Rear Roll Resistance Never Needed

problemchild
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Re: Rear Roll Resistance Never Needed

Post by problemchild »

CitationFV21 wrote:

Not to hijack the thread, but since you drive both, does a FST not handle better than a FV given new tires?

ChrisZ
Sorry for delayed response, I have not read this in a while. FST cars do everything the same as FV cars but with $50 intakes and tires that last for the season. They all have an inherent push and success depends on how you manage that push. A top National FV package will kick the shit out of a top FST package through the corners at a track that is tire dependant. I expect someone like Loughead could even do faster laps at a track like Grattan. The extra power lets the FST car be faster at most tracks but it is not cornering grip. It really is an awesome spec tire!

The dominant FST cars have been Autowerks powered but also have high-end front shock treatments. At this time, most of the FST guys use economy shocks which limit the front grip potential, and leave them "unsticking" the rear ends with monster spring rates, etc. This works OK on some slower tracks but make the cars very difficult to drive on high-speed and bumpy tracks. Of course, this would be my opinion, and the official FST propaganda may differ.

Cheers!

PS .... Same challenge in FST as FV but a few more tools to work with. Minimal tire degradation helps too as the tires are a virtual constant in the handling package.
[ external image ]
Greg Rice
"Happy 50th Birthday"
P-2 Mark
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Re: Rear Roll Resistance Never Needed

Post by P-2 Mark »

Greg,

Although I only drove a FST for two years, I have to agree with your opinion regarding the difficulty of driving
them on a bumpy track. I'm usually competitive at Nelson Ledges (regionally) in a vee but was very uneasy
about driving the FST there, even with fairly new tires that didn't seem to give ME the feedback the Vee tires
provided and my times suffered as a result. I also raced it at Beaverun and the switchbacks provided problems
because I couldn't get enough front end bite so each subsequent turn in the series of turns made it harder and harder
to maintian speed etc. Then again our front shocks weren't of the highest quality and this may explain our issue's.

Mark
problemchild
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Re: Rear Roll Resistance Never Needed

Post by problemchild »

Mark,
That is a common problem with FVs and FSTs at Nelson. Over half the cars out there are so out to lunch on their shocks that the guys would just drive around in the middle of the track. I lost 3 races there because I could not lap cars that were 8 secs per 72 sec lap slower. I could drive anywhere and my car was hooked up but the slow cars are so busy hanging on to look in their mirrors .... and then afraid to get within 5 ft of the track edge. It is mainly shock absorber settings ..... too stiff in front and too soft in the rear. I even ran front-preload when I did the 1.11 so its not a shock travel issue. I also ran the same rear settings as at any other track .... I ran the same rear set-up for 4 years .... but worked to make the front better with springs, bars, and shocks. The one thing FV does better than FST is steering geometry. I would absolutely recommend keeping the steering box over the rack if converting to FST.
Greg Rice
"Happy 50th Birthday"
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

1) Bruce, "If I ran 0 degrees droop would I still corner "on the limiter"? I don't have any data to say yes or no. But I will speculate that I would still be on the limiter by mid corner more often than not."

Off coarse you have your droop limited a what is probably the idle rear camber for our tires, 2 deg total. ASSUMING a static camber that gets me to 2 deg fully jacked, what force is going to raise (jack) the rear of the car higher? How can the rear generate more cornering force (and jacking force) if you are starting to travel back down the camber curve as you continue to rise, say to 1 deg total camber? My data system says it does NOT happen. With my droop at 0, I jack up to 2 deg and stay there. I'm assuming a smooth track in this discussion. You guys sound like you still believe the World is flat, that you will go fly off the edge of the World with out the security of a droop limiter.

2) "For those that struggle to reduce understeer in their FV and believe their front grip is already optimized"
You are wrong, if you are under-steering there is still plenty of adjustment available at the front of a FV. Your front camber is still going positive in the middle of the turn, CORRECT? How can this be refuted?

3) "From the rear shot there is jacking but also roll. Could it be that a lot of roll offsets some of the camber change? I think there is a lot going on than the traditional swing axle jacking."
You will find that roll only changes the "jacked" camber setting by about a 1/4 degree.

4) "Thinking about it there is a difference between the Corvair and the VW. The Corvair (and Triumph Spitfire) had external universals. The VW has internal pivot points. Not a big difference but in this case VW (Dr Porsche) might have gotten it right - size does matter"
Of coarse most all suspensions have jacking. I think the Corvair and Spitfire might jack a little less. The axles are shorter and maybe the roll center lower.

