No down force in CM.

I keep thinking about front down force in my FF. I know it isn't legal in CM, but would it even help to get more grip from the front end of a CM car? If more front grip would be helpful, then might it work by putting some weight in the crush box(my car is way under legal weight anyway and 58% rear to 42% front) which is in front of the front axle? I am going to try it. Maybe someone who is a better engineer than I can say how much weight would be a good starting point and figure out if the extra weight would cause a momentum problem greater than the extra down force.
Mark

aero downforce, and ballast weights, are two completely different things.

food for thought, "downforce" is not a "sprung weight" - i'd rely on the AutoX guys in class on advice on weight distribution and set up in order to dial out understeer. and suspect after a good static set up, damper/ARB is the first place to start.

unrelated note - i have some good but used AutoX hoosiers to sell :)

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Originally Posted by mwizard

I keep thinking about front down force in my FF. I know it isn't legal in CM, but would it even help to get more grip from the front end of a CM car? If more front grip would be helpful, then might it work by putting some weight in the crush box(my car is way under legal weight anyway and 58% rear to 42% front) which is in front of the front axle? I am going to try it. Maybe someone who is a better engineer than I can say how much weight would be a good starting point and figure out if the extra weight would cause a momentum problem greater than the extra down force.

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First you have to understand that rubber on asphalt/concrete/etc. is not a linear friction system.

If you double the load on a tire you get more friction, but not twice as much more.

So if you add that load by adding or rearranging ballast, you get not quite as much friction, but you all of the additional MASS of the ballast to accelerate. Moving weight forward is going to make the car have worse understeer (or less oversteer).

Aerodynamic downforce gets load on the tires WITHOUT adding mass that must be accelerated laterally by those tires. That's why it works.

Another thing to consider is that by putting the ballast you need (i.e. it's not additional mass, just where you position what you need meet the legal minimum weight) at the ends of the car, you will have a larger polar moment of inertia than if you placed the ballast between the front and rear axles. The larger the polar moment of inertia, the more your car will resist turning (ie, makes it stable in high-speed large radius turns, and not so much in the tight quick transitions, like slaloms). You will get the lowest moment of inertial by placing it near the CG of the car (likely under the fuel cell -- or in the fuel cell if you use extra fuel as the ballast)

You can still change your weight distribution to the front by putting your ballast under your legs, I would suggest you try that first.

An interesting experiment would be to do fun runs on the same day with ballast in the nose one run, under your legs for the next and finally in the fuel cell to see how in affected the handling.

Mark,
1) From what I understand, it is easy to have lift on those cars. Killing frontal lift may help you. Look at underbody rake angle at speed first.
2) Look at the exact wording in the rules to see what you can actually do.
3) I've seen some FF bodies over the years that clearly were designed for downforce and I'll bet there wasn't one dang thing anyone ever did about them.
4) Getting weight off the rear supposedly has helped some. Getting the Alum head not only allows you to take weight off the rear but to have it lower on the car.

-Chuck

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Originally Posted by David Ferguson

Another thing to consider is that by putting the ballast you need (i.e. it's not additional mass, just where you position what you need meet the legal minimum weight) at the ends of the car, you will have a larger polar moment of inertia than if you placed the ballast between the front and rear axles. The larger the polar moment of inertia, the more your car will resist turning (ie, makes it stable in high-speed large radius turns, and not so much in the tight quick transitions, like slaloms). You will get the lowest moment of inertial by placing it near the CG of the car (likely under the fuel cell -- or in the fuel cell if you use extra fuel as the ballast)

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+1. This is a huge player in dynamic response. If you want to move the weight distribution forward do it by keeping the weight more toward the CG to keep your yaw inertia low.

What issue are you trying to solve? Does the vehicle lack ultimate grip or is the handling balance the issue? Start there in your search for a solution because they are likely not the same development paths. Balance is a relatively ease thing to chase, but increasing overall grip can be more involved.

For nationals in the past I have placed weight under and in front of the seat. I was hoping that I could get a bit better bite from the front wheels for turn in and speed before push. I have only 42 % weight on front wheels. Some have said rake can be used for better front end response. How does this work.
Mark

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Originally Posted by mwizard

For nationals in the past I have placed weight under and in front of the seat. I was hoping that I could get a bit better bite from the front wheels for turn in and speed before push. I have only 42 % weight on front wheels. Some have said rake can be used for better front end response. How does this work.
Mark

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If turn in response is your goal there are other ways, with less compromise, to get there. One would be running toe out in the front. Another would be increased low speed compression in the damper. You could also do it with a stiffer front ARB or springs although those will have effects elsewhere in the corner.

Ackerman.

