one Vee.

I hear all this talk about how brilliant this set up is, but it is yet to finish well in a race.

You can't expect to have that stiff of a setup and have a car handle the conditions of a 35-40 minute race. Unless maybe you put Raikonen in the car. :-)

Think about it, in order to not have at least 2 inches of compression on the spring, you have to be running 400 pounds or so of spring. (depending on the motion ratio of the rockers of course, but bottom line is, it has to hold up about 550 pounds of car) that gives the rear almost no ability to absorb road irregularities and makes the tire essentially the only absorbing agent. I'm just not seeing that work.
I really don't know how stiff Brian's suspension is to be able to make an intelligent comment on that, nor do I know whether the car is undriveable or not. Apparently it is drivable, since he's had it like that for some number of years. Let me just say that I'm just glad the makers of my car aren't judged by my results! With all due respect to Brian, I'm not sure anyone has claimed his design was brilliant, including Brian, but if people more knowledgeable than myself on such matters are making such claims, then perhaps there's some truth to it, and we should listen.

Getting back to the subject, thanks for answering my question. You made some good points, although I could not see where your numbers suggested the car was too stiff - not enough information. I read your latest post, too. With all due respect, it does seem like you don't understand Brian's suspension. That's okay, I'm not sure I do either. The scenario you were suggesting - the car coming off the spring - could happen, but I don't see a reason why that would have to be the way Brian's design is set up to get what the result he wants.

I agree with you that the spring rate or effective spring rate is important with regard to soaking up road irregularities and should be sufficiently soft that the tires aren't the main springs. And I can believe that if it's too stiff then it would make the car less drivable for many, or perhaps most drivers. But, when you're up against a droop limiter, you have effectively increased your spring rate to a very high level. The droop limiters are highly nonlinear springs. If Brian is running a roughly linear spring rate that runs the full range and has the appropriate static droop and also can get to 2 degrees droop (negative) at full jacking, then I believe that he has to have a lower spring rate when he's at the 2 degrees (typically full droop) camber point than a car using a droop limiter. If that's the case then Brian's suspension is less stiff at full droop than a car with a limiter.

I think the difference between my version of "Brian's suspension" and yours is that in my version the shock does not come off the spring.

John
problemchild
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Re: Zero roll "bump" limit?

Post by problemchild »

Brian should start a thread about his car and rear suspension. Maybe people can help him sort out his problems.
For the purposes of this discussion, Brian's car is unlike the other 99.9% of the FVs in North America and talking about it in the general context of FV is very misleading and confusing for people trying to follow casually or who are unfamiliar with Brian's history. People like Scott and Stephen, have well-prepared competitive equipment with the lastest technical applications in this 49 year old class. Focus and discussion about their cars and programs, in comparison to lesser cars and programs, can be very educational and a real service to the general FV community.
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Re: Zero roll "bump" limit?

Post by Mystique Racing »

John, not quite sure I understand this statement.
jpetillo wrote: when you're up against a droop limiter, you have effectively increased your spring rate to a very high level. The droop limiters are highly nonlinear springs.
The spring "rate" never changes, the force that it is resisting will change depending on how much the spring is compressed. If your up against the droop limiter then I would think that the spring force would decrease or be resisting the least amount of force, that would be based on a typical zero roll rear suspension. I think in Brian's case, based on his description of how his rear suspension works, if his car is up against the limiter then his spring is completely unloaded which would mean that the rear of his car is completely unloaded.
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Re: Zero roll "bump" limit?

Post by FV80 »

Mystique Racing wrote:John, not quite sure I understand this statement.
jpetillo wrote: when you're up against a droop limiter, you have effectively increased your spring rate to a very high level. The droop limiters are highly nonlinear springs.
...
The difference here is that Brian says his system operates in a 'no xxbumpxx DROOP limit' configuration. Although the shock is an "ULTIMATE" limiter, IIRC the normal full extension (completely unloaded) position of his spring is still well below the shock limitation. Therefore, his car gets to "AIR" before the limiter. In a normal xxbumpxx DROOP limit car, the suspension hits the xxbumpxx DROOP limit BEFORE full extension of the spring - ergo the xxbumpxx DROOP limit ADDS to the spring rate ... and the xxbumpxx DROOP rubbers ARE, as John says, quite non linear - more like exponential in spring rate.