5) "Then again our front shocks weren't of the highest quality and this may explain our issue's."
Your shocks only function during the first and last quarter of an average turn, when the chassis is rolling. There is a 50% chance the shocks are not the problem. Most FV drivers can't tell what is happening with there shocks. It is very easy for then to be sold on their importance if they have no clue.

6) low speed push is not unusual for ANY non-winged race car. If a car is perfectly balanced in the high speed turns (normal driver preference), then it is quite normal to have a little push in the slow speed turns. Nothing about a FV is different in this regard.

Brian
CitationFV21
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Re: Rear Roll Resistance Never Needed

Post by CitationFV21 »

hardingfv32-1 wrote: 4) "Thinking about it there is a difference between the Corvair and the VW. The Corvair (and Triumph Spitfire) had external universals. The VW has internal pivot points. Not a big difference but in this case VW (Dr Porsche) might have gotten it right - size does matter"
Of coarse most all suspensions have jacking. I think the Corvair and Spitfire might jack a little less. The axles are shorter and maybe the roll center lower.

http://pages.cthome.net/czracing/FV/fvroll2.jpg

Excuse the crude drawing but moving the pivot point closer to the center-line decreases the roll center. It can never be lower that the pivot point, but less is less.

ChrisZ
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

You are correct and probably the shorter axles make for better leverage and more lifting for a given side force? So VW did the best they could.

Brian
cendiv37
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Re: Rear Roll Resistance Never Needed

Post by cendiv37 »

Brian,

What I was trying to get at is the unstable nature of a swing axle suspension without a droop limiter. The jacking can go "over center" because there is a source of positive feedback in the jacking forces. As the car jacks, the roll center is moving: UP. This is causing MORE weight transfer to the outside tire which causes MORE grip at that contact patch and therefore MORE jacking. This is positive feedback. What happens on swing axle VW's and Corvairs and Spitfires is that once the jacking reaches a certain point the system goes into this positive feed back mode and the car jacks right over the top of the tire causing a SUDDEN loss of rear grip. This usually results in an off road excursion, back end first. Or a roll over, especially if the CG is high enough and the tires sticky enough. (I raced a swing axle VW sedan - with a camber compensator- and co-drove it with a number of other drivers. I think we flipped that car 3 times.) With a relatively wide flat tire, there is even an increase in the leverage of the contact patch as the contact patch moves towards the outside of the tire. Whether the tire deflects enough as the contact patch gets to the edge to counter that, I'm not sure.

Your suggestion that the rear will lose grip before the car jacks further is indeed an opposing effect, but the rising roll center is doing the opposite to that outside rear. So which will win? It probably depends on the spring rate, shock rates and how bumpy the track is. But if you let it jack so high as to lose good camber, you still have just lost rear grip and at least gotten loose if not actually spun out. Bottom line: why would you want to let the rear jack to a point that it is losing grip because of anything?

Like I said, try your car with 10 degrees positive as it's droop limit and a reasonable static camber. Let us know what happens!

As Chris showed in his sketches, the wider the pivot points, the greater the jacking, Thus a VW sedan is probably better than the Corvair and Spitfire in this regard. However, it's higher center of gravity and relatively soft springing probably make it just as nasty when jacking does occur. I believe Mercedes made a swing axle rear that pivoted in the center. I never dug into how exactly they did it, but that would be about the best you could do. That puts the roll center at the center of the axle and it stays there.
Bruce
cendiv37
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

Bruce,

Thanks for the rising CG thought. I have been a little uncomfortable with my model, as it seems that the jacking phenomena was self regulating. This seemed a little too simple. I wondered about the interaction with the rear spring, but never gave the CG movement any importance. I'm still have my actual experience of using 0 droop and never jacking beyond 2 deg negative as recorded by the DA system. I do not see the purpose of testing with positive droop if I never exceed 2 deg. Such an experiment is not possible for me without a new shock. I also have my new features to refine on my new car before any such testing is possible.

In this case I think I can come to an appropriate conclusion with the use of logic and the help of this discussion. I can learn something every so often from these threads. It is not all self absorbed chatter.

Brian
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

Bruce, I challenge your statement that jacking can go "over center" or lift beyond the max jacking point.

