The Mark Ortiz Automotive

CHASSIS NEWSLETTER

November 2014

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WELCOME

 

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions.  Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by    e-mail to: markortizauto@windstream.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

EFFECT OF SPRING SPLIT IN THE PRESENCE OF UPWARD JACKING

 

Mark, I was going to ask you a question about Panhard bar rake, but found you did a piece on it back in August 2006 in your newsletter. I have copied the text below, but what I am wondering is what the change in tire loadings are from the jacking force? I see you mention itís not significant? Also regarding spring split your say if the RR spring is stiffer than the left and you have the Panhard bar sloped down to the frame attachment that the LR will gain load and diagonal cross weight percent. This confuses me because I would think the stiffer spring would carry more of the load than the softer one. Can you explain the further?

 

Hereís the 2006 text:

 

ďIn a NASCAR oval track chassis, a Panhard bar that slopes down from its attachment on the left axle tube to its frame attachment on the right does create a force trying to lift the rear of the car.  This force is present through the entire turn, not just during entry.  This force does not just load the left rear tire.  It does pull down on the axle on the left, but it also lifts up on the frame on the right.  Its effect is most commonly modeled as a force spreading the axle and frame apart, acting at the midpoint of the bar's span, approximately in the middle of the car.

ďIf the car has little or no rear spring split, a force in the middle of the car, lifting the frame away from the axle, gets the rear spoiler up in the air but does not significantly change wheel loads, except by aerodynamic effects.  However, current NASCAR setups use considerably stiffer springs at the right rear than at the left rear, so there is some increase in left rear load, and diagonal percentage, because of that.  If the car has a left-stiff rear spring combination, the effect reverses, and the jacking force actually increases right rear tire loading and reduces diagonal percentage. 

ďAgain, these effects persist through the entire turn, and only go away when the rear tires cease making lateral force.Ē

 

 

Okay Ė does a jacking force change wheel loadings?  Disregarding secondary effects from aerodynamics and small effects due to c.g. movement, jacking forces can significantly change diagonal percentages but not front, rear, left, or right percentages.

 

The suspension linkage and springs canít press down on the axle harder than the rest of the car presses down on them.  When the linkage generates a force trying to lift the frame with respect to the axle, that takes some load off the springs.  The linkage is then partially supporting the frame.  Since the springs have less load on them, they extend.

 

Remember that rate is not force.  Itís the amount of force change per unit of displacement.  A stiffer spring exhibits a greater force increase per unit of compression, and also a greater force decrease per unit of extension.  When a jacking effect adds load to both rear springs similarly, the stiff one gains more load.  When a jacking effect unloads both springs, the stiff one loses more load.

 

Actually, itís a bit more complicated than that.  The front suspension affects things too.

 

Some extreme cases may serve to illustrate.  Suppose we have a beam axle rear suspension with a 200 lb/in spring on the left and a 300 on the right.  Suppose we take the front wheels off the car and support the front end on a jack stand with a piece of angle iron on the top of it, under the middle of the front crossmember, so that the front end is held up but canít resist roll, and we have the rear sitting on the tires as usual, on wheel scales.  Suppose thereís a crossmember above the axle and we can put a bottle jack on the axle and lift the frame, midway between the springs.  Then suppose we exert a 500 pound force with the jack, spreading the axle and frame apart.

 

The springs will each unload by 250 pounds.  The left one will extend an inch and a quarter (250/200).  The right one will extend 5/6 of an inch (250/300).  The car will rise and roll to the right.  There will be no significant change in the scale readings.

 

Now suppose we do the same thing, except now we have the front end on its wheels as well, on wheel scales.  Suppose the front end has an infinitely stiff anti-roll bar, and the tires and the frame and all other parts are perfectly rigid, so that the front end prevents the car from rolling at all and the frame canít twist.  The rear springs will want to extend unequally as before, but they wonít be able to.  They will have to extend equal amounts, since the car canít roll.

 

The springs have a combined rate of 500 lb/in, so they will both extend an inch.  The left one will then unload by 200 pounds and the right one will unload by 300 pounds.  The right rear wheel will show a load decrease.  The left rear wheel will show an equal load increase.  The front wheels will show approximately equal load changes, the opposite way.  The diagonal percentage will increase.  The left and rear percentages will not change significantly.