Personally, I have never tried a setup like Brian suggests. It's worth thinking about ... but I probably WON'T try it as I can see consequences that I don't want to explore.
Steve, FV80
edit: exchange 'bump' for 'droop' (I was thinking of BUMPING the DROOP limiter :oops: :mrgreen:
Last edited by FV80 on January 5th, 2012, 4:48 pm, edited 1 time in total.
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Re: Zero roll "bump" limit?

Post by smsazzy »

jpetillo wrote:But, when you're up against a droop limiter, you have effectively increased your spring rate to a very high level. The droop limiters are highly nonlinear springs. John
The droop limiter does not need to absorb road irregularities. It only works in extension (rebound) not compression. So if you are on the droop limiter, and you hit a bump, the suspension still goes into compression. Look again at Scott's data graph and you will see the small oscillations (looks like noise) in the signal. Those are road irregularities.
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

problemchild wrote:Brian should start a thread about his car and rear suspension. Maybe people can help him sort out his problems.
For the purposes of this discussion, Brian's car is unlike the other 99.9% of the FVs in North America and talking about it in the general context of FV is very misleading and confusing for people trying to follow casually or who are unfamiliar with Brian's history. People like Scott and Stephen, have well-prepared competitive equipment with the latest technical applications in this 49 year old class. Focus and discussion about their cars and programs, in comparison to lesser cars and programs, can be very educational and a real service to the general FV community.
1) No need for another thread, you guys get bored to easily too actually get anything resolved.

2) I THINK my latest car's issue was related to test/development system that was not correlated correctly to the car. I was making design decisions based on incorrect results. I have resolved the correlation issue.

3) My car is in fact very different than any Vee in the world. On the whole, it is more sophisticated than any Vee ever built in the world that I am aware of. I derive most of my enjoyment from the sport from these facts. That said I make no representation to my car being the fastest Vee every built. The majority of my racing takes place in the shop where I spend my all my time. I have limited interest in traveling great distances as I must to find good tracks and competition.

4) Focus on what ever meets your needs. I focus on sophistication because the std FV and even race preparation is boring to me. I work on the car every day, so boredom is not an option.

5) No upside to exposing all the features on my cars. Most of you would not appreciate the design features. I only derive pleasure from admiration of people that I know have the technical knowledge to make judgments about the car.

Brian
Last edited by hardingfv32-1 on January 5th, 2012, 3:25 pm, edited 1 time in total.
Mystique Racing
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Re: Zero roll "bump" limit?

Post by Mystique Racing »

I agree with Stephen.

On another note: I have used elastic droop limiters and hard limiters (like the shock) as my droop limiter and have found no difference in performance.

I currently use the shock as my droop limiter and the data that I posted reflects that.
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

smsazzy wrote:The droop limiter does not need to absorb road irregularities. It only works in extension (rebound) not compression. So if you are on the droop limiter, and you hit a bump, the suspension still goes into compression. Look again at Scott's data graph and you will see the small oscillations (looks like noise) in the signal. Those are road irregularities.
AND what happens it the road irregularity is a dip? Does your suspension track into the dip on leave the ground?

Brian
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Re: Zero roll "bump" limit?

Post by BLS »

OK, now I am officially confused (not hard to do :roll: ). My original question and title referring to "bump" limit had to do with the opposite of the "droop" limit. As I understood the response, there is no physical "bump" limit except for the axle tube bell hitting the tranny when fully compressed. What I call the droop limiter is what what hold the axles near parallel or a negative degree or two when you jack the car up unweighting the rear. A rod or cable or some other mechanism that ties the left and right side together, allowing any negative camber but hitting the stop to prevent positive camber. Seems simple enough (mechanically) as I understand it.

