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PostPosted: September 1, 2014, 2:40 pm 
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so my memory is fading fast, in a rear wheel drive car:-

when designing a suspension system for autocross, in a car with 50-50 weight distribution, including driver, static state is something that only occurs immediatly after braking has finished and turn in has not started. AM I CORRECT IN THIS ASSUMPTION?

would braking and turning at the same time be advantageous in autocross?

if i assume that it would be, then steady state is of no use in corner grip calculations?

given the above, it would not be of benefit in straight line braking either when the weight is being transfered to the front?

so given all of the above, do we need to radically re look at a perfect setup in real life situations on an autocross course where during heavy braking and turning in simoltaniously, we still need to get round that corner and into the next as quick as possible.

when braking, the line of force is axial through the chassis, if braking is optimized front and rear, then maybe the rear of the car is not going to rise in relation to the front if the center of gravity is below the spindle hight of the wheels?

likewise, if the center of gravity is above the spindle hight, then the rear will rise and the front will fall, causing the weight to be moved forward thus compounding the difference in front to rear ride hight on braking.

i am trying to establish the attitude of the car in this critical moment before turn in occurs and where the suspension is before attempting to input cornering moments of momentum at a tangent to the axial force of the car.

i was struck by the performance of a kart with no suspension and only polar momentum to deal with in corners, in this situation however there is no suspension inputs to consider as it lacks any and must corner with just caster, camber, toe and tire grip.

it should be clear that suspension should be an advantage when cornering but using a steady state situation to base line an autocross car requires the old addage of "only brake when travelling straight" to be employed before the turn in occurs and a suspension system that reacts fast enough to bring the car back to a steady state almost instantaniously when the brake pedal is released before turning the wheel.

we could fix this by removing the suspension and setting the geometry to corner in steady state and just increase the tire size until it cornered, Colin Chapmans words echo in my ears "any suspension will work if you don't let it" but could a human drive the thing?

i think that what we need is a vehicle that is in the perfect attitude to corner when subjected to maximum braking and is ready to take the corner and indeed must have the brakes applied to be cornering to the apex.

here's the rub, half way round the corner, you need to apply the throttle, if not before.

so if we can come up with a car that has fully functioning suspension in the corners but during braking has no attitude changes at all, it would only react to tangential forces to the axis of the car.

just the ramblings of this old guy in a shed in the desert.

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PostPosted: September 1, 2014, 3:05 pm 
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Pretty much everything you said is why I'd say 50/50 is a great marketing tool and 45/55-40/60 is what you want.

If you have 50/50 with a rear wheel drive car you have less weight on the rear which means less grip to accelerate. It also means the rear end will be light under hard braking due to weight transfer. If the power to weight ratio is low this probably doesn't matter as much but your typical Locost can benefit from a little more rear weight.

The front diving/rear rising has nothing to do with CG above the spindles but rather distance between CG and the lines of action of the anti-drive/anti-squat geometry. If you have no anti-geometry then it ends up being the distance between the pavement and the CG. Unless your CG is below the ground the front will always dive and the rear will always rise under braking since the force is being applied to the mass at the contact patches.

I think the main difference in how much you trail brake depends on the type of car. You always want to trail brake some otherwise you'll never use the entire traction circle and you'll be slow.

Just keep in mind that chassis movement and weight transfer are two totally different things. Just because the suspension may not move under braking doesn't mean weight isn't move from one axle to the other. The primary goal behind limiting chassis movement is to keep the tires at an acceptable camber angle.

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PostPosted: September 1, 2014, 4:26 pm 
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The pitching force comes from the CG height above the contact patches. A formula car can have it's CG under the hub height and still pitch forward under braking.

Steady state seems to occur after the braking zone and up to nearly the apex. During this time the driver is making corrections for any errors just made, either more brakes in a bad situation or trying to get on the throttle. You can see the driver doing wheel adjustments and perhaps giving the gas pedal nudges.

Trail braking is perhaps most useful just to keep traffic from trying to pass you on the inside? Evan on a road course the braking doesn't last very long. If the car is light and/or stiff the transitions don't last very long. I'm planning on setting up for hard braking and then cornering and will hope the transitions take care of themselves. For fine tuning the transition in and out of the corner I will try to play with rear roll center movement when the car pitches.