 

Real world situations fall somewhere in between these extremes.  We can definitely say that any time anything in the rear suspension creates a roll moment, the resulting change in diagonal

 

percentage becomes greater as we increase the relative elastic roll resistance at the front and becomes less as we increase the relative elastic roll resistance at the rear.  This applies even to torque roll from driveshaft torque.  This explains why we can improve the drag strip times of a powerful live axle production sedan by disconnecting the front anti-roll bar.

 

We can generalize this a bit further.  Anything that produces a roll moment at only one end of the car changes diagonal percentage.  The magnitude of that change increases as we add relative elastic roll resistance at the opposite end of the car and decreases as we add relative elastic roll resistance at the same end of the car.

 

 

UK GP MIDGETS

 

I have really enjoyed your articles in Racecar Engineering and I hope you would be able to help with the suspension design of our new GP midget chassis for the 2018 season.  We are designing the car from the ground up and given the rules book for UK GP midget oval racing is very open in regard to suspension we really could do with a hand with what direction to go in for the suspension.  Our current car uses conventional double wishbone suspension with large coil over dampers at each corner.

 

My plan for the new car is to make much better use of aero which will be a first in the class. Most of the current competitorsí cars run little bodywork and fit a large F1 oval style wing to the upper roll cage. I am thinking along the lines of an Indy/F1 car style body, front and rear adjustable wings, side pods, flat floor and diffuser, with inboard dampers front and back to improve aero/packaging. The ovals we race on tend to be rather small with around 15 to 17 second laps with a varying degree of banking on tarmac.  The car will be running a ~180bhp 1.4 Vauxhall Ecotec in a rear engine, RWD configuration.  Tire wise we run Avon crossplys 10/20/13 on the back and 9/20/13 on the front but the sizes could vary from this if needed.  I have attached a link to the technical regs to help. 

http://www.ovaltrack.co.uk/gpmidgets/starting_out.php

This is a completely new class to me.  Iíd never heard of these cars.  The question came in back in August.  I was hoping to have some consulting work out of this but the questioner hasnít been replying to my e-mails so I guess Iíll use this for publication, and expand a bit on my original reply.  Also, the link to the rules worked back in August but now appears to be dead, so I donít know whatís going on with the sanctioning body or the class.  Regardless, itís interesting to contemplate how youíd build one of these.

 

I read the rules.  A few initial thoughts:

 

Remember that the bodywork and wings and undersides of current F1 and LMP cars are the result of their rules, not what works best.  They use flat floors and diffusers because they are no longer allowed to use tunnels.  They donít use big overhead wings because they arenít allowed to.  They

 

 

need relatively low drag because they have long straightaways.  You are running short ovals and have none of those rules.  You need high downforce, almost at any price in drag.

 

If I were doing this, I wouldnít build a miniature Indy or formula car.  Iíd build more of a miniature Supermodified Ė engine way to the left, between the left wheels, alongside the driver.  Indy cars have to run on road courses.  You run only short ovals.  You need to maximize left percentage.  If you do put the engine behind the driver, you still want to offset as much mass to the left as you can.  The only constraint I see in the rules is that you can only offset the driver ten inches.

 

Most of the legal engines are from front-drive sedans with transverse engine mounting and have integral transaxles.  One exception is the old BMC A Series engine, which was used in that sort of layout and also in conventional rear drive cars like the Spridget and Morris Minor.

 

The cars are required to have starters and reverse gear, unlike US-style midgets.  It might be possible to mount a front-drive powertrain far to the left, rotated 90 degrees, substitute a spool for the diff, and run a driveshaft back to an open tube quick change rear axle, if the gear ratios could be made to work.  The car would have to run in high gear at the transmission, and the rear axle would have to be toward the tall (numerically low) end of the available ratio range.  Possibly the gearing would be too short even then.

 

I also donít know how much stagger can be obtained with legal tires.  Thatís important with any kind of locked axle.

 

With the driver and engine offset to the left, it might be possible to have a single tunnel to the driverís right.  Iíd have a rear wing to help drive that, whatever wing would fit at the front, and an overhead wing on the roll cage.

 

Small cars like this on short tracks can go really fast and be a lot of fun, without costing an arm and a leg.  I wish this class well.