Steve, you are using the term "bump" and I'm thinking droop. Am I backwards?

From reading the other thread some time ago, it was my opinion that Brian's car does not have a droop limiter, at least not like most cars. He has an ultimate droop limiter in the maximum shock extension that would be similar to a normal droop limiting rod. It does not, if I understand it, come into play in normal use. Rather, as the rear end is unweighted, or the jacking force pushes the car up relative to the axle horizontal, the very stiff short stroke spring is no longer compressed, adding no further rear weight counteracting force. Once it reaches that point there is no further movement towards the positive camber zone. So, in that case it has reached the droop limit without an actual mechanical limit.

Am I wrong? Am I confusing terms? Am I confused? Enquiring minds (whatever is left) want to know.

While Brians car may be quite different and may or may not work well, it is still an interesting idea. Even bad ideas sometimes lead to a better understanding. I'm not passing judgement on good/bad design. I really do not know.
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Re: Zero roll "bump" limit?

Post by Mystique Racing »


AND what happens it the road irregularity is a dip? Does your suspension track into the dip on leave the ground?

Brian

Depends where (in the overall travel of the suspension) it is when it hits this irregularity....... Is it at full compression? is it at full droop? or somewhere in between?
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Re: Zero roll "bump" limit?

Post by BLS »

Well, more posts occured before I could put my last one up, which I think answers my question.

AND what happens it the road irregularity is a dip? Does your suspension track into the dip on leave the ground?
Brian, IF I am understanding how your design works, it is that the weight of the axle/wheel/tire is free to fall into the dip (at least until it hits the shock extension limit) while a conventional design has the whole car trying to fall into the dip. More mass of the conventional system probably means it see's air.
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Re: Zero roll "bump" limit?

Post by smsazzy »

BLS wrote:OK, now I am officially confused (not hard to do :roll: ). My original question and title referring to "bump" limit had to do with the opposite of the "droop" limit. As I understood the response, there is no physical "bump" limit except for the axle tube bell hitting the tranny when fully compressed. What I call the droop limiter is what what hold the axles near parallel or a negative degree or two when you jack the car up unweighting the rear. A rod or cable or some other mechanism that ties the left and right side together, allowing any negative camber but hitting the stop to prevent positive camber. Seems simple enough (mechanically) as I understand it.

Steve, you are using the term "bump" and I'm thinking droop. Am I backwards?

From reading the other thread some time ago, it was my opinion that Brian's car does not have a droop limiter, at least not like most cars. He has an ultimate droop limiter in the maximum shock extension that would be similar to a normal droop limiting rod. It does not, if I understand it, come into play in normal use. Rather, as the rear end is unweighted, or the jacking force pushes the car up relative to the axle horizontal, the very stiff short stroke spring is no longer compressed, adding no further rear weight counteracting force. Once it reaches that point there is no further movement towards the positive camber zone. So, in that case it has reached the droop limit without an actual mechanical limit.

Am I wrong? Am I confusing terms? Am I confused? Enquiring minds (whatever is left) want to know.

While Brians car may be quite different and may or may not work well, it is still an interesting idea. Even bad ideas sometimes lead to a better understanding. I'm not passing judgement on good/bad design. I really do not know.
Bump is when the spring compresses on a standard pushrod vee (car goes down/wheel goes up). Droop/rebound are when the spring extends (car goes up/wheel goes down).

The bump limiter is coil bind of your spring. I would think you would have this before the bellhousing is hit by the axle.
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Re: Zero roll "bump" limit?