Analysis:
1) Tire camber sensitivity data is very hard to find. I found one graph on an unspecified Avon tire. It indicates a lateral force loss of about 6 lbf at a normal force level of about 400 lbf when going form 2 deg camber to 1 deg.
2) From your weight transfer spread sheet that you developed, a 1/4" ride height gain equates to 4 lb additional weight transfer at the rear (at 1.5 g). 1/4" ride height change should be good for more than 1 deg of camber with the normal zero roll rear suspension. Could this mean less than 4 lbf addition lateral load at a 1 g load?
3) Obviously when the rear suspension is jacking, the rear spring is in rebound, and thus providing less lift to the equation. Using a 100 lb per inch wheel rate as normal for a zero roll suspension, a 1/4" ride height increase equates to 25 lb less lifting force.

Conclusion:
I'm not sure how all the units discussed equate to each other. At a minimum it would seem that the there is not enough jacking force being created to replace the springs reduction in lifting force.

Please critique.

Brian
cendiv37
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Re: Rear Roll Resistance Never Needed

Post by cendiv37 »

I'll try to get to my spreadsheet tonight. It's been a while. I can't remember whether the roll center is fixed or changing automatically. same for the CG height (which was just a swag). But don't forget that both the CG AND the rear roll center are rising as the car jacks. Both of these are increasing weight transfer at the rear and thus increasing vertical loading on the outer rear tire. Increasing vertical force yields increasing lateral force (less camber change losses - once over 0 degrees anyway). The springs lifting force is indeed going down and the shock is slowing down any suspension motion, but the fact that our cars rise at the rear at mid corner indicates that the net result of the lateral forces acting at the rear axle does jack the rear (at least up to near 0 camber). Exactly how it all balances out, I'm not sure. Maybe anti-jacking is also part of why we usually run more rebound than jounce on the rear shock. Another thing to investigate in testing: the relationship between rear rebound setting and how the car "settles" mid corner and at what total camber.

By "over center", I mean that the the outside wheel can go into positive camber fairly suddenly as illustrated by plenty of photos taken (on slalom courses, etc.) of swing axle cars spinning out with both rears tucked under with huge positive camber. Snap oversteer is the common term for the behavior.

Later,
Bruce
cendiv37
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

In the most simple of cases, the jacking force could be replacing all of the lost spring force, the spring being in rebound. The static camber setting is a level that get us to the ideal jacked camber of say 2 deg total. I suspected a higher static number could lift you to a point beyond idea, but still a steady jacking level. All things being normal, I don't see what gets higher than the max jacking level. My data shows the jacking is at maximum as soon as the car reaches steady state in the turn, seldom later than the 25-30% mark of the turn.

Let's keep shocks out of the the model at this point.

Brian
jpetillo
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Re: Rear Roll Resistance Never Needed

Post by jpetillo »

hardingfv32-1 wrote:You are correct and probably the shorter axles make for better leverage and more lifting for a given side force? So VW did the best they could.
Brian
Yes!
jpetillo
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Re: Rear Roll Resistance Never Needed

Post by jpetillo »

Let me ask a few questions and make some comments.

Brian H., when you jack in a corner, do you hit up against your shock stop? I'm guessing that you don't, but am asking because some of the posts by other seems to suggest that they believe you do. If you don't, then that's proof that a droop limiter is not needed. Maybe Steve S. can comment on whether he knows if his hits the shock limit in corners.

I don't see why it's so hard to believe that a droop limiter is not needed. It all depends on the spring rate. The jacking force at 1G is a few hundred pounds and it changes fairly linearly with camber, and slowly. It has to since we are only allowing the camber to change a few degrees (let's assume droop stop or not). The car in the rear "weighs" about 600 lbs. If at some point during the jacking we have a jacking force that slowly rises past 300 lbs and we have a shock that reduces it's force faster, then the jacking stops. There is no positive feedback to it. Positive feedback means that it would go unstable on any provocation. It doesn't. There is no magic camber angle we come close to that anything changes dramatically to slip the system into an unstable regime.