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PostPosted: September 1, 2014, 7:50 pm 
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I think the most important thing to do when designing suspension is have lots of adjust-ability. That way once you actually start to drive the car you can feel what it is doing an adjust accordingly. Autocross driving is just like track driving only much more aggressive and much quicker inputs. I try to get most of the braking done in a straight line then ease off into the corner and onto the gas.

I always like to look at the tires as having 100% grip. If you are threshold braking (100% in a straight line) then you cannot turn as you have already used you 100%. As you reduce brake force to 80% you can use 20% of the grip to corner. You reduce braking until you are using 100% of the grip in cornering forces (your tires should make a nice squeal noise but not screeching). Then as you start to straighten the wheel you can start to feed the gas as much as possible without losing too much rear traction (a little wheel spin is ok on corner exit as long as the car does not rotate too much).

If you are serious about driving you should also learn to left foot brake especially for autocross as it is typically all in 2nd gear (Maybe 3rd in a Locost). Either way you don't need to use the clutch much so brake with the left foot. This gives a huge advantage in balancing the car on sweepers as you can use the brake to rotate the car while still holding the gas steady. When changing from braking to acceleration you can do both at the same time for a split second to get the engine up to speed quicker. Rather than your foot coming completely off the brakes moving across to the other pedal and then onto the gas (The time saved is tiny but if you can shave a tenth of a second off every corner your dropping seconds off your lap). Also heel toe/blip the throttle on a down shift so you do not lock up the rear tires under heavy braking. Left foot braking can be a challenge at first but practice makes perfect. If you are going to do it on the street make sure there is nothing behind you when you press the brake.

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PostPosted: September 1, 2014, 8:30 pm 
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if you could completely disconnect the front brakes and apply full braking with just the rear,
would the weight transfer still happen, i know that if i apply the park brake only in a straight line, the rear squats, now is the weight still being transfered to the front or to a point infront of the rear wheels?

i can see that where if you add more tire to the rear, load over contact patch area will reach a point where there is just not enough weight pushing down on the contact patch square inches and a breakaway will occur but with no front brakes will the weight still transfer forward lifting the rear and breaking away?

i guess what i am asking is will the car stop quicker with only rear brakes because the weight is not lifted off the rear tires and the nose doesn't dive?

if we then add a lot of weight to the rear say as suggested 40-60, can we exploit this phenomina if it indeed does exist.

imagin if you will a car with the front wheels and brakes added to the back wheels so all the tire and all the brakes were at the rear, in the front just to stop it from dragging on the ground we had some casters with some rudimentary springing, now would this car stop in a straight line quicker or slower than a normal car, the contact patches would add up to the same and the brakes would be the same?

and would the front still nose dive at all?

or are we really dealing with a flywheel, dirt bikes can adjust there attitude when in mid air by braking to lower the front or throttle to raise the front by using the rear wheel as a flywheel?

if we take moments about a pivot called the rear axle and there is "X" energy stored in the wheel and a turning force from the road, by applying the brakes we impart a torque on the chassis which could be measured at one foot intervals along the chassis and expressed in foot lbs. by using a spring of known lbs.inches rate, when this measuring point spring is compressed equal to the weight of the car pressing down on the contact patch will the car pivot around the measuring point and looose drive from the road?

before that occurs, the further we move the spring to the front, away from the axle, the less the spring will be compressed, this would suggest to me that if the spring were far enough away from the back that almost no effect would be seen at the spring.

does the team think that porsche 911 and its derivertives are exploiting this effect?

would this effect, if it exists, leave the front suspension free to control the cornering when under braking?

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PostPosted: September 1, 2014, 11:07 pm 
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Quote:
if you could completely disconnect the front brakes and apply full braking with just the rear,
would the weight transfer still happen, i know that if i apply the park brake only in a straight line, the rear squats, now is the weight still being transfered to the front or to a point infront of the rear wheels?


The weight transfer still happens just the same, it cares not why the car is slowing. The center of gravity wants to continue moving and it creates a force because it is above the tire contact patch. These forces are the same in a lengthwise sense as cornering forces are sideways. Just like there are roll centers, there are also pitch centers. I'm not sure if they use that word though. It's a little more complicated especially with a solid rear axle because the brakes and the diff put torque on the axle.

So that's what you are seeing when the rear drops with the emergency brake. Your car has anti-lift, I think its called. So the torque from the brakes is made to pull the rear down, that reduces the dive under braking. The front brakes are strong though and may overcome that effect.

I'm not sure how to answer your other questions, but hope this first answer helps.