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Originally Posted by mwizard

For nationals in the past I have placed weight under and in front of the seat. I was hoping that I could get a bit better bite from the front wheels for turn in and speed before push. I have only 42 % weight on front wheels. Some have said rake can be used for better front end response. How does this work.
Mark

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corner weights are just like you standing, if you shift your weight to the side you put more weight on that leg, but your total weight never changes, so as you change 'rake' front to rear, or left to right, you are changing how much the car is standing on all four feet. more rake to the front (lower) will transfer more weight to the front. if you can imagine you raked the front end down until the car was vert. you would have 100% of the weight on the front.

in reality you likely want to set your front end ride height where it needs to be and adjust the rear.

unrelated side note.... again... i have 3x adjustable ackerman upright tabs (ears) for swift db-1.

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Originally Posted by 2BWise

If turn in response is your goal there are other ways, with less compromise, to get there. One would be running toe out in the front. Another would be increased low speed compression in the damper. You could also do it with a stiffer front ARB or springs although those will have effects elsewhere in the corner.

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I can go for more LSC, pretty easy and toe out as well. Is an 1/8 in per side a reasonable amount? I can also lower the front a bit, too.
I have heard the term Ackerman, but know nothing about it. Any help?
Mark

Hey Mark,

The next event you go to that maybe we can both go to I'll show you what there talking about.

Ben

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Originally Posted by mwizard

I can go for more LSC, pretty easy and toe out as well. Is an 1/8 in per side a reasonable amount? I can also lower the front a bit, too.
I have heard the term Ackerman, but know nothing about it. Any help?
Mark

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1/8" would be a lot for road racing but could work for autox. Back when I was auto crossing fsae I would go as far as 1/4" but at that point it was more about heating up the tires earlier in the run.

Due to the way SCCA Safety Stewards look at course design and often mandated top speed allowable considerations..........for most speeds of SCCA AutoX, aero is a very negligible factor...........unless you go way over the top like some do in the M classes where they do hang virtually a set of venetian blinds on the car

softer front springs for weight transfer, maybe more rear sway bar to force the rear to rotate, more seat time and shhhhhhhhh...............because you got to 'cheat legal' use a LOT of tire softener at home and don't bring it to an event.

^^ The above is a common misunderstanding. Aero plays a significant role in autocross cars. Keep in mind we are not using road race profile wings. For CMod, IF your car has significant positive lift, your situation can be improved.

I feel like typing this morning.....
Ackermann geometry involves the angle of the inside tire vs. the outside to make the tires EACH follow the correct radius in a turn. The original presumption was that the inside tire had to turn sharper than the outside tire to avoid "fight" between the tires and put each tire on its proper radius in a turn. If one were to project the steering arm angles so they intersected at the rear axle centerline, that was geometrically optimum for a car with rigid tires.

However, once you have a suspension system with flexible rubber as well as different forces on that rubber which affect slip angle vs. speed from aero down-force, what works best for you can be different from that standard Ackermann picture.

Playing with changes to static toe is good to start off with, since it is so easy.

Many (not all) UN-modified older road race cars have too little positive Ackermann for our corners, so toe-OUT helps your grip.

Static toe changes may only really "fix" the overall geometry for a single radius turn; they may not help the ability of the car's steering geometry to provide a correct high grip solution in all your radii turn cases.

But, for many, it may be good enough for their type of racing.

It is also possible to have so much positive Ackermann effect from the car's steering geometry that you need to add static toe-IN to improve low speed tight radius grip.

Years ago, I worked on a stock class rear-heavy street car that gained significant front end autocross grip from a large amount of static toe-IN.

The location of the rack fore and aft vs. front axle centerline also has a significant effect upon Ackermann.

One of the classic improvements to front end grip on Swift DB-1's and 3's was to move the rack and not change the steering arm angles.

Ok, Lots of good info here. In order to get good data(not too many variables in play), I only increased the low and high speed comp. dampening. The low was at about the middle of 30 clicks so I increased it 4 clicks. The high was at 1/3 of the way to max(24 clicks) so I added 2 toward max.
I will get results tomorrow.
Mark

If you really want to experience the effects you should try bigger changes that just a couple clicks. Make big swings early in to see what they do and how much authority they have on performance. You'll make slow progress doing a click or two at a time.

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Originally Posted by mwizard

Ok, Lots of good info here. In order to get good data(not too many variables in play), I only increased the low and high speed comp. dampening. The low was at about the middle of 30 clicks so I increased it 4 clicks. The high was at 1/3 of the way to max(24 clicks) so I added 2 toward max.
I will get results tomorrow.
Mark

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The most effective development method is to make only one change, every single time the car comes to a stop. Like that, every run is a development iteration.

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Originally Posted by 2BWise

If you really want to experience the effects you should try bigger changes that just a couple clicks. Make big swings early in to see what they do and how much authority they have on performance. You'll make slow progress doing a click or two at a time.