Post by BLS »

OK, it is as I thought. My original question was what the limit for bump is which I figured was the shock fully compressed (or the spring). However, if you then roll the chassis you could get to the bell. It's been answered and I understand it. No need to go further. The rest is much more interesting 8)

I think Stevan's post confused me regarding the droop and bump terms. I just wanted to clarify it.
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

FV80 wrote:The difference here is that Brian says his system operates in a 'no bump limit' configuration. Although the shock is an "ULTIMATE" limiter, IIRC the normal full extension (completely unloaded) position of his spring is still well below the shock limitation. Therefore, his car gets to "AIR" before the limiter. In a normal bump limit car, the suspension hits the bump limit BEFORE full extension of the spring - ergo the bump limit ADDS to the spring rate ... and the bump rubbers ARE, as John says, quite non linear - more like exponential in spring rate.

Personally, I have never tried a setup like Brian suggests. It's worth thinking about ... but I probably WON'T try it as I can see consequences that I don't want to explore.
Steve, FV80
1) Steve raced against this suspension at VIR and can attest that it was not a disaster.

2) What you say is not completely true. The rear spring and jacking force COMBINE to lift the car in a turn. We can accurately calculate the jacking force for a given G load and then knowing how much the car weighs and what camber level you want to jack to provide you with a spring force number. This is not a spring rate, but just the strength the spring must have to reach your optimum jacked goal. Almost any spring rate can accomplish this depending or the linkage system.

Now if you don't want to do the calculations them you make sure that you have excess spring force available when fully jacked and use a droop limiter to make sure you don't over shoot the limit.

With my stiff suspension the spring develops just the right amount of force to reach the optimum jacking level. The spring is NOT at a zero force level. There simply is not enough combined force, jacking and spring, to raise the car any higher.

3) That said, this is not why I use a stiff system. The above explanation was developed in a previous thread as an interesting feature of this type of setup. If there are any benefits from such a feature it is an accident on my part.

4) You guys are missing the big picture on the stiff suspension. Without doubt a stiff suspension causes less compliance, but there are other areas of performance improvement that MIGHT make it a net performance benefit. Without a valid tire model I simply can not demonstrate the net benefit.

5) Steve, I am interested in the 'consequences' that you are concerned with. There could be something for me to learn.

Brian
Last edited by hardingfv32-1 on January 5th, 2012, 5:46 pm, edited 2 times in total.
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Re: Zero roll "bump" limit?

Post by smsazzy »

hardingfv32-1 wrote:
smsazzy wrote:The droop limiter does not need to absorb road irregularities. It only works in extension (rebound) not compression. So if you are on the droop limiter, and you hit a bump, the suspension still goes into compression. Look again at Scott's data graph and you will see the small oscillations (looks like noise) in the signal. Those are road irregularities.
AND what happens it the road irregularity is a dip? Does your suspension track into the dip on leave the ground?

Brian
A fair point. If you were completely on the droop limiter, the car would either have to travel down to meet the road or the tire is airborne. I do not think this happens very often though due to the zero roll nature of the car. You certainly have limited the grip of that tire during that instant, I do not argue with you there. I *think* the lifting of a tire would tak e a very large dip (such as the apex of turn 2 in Seattle) which is why you would stay off that section of the track. A dip is always worse than a bump in terms of loosening the car in my experience. I would be curious if your car expoeriences the opposite.
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

BLS wrote:Brian, IF I am understanding how your design works, it is that the weight of the axle/wheel/tire is free to fall into the dip (at least until it hits the shock extension limit) while a conventional design has the whole car trying to fall into the dip. More mass of the conventional system probably means it see's air.
Yes, this should be true, but remember this is a very dynamic situation. The reduction in grip from the dip will also reduce the jacking force which depends on lateral grip levels. I am not sure how that would effect the vertical force level at the contact patch.

Brian
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

smsazzy wrote:A fair point. If you were completely on the droop limiter, the car would either have to travel down to meet the road or the tire is airborne. I do not think this happens very often though due to the zero roll nature of the car.
Of coarse we really are not that concerned with the wheel leaving the ground as much as we are with changes in the vertical load levels at the contact patch. Reductions of loads that decrease grip levels. So it could be a valid plus of a stiff suspension that maybe negates the loss of compliance from a stiffer suspension. Again, you have to look at the big picture.