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PostPosted: September 1, 2014, 11:10 pm 
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I was wondering why I never left foot brake and realized the steering shaft is between my feet in my car...

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PostPosted: September 2, 2014, 7:53 am 
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I think you may be over thinking it a little bit. When driving if the rear brakes do more work than the front they are going to lock up first. If the rear locks up first the car become extremely difficult to control an typically the rear of the car will wander off course and can cause the car to rotate. This can be an advantage in some situations as you can use the brakes to rotate the car into a corner and get a tighter turn radius then in theoretically possible when looking at the suspension/steering/speed alone.

You can try to build in anti dive to the front suspension and also add stiffer springs which will also prevent the roll from moving forward. Unfortunately the dive is what creates a lot of the breaking force. This is why you squeeze the brake pedal not stamp on it. As the weight moves forward there is more weight on the front tires and they offer more grip.

Lets say the car weighs 1000lb and is 50:50 balanced. The first instant you brake the front tires have 500lb of force on them as the braking increases as does the load on the front tires lets say 80% of the weight is on the front tires. They now have 800lb of force, squashing the tires into the ground which will cause the car to slow down faster. (Not sure exactly how to describe why it work but it does). The point is that the more weight is on the tires the more grip they will provide. Our light cars need super soft tires as they do not have the weight to compress a harder tire that would work well on a 4000lb car.

I know I am not really answering your questions but when racing it is all about getting the most out of the tires and balancing the car when cornering to allow you to get the power down. I always like to have my car neutral with a little over-steer which can be corrected with the gads pedal.

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PostPosted: September 2, 2014, 12:03 pm 
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all i'm trying to do is establish a situation that prevents loss of grip during braking at the rear, if there were no front brakes by law, we would have to deal with it.

do you not think that in a conventional setup, that "most of the braking is done by the front because the back gets light" or "the back gets light so you have to use the front brakes"

either way, if the front brakes are applied to the point of lock up and the car didn't stop, only having 20% of the braking on the rear is counter productive,(and you lost the abillity to steer as well).

as i see it front brakes have a lot of bad points, they will and do lock up especially in a light car, they cause the front suspension to dive, no matter how much anti dive you build in to the geometry, the front tires are over stressed because they are doing two jobs and perhaps are larger than they need to be, if you drive a car with narrow tires and turn the steering, it will turn, now do the same and apply the brakes, the car will not turn, take your foot off the brakes and what do you know, it turns.

and last of all they cause the back to rise so you can't use the rear brakes to the maximum of the contact patch, this is why the car becomes unstable, not the rear brakes.

in my description above i stated that the torque applied to the chassis by the rear brakes got less as you moved the measuring point forward under the chassis, therefore close to the axle it is maximum, applying a load just infront of the axle centerline and well within the contact patch, applying the front brakes, however causes the weight to be moved away from this point towards the front and your rear tires loose grip.

where would you like the center of mass to be in an autocross car, i think i would like it to be in the middle.

my locost has the brake bias bar biased to the rear because it has 40-60 weight distribution, and it stops better like that than with no bias, the acid test is on a dirt road, which end consistantly locks up under hard braking? i will almost gaurentee that on most cars the fronts lock up, this to me indicates that when the fronts lock, the backs still have more to give that is not being used, i do understand that on a dirt road the tires break away before weight transfer can occur.

wouldn't all four wheels locking up be good if they did it at the same time, then you would know that you were getting the best stopping available before the tires break away.

all of you that race, i have a question,"which tires are more prone to flat spotting, the fronts or the backs?"

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PostPosted: September 2, 2014, 12:54 pm 
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Long story short: Sorry, but your basic assumptions are incorrect.

Drivers are taught to brake separately from turning, as you cannot master the art of doing both without mastering each independently first. However, for a skilled driver, there would ideally be no period of time during which the car is at a 'static' state without any lateral or longitudinal acceleration. However, braking while turning also does not mean large amounts of both at the same time either. The tires have a finite amount of grip available, all of which should be used under braking in a straight line. Use a little less longitudinal grip from braking, gain a little lateral grip to use for beginning to steer. Use a lot less longitudinal grip, gain a lot more lateral grip.