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I know about the big swings theory. I prefer to do it several times in smaller increments. Also, I am a bit slow to pick up on the changes so need lots of runs to get good data.
So the car seemed to handle better today. I charged the back 1/2 in 4th gear with no protests from the suspension or tires. Unfortunately the motor had another opinion and on the last(9th)run, it blew(see photo). There is a corresponding hole in the block just in front of the distributor. Been nearly 45 yrs since I last blew up an motor. Maybe I can live long enough to do it again.:D
I had already started on a new motor. Now just need to up the priority.
Mark

Wow that really sucks Mark, good you have a spare motor but I feel your out for the year? let me know if you need any help.

Ben

ouch mark... that sucks..

So sorry to hear about the motor misfortune.

Was this at Crow's Landing where you might have seen some prolonged high revs?

How long are the runs there typically for you and other fast guys in your class as well as Tom and Matt in BM?

All clubs running about the same length and time + top end speed course there?

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Originally Posted by mwizard

I know about the big swings theory. I prefer to do it several times in smaller increments. Also, I am a bit slow to pick up on the changes so need lots of runs to get good data.
So the car seemed to handle better today. I charged the back 1/2 in 4th gear with no protests from the suspension or tires. Unfortunately the motor had another opinion and on the last(9th)run, it blew(see photo). There is a corresponding hole in the block just in front of the distributor. Been nearly 45 yrs since I last blew up an motor. Maybe I can live long enough to do it again.:D
I had already started on a new motor. Now just need to up the priority.
Mark

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I may have a block if you need one.

Dan

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Originally Posted by B17overhead

So sorry to hear about the motor misfortune.

Was this at Crow's Landing where you might have seen some prolonged high revs?

How long are the runs there typically for you and other fast guys in your class as well as Tom and Matt in BM?

All clubs running about the same length and time + top end speed course there?

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The runs out at crows landing are using not longer than 60 seconds for any given class. Tom and Matt go plaid in just about every event we have out there and they always seem to have a gear ready to go faster...

In CM it really depends on the gear ratio setup we run. I used to run a short setup that required more shifting, but even on the longer courses you were between 3rd and 4th gear. I can't speak for Mark, but I now run a long gear setup where I am staying in 3rd gear for a longer period.

With either gear ratio setup, we don't seem to stay in the higher revs, or bounce off the rev limiter as much as you would expect. If I hit the rev limiter it's only activating maybe 2-3 times before you're braking for the next corner

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Originally Posted by B17overhead

So sorry to hear about the motor misfortune.

Was this at Crow's Landing where you might have seen some prolonged high revs?

How long are the runs there typically for you and other fast guys in your class as well as Tom and Matt in BM?

All clubs running about the same length and time + top end speed course there?

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It was at Alameda, a Porsche club event and my best time was 38.3. Previous run I had gone to 4th on the back stretch with positive results (a max of 88 mph which I did not hit), the cones come at you real fast on such short courses at that speed.
I am running like Chris, usually in 3rd with 2nd and 4th in reserve depending on the course. I am very impressed by the size of the power band in the Kents.Mark

Chris, what kinda times should a BM car expect there? 50 sec typical per side, 115 mph top end?
Just trying to get a feel for how much harder compound compared to Marina as well as top end to plan fur.

Thanks

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Originally Posted by B17overhead

Chris, what kinda times should a BM car expect there? 50 sec typical per side, 115 mph top end?
Just trying to get a feel for how much harder compound compared to Marina as well as top end to plan fur.

Thanks

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Here the results from the last event where Mark and I ran against Tom & Matt in their oh so fast BM car.

http://www.americanautox.com/wp-cont...10-updated.pdf

Tom won the modified class (AAS doesn't use the SCCA classing) with a 38.1, I topped the CM class with a 42.3. Tom is typically 3-5 seconds faster on any given course at this point, but we have some good drivers coming into CM which is really upping the game for Mark and I.

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Originally Posted by B17overhead

Chris, what kinda times should a BM car expect there? 50 sec typical per side, 115 mph top end?
Just trying to get a feel for how much harder compound compared to Marina as well as top end to plan fur.

Thanks

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The last event, Tom got 33.6. I was trying to stay within 4 sec of him.

You guys sure are quick in CM, but like you indicate, those 2 in their Cheetah are tough acts to follow.

My Avons are close to 5 years old now, so trying to decide on new shoes for the car. For Crow's, looks like it has to be A11.

Lesson learned by this old fool: I spent WAY too many years designing and building my BM car.

Maybe its time now to buy a rocking chair?

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Originally Posted by B17overhead

Maybe its time now to buy a rocking chair?

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As long as that rocking chair has 4 wheels and has awesome cornering ability. :beer:

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Originally Posted by B17overhead

You guys sure are quick in CM, but like you indicate, those 2 in their Cheetah are tough acts to follow.

My Avons are close to 5 years old now, so trying to decide on new shoes for the car. For Crow's, looks like it has to be A11.

Lesson learned by this old fool: I spent WAY too many years designing and building my BM car.

Maybe its time now to buy a rocking chair?

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No. Bring it out! We want more mod cars.
Mark