I'm not suggesting that a droop stop is a good or bad idea, I'm just arguing that you don't need one to stop the car from jacking. You just have to have a spring rate that reduces force faster than the jacking force can climb, and it has to happen in the camber range we want to live in. John
problemchild
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Re: Rear Roll Resistance Never Needed

Post by problemchild »

I thought Ralph Nader proved all this in the sixties. And I always blamed the broken axle strap (limiter) when I crashed my Renault Gordini into that bridge. Droop limiters or droop control not being required is just more Harding BS.
Greg Rice
"Happy 50th Birthday"
jpetillo
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Re: Rear Roll Resistance Never Needed

Post by jpetillo »

Greg, if your car had a droop limiter to contain jacking, it was designed to work with one. In that case it failing was probably to blame for the accident. My Vee sits against the droop limiter and needs one to contain its camber - it was designed to use the limiter under normal operations. That doesn't mean that I can't modify my suspension so that doesn't need a limiter to contain camber under typical operating conditions. John
Ed Womer
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Re: Rear Roll Resistance Never Needed

Post by Ed Womer »

It is interesting following this post and hearing some competing discussions on droop limiters but if I am correct I think there is a rule requiring something to stop the axles from going positive and effectively rolling over causing the car to possibly flip/roll. Brian's approach in not using a droop limiter per say but the full extension of the shock is still a limiter just different than the common one which is usually a rod wit a rubber to control it from coming to an abrupt stop. The first ones were camber compinsator springs which my first vee had and the cable system. Since I wasn't around when the class first started I don't know all of the limiters that might have been tried but the rod with it's stop appears to be the preferred method.

Someone mentioned intenionally letting the axles go positive and see how you like it might end up with a not very pretty outcome. Without a limiter of some kind the outside wheel will be forced down or drop because of the G force trying to build will have to go somewhere and without the limiter the car should start to jack and will continue until the G force stops which could be straighting out or the tire/wheel rolling over and causing a roll/flip when it goes to far.

As for the understeer that is inhearent in vee's to varing degress, one option is to actually toe out the front which will give you good turn in and unless it is excessive and you have castor you proabably won't notice it. With most chassis having 6-7 deg of castor built in I think it might cause the understeer to some extent but for me the worst thing is the very heavy steering it causes just to give you the self centering effect. I build my chassis without castor and if you want it just ad shims to obtain it.

Ed
smsazzy
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Re: Rear Roll Resistance Never Needed

Post by smsazzy »

It is the tire itself that jacks the rear on a swing axle suspension.

The side load on the inside tire is pushing the bottom edge of the tire in towards the car. Nothing is pushing the top of the tire. This "lever" action is lifting the car at the pivot point in the tranny.

The motion ratio on it's own I do not know. However, this upward lift is what jacks the car.

The cars own weight is already balanced by the spring, so any additional lift will raise the back. This is only counter acted by the weight transfer in the car. Where those balance and which one wins would take some data.

Brian H. Are you running a potentiometer on your rear shock to tell you if it is or is not hitting the droop? If not, then how do you know for sure it does not hit?

Is anyone out there who IS running a potentiometer on the rear shock saying their car does not ride the droop around the corner? Everyone I have spoken with that runs a shock potentiometer tells me they are on the droop in the corners.
Stephen Saslow
FV 09 NWR
smsazzy
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Re: Rear Roll Resistance Never Needed

Post by smsazzy »

And another thing, on the droop or not, what does this have to do with roll resistance?

I thought we were discussing whether or not it was a good idea to control the rate at which the car rolls in the rear?
Stephen Saslow
FV 09 NWR
problemchild
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Re: Rear Roll Resistance Never Needed

Post by problemchild »

For those not technically inclined, please do not remove your droop control device and try and drive your FV at speed based on info provided here. It might work for a while if you run about 10 deg total Neg camber and keep your foot on the gas all the time ..... but eventually you will be testing your helmet out.
Greg Rice
"Happy 50th Birthday"
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

Ed

I have stated many times in my posts that droop control is needed for safety. Rear camber can't be allowed to go positive. That said, there has not been a logical thesis presented that indicates the there is any way to jack the rear suspension above the max jacking point of 2 deg neg. under NORMAL racing conditions in a standard turn. I always limit the variables to make it simpler to get to a conclusion on this one narrow point.

Bruce has presented a good thought that I have challenge. We are waiting on his response.

I still stand by my statement that droop control is shrouded in ignorance and FV tradition. This is clear from some of the responses. If you have doubts, ask questions that we can attempt to answer. That is how we can learn from this discussion.

Brian
smsazzy
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Re: Rear Roll Resistance Never Needed

Post by smsazzy »

smsazzy wrote:It is the tire itself that jacks the rear on a swing axle suspension.