Your premise focuses on the suspension attitude, when maximizing use of the available tire grip at any given point in time is what turns the fastest lap times...Due to the intricacies of load paths through the suspension geometry these two are not necessarily correlated at any given point in time. The anti's (dive/squat/roll) change how the weight is transferred, but not how much. Consider a drag racing car with >100% anti-squat. Under full acceleration the rear suspension can actually be designed to rise...Yet at the same time 100% of the weight is also transferred to the rear tires, as it pulls a wheelie. The same basic principle applies when your car (or a motorcycle) squats when applying brakes to only the rear, yet the weight and available grip is still transferring to the front.

As far as brake bias goes, if the front tires lock first the loss of control will be snap understeer. If the rear tires lock first the loss of control will be snap oversteer. Take your pick. The theoretical ideal might be to have all four lock at the same time, but in the real world conditions vary from braking zone to braking zone and from day to day...So ultimately the goal is to have the fronts lock just before the rear under the conditions most prone to rear lockup, and live with the slightly less ideal use of the rear brakes everywhere else.

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Last edited by Driven5 on September 2, 2014, 2:40 pm, edited 2 times in total.

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PostPosted: September 2, 2014, 2:36 pm 
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john hennessy wrote:
if you could completely disconnect the front brakes and apply full braking with just the rear,
would the weight transfer still happen, i know that if i apply the park brake only in a straight line, the rear squats, now is the weight still being transfered to the front or to a point infront of the rear wheels?


As has been said, yes it will transfer *but* as you transfer weight off of the rear, you have less grip, which means less available braking. The amount the brakes can decelerate the vehicle will be significantly low.


john hennessy wrote:
my locost has the brake bias bar biased to the rear because it has 40-60 weight distribution, and it stops better like that than with no bias, the acid test is on a dirt road, which end consistantly locks up under hard braking? i will almost gaurentee that on most cars the fronts lock up, this to me indicates that when the fronts lock, the backs still have more to give that is not being used, i do understand that on a dirt road the tires break away before weight transfer can occur.

.......

all of you that race, i have a question,"which tires are more prone to flat spotting, the fronts or the backs?"


The fronts tend to lock more since the bias is typically in that direction. If a car is 40/60 weight distribution, you probably won't want 60% of your braking effort on the rear since it loses weight as the brake pedal is applied. Many factors effect how much weight transfers forward under braking but lets say you have 20%; in this case you want 60% of your braking to come from the front wheels since they have the extra weight.

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PostPosted: September 2, 2014, 3:12 pm 
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John,

I can't say anything about autocross, but can tell you that most of the things you read that say "steady state" or similar are idealized conditions that permit simplifying assumptions to be made by engineers and physicists. Usually, it's for the purpose of making the mathematical models they're using simpler and more manageable.

For example, in a steady state (essentially equilibrium) you can assume all forces, or moments, or accelerations involved sum to zero. It doesn't happen in real (automotive) life too often.

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PostPosted: September 2, 2014, 7:13 pm 
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I also plan to set up my car so it all 4 wheels lock up as the same time. I have the bias valve right next to the steering wheel so adjustments are easy to do on the fly. Not as easy in reality due to the different track conditions. In autocross you get several runs so you can test and tune as the event goes on especially if Saturday is a test and tune day. If you race as the same place all the time you can get your car dialed in and maybe take notes for each track.

There are several things that change on the track also that affect car setup. On concrete surfaces you generally have a lot more grip so the fronts will provide more stopping force prior to locking. Asphalt/tarmack the grip is typically much less so the brakes lock up easier. The first few runs in the morning often have more dust on the track that will eventually clear. Going off line can also be an issue with dust causing loss of traction often to just one wheel. Typically during the day the track will warm up and as it does it gets faster, however it can also get too hot and make the car feel mushy. Also moisture maybe rain, humidity, morning dew, snow all change the characteristics of the car.

Other things that affect the cars handling are tire types. Typically as tires wear my car will stop being neutral and start to over-steer or under-steer so I need to adjust accordingly. Same thing when I get new tires. Typically changing tire pressures and shock settings will get the car to handle the way you want at the track. As I start to run our of shock adjustment (ie running full stiff) it is time to make other adjustments to get back into the middle of the shock and have adjustments.

Different track setups will also affect the car especially how you will compare against other cars. Power courses, speed maintenance courses, tight twisty tracks, short courses long courses all need a slightly different setup and give an advantage to different types of car. In my Rx8 I can go to the same track one week and finish top 3, then next month go back and not even make top 10 due to a different course layout.

If you want to look more into rear only braking might want to find some GoKart guys as the Non shifter karts only have rear brakes.

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