Brian
Last edited by hardingfv32-1 on January 5th, 2012, 5:44 pm, edited 1 time in total.
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Re: Zero roll "bump" limit?

Post by FV80 »

BLS wrote:...

Steve, you are using the term "bump" and I'm thinking droop. Am I backwards?
NOPE - I am - sorry - I've edited my post for the correction...
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Re: Zero roll "bump" limit?

Post by BLS »

Steve,

Not a complaint by any means. When you speak I listen 8)

I just wanted to be sure...

4) You guys are missing the big picture on the stiff suspension.
Ha, I'm quite certain I am. However, I appreciate the further explanantion. I actually thought the design was to reach the "optimum jacking level", not just a side feature. Thanks Brian.
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Re: Zero roll "bump" limit?

Post by hardingfv32-1 »

BLS wrote:Ha, I'm quite certain I am. However, I appreciate the further explanation. I actually thought the design was to reach the "optimum jacking level", not just a side feature. Thanks Brian.
1) Almost any rear suspension can get you jacked up into the droop limiter. A stiff suspension allows for less ride height and thus, a lower CG. This lower CG comes at the expense of tire compliance at the contact patch. Again without a valid tire model I simply can not calculate the numbers for you. It seemed to be a compromise ALL the other race cars were making, so I proceeded based on that observation. Because of the relationship of the rear axle camber to ride height, you have to design the car from the outset with this in mind.

2) There is also the benefit of less suspension travel for a given set of load changes. My suspension moves about half as much as the std FV. So if you are starting with 4 deg static camber and going to a 2 deg droop limit, I would start at 3 deg. The droop limit of say 2 deg is considered the optimum camber for the rear tires, so ideally you would like that camber setting at all times. With a stiff suspension you are closer to that goal than with the std soft setup.

You will note all the movement of the rear shock in the data shown earlier. Those all represent camber changes in a FV.

3) Based on the car's performance when ALL else is satisfactory, the compromises presented by the stiff suspension are a benefit. Unfortunately my car is in a constant state of being updated. I have 2-3 new things for the chassis next season and one for the engine that will require all my engines to be rebuilt. But this is how I have fun and stay creative.

Brian
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Re: Zero roll "bump" limit?

Post by jpetillo »

Mystique Racing wrote:I agree with Stephen.

On another note: I have used elastic droop limiters and hard limiters (like the shock) as my droop limiter and have found no difference in performance.

I currently use the shock as my droop limiter and the data that I posted reflects that.
Scott, yes Stephen got it right in terms of how the "springs" combine. John
Last edited by jpetillo on January 5th, 2012, 11:07 pm, edited 1 time in total.
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Re: Zero roll "bump" limit?

Post by smsazzy »

Brian,

I think the lower CG has merit. That is something i can't change very easily - for the reason you point out.

I think you can do a lot of the camber control with high speed and low speed compression valving in the shock. You can have a stiffer low speed circuit that keeps the car close to preferred camber during cornering, but the high speed circuit is still soft enough to absorb road irregularities. This is the end goal I was after when I re-did my suspension last year. I made a significant upgrade to the equipment before the Seattle race and it was night and day different.
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Re: Zero roll "bump" limit?

Post by jpetillo »

hardingfv32-1 wrote:
smsazzy wrote:The droop limiter does not need to absorb road irregularities. It only works in extension (rebound) not compression. So if you are on the droop limiter, and you hit a bump, the suspension still goes into compression. Look again at Scott's data graph and you will see the small oscillations (looks like noise) in the signal. Those are road irregularities.
AND what happens it the road irregularity is a dip? Does your suspension track into the dip on leave the ground?
Brian
That was going to be my question. Okay, I need to read the rest of the posts before responding again - sorry. John
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