The side load on the inside tire is pushing the bottom edge of the tire in towards the car. Nothing is pushing the top of the tire. This "lever" action is lifting the car at the pivot point in the tranny.

The motion ratio on it's own I do not know. However, this upward lift is what jacks the car.

The cars own weight is already balanced by the spring, so any additional lift will raise the back. This is only counter acted by the weight transfer in the car. Where those balance and which one wins would take some data.

Brian H. Are you running a potentiometer on your rear shock to tell you if it is or is not hitting the droop? If not, then how do you know for sure it does not hit?

Is anyone out there who IS running a potentiometer on the rear shock saying their car does not ride the droop around the corner? Everyone I have spoken with that runs a shock potentiometer tells me they are on the droop in the corners.
Here is my thesis. (Although I believe you meant hypothesis) :-)
Stephen Saslow
FV 09 NWR
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

smsazzy

Yes, my data comes from measuring the rear sock length. I never go higher than 2 deg, my droop is set at zero. In a back to back driver BLIND test at Laguna Seca, at lap record pace, the times were the same and the driver could not sense any difference with or without a droop limiter (set at 2 deg neg).

Other people have stated that their data shows that they get to the limiter, generally SET in the 2 deg range. Of coarse this does not help with our understanding of this issue. We know the suspension wants to get to 2 deg, The question being raised by everyone is: Will the rear suspension continue raising?

I think my flat World analogy is very appropriate here!

jpetillo has done a force study of the rear suspension in jacking that shows that the jacking force is replacing all the lost spring force during jacking. The rear spring is in rebound and creating less lifting force. So we think we have a clear picture of that part of the subject.

THIS IS THE HEART OF THE ISSUE WE ARE DISCUSSING:

1) We KNOW the rear spring force must be replaced for the rear suspension to continue jacking.
2) ASSUMING rear camber is optimum at 2 deg (max jacking point?), them the corning force created by the rear tires will go down as the camber level is reduced if we assume jacking continues to take place. Reduced corner force means reduced jacking force.

This all means that we do not have any force to compensate for the decreasing rear spring force if we continue jacking.

Conclusion: The rear suspension will not jack beyond the max jacking point, 2 deg neg.

Questions????

Brian
brian
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Re: Rear Roll Resistance Never Needed

Post by brian »

I'm having trouble following this, my spell check just caught on fire. Brian, are you saying that a vee won't go higher than 2deg. negative even without limiters? And, Are you saying that after you set droop at 0, you are able to measure the car at the least 2 neg. at max cornering? I kinda agree with Greg having had z-bar and droop stop break , it'll scare the sh*t out of you if not put you in the fence.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
P-2 Mark
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Re: Rear Roll Resistance Never Needed

Post by P-2 Mark »

Two weeks ago, I accidently went out onto the track (M-O) without properly adjusting up my Droop limiter
and was qualifying with 3 degree's positive camber in droop and 4.5 degree's negative camber.in total
I ended up spinning at turn 7 twice (end of back straight)and knew something wasn't correct since
I rarely if ever spin there. We discovered the error after the session, and I had trouble in other corners but didn't
spin, even when entering "Thunder Valley" where you would think the rear unloading would cause a problem?

Mark
hardingfv32-1
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Re: Rear Roll Resistance Never Needed

Post by hardingfv32-1 »

Everyone... Stop telling me the World if flat! Disprove my statements with some form of logic.

THIS IS THE HEART OF THE ISSUE WE ARE DISCUSSING:

1) We KNOW the rear spring force must be replaced for the rear suspension to continue jacking.
2) ASSUMING rear camber is optimum at 2 deg (max jacking point?), them the corning force created by the rear tires will go down as the camber level is reduced if we assume jacking continues to take place. Reduced corner force means reduced jacking force.

This all means that we do not have any force to compensate for the decreasing rear spring force if we continue jacking.

Brian

FACT: my car never goes high than 2 deg neg with droop (shock) set at 0. That is for any radius average flat turn.

Mark

I have people that are having a VERY hard time with my very restricted everything average and smooth track only discussion. Let us try to develop a consensus of this simple example before moving on to more complex situations. I'm well aware there could be exceptions to the simple rule I am proposing. I'm have never said you can't drive around on the limiter (possibly your normal setup), only that it is not necessary.

Brian
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