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Mark Ortiz Chassis Newsletter

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    Mark Ortiz Chassis Newsletter

    I'm on the mailing list for Marks newsletter. Every now and then there's a newsletter that directly relates to the 4x4 world. Most of the time it's for the circle track guys. All of the time there is a tidbit of info that helps to better understand how changes in chassis/suspension geometry effect how a vehicle drives.

    So I'll post them up when he puts them out.

    June 2020
    Reproduction for free use permitted and encouraged.


    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: Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.


    I was looking at the pic of the DSR racer in your column in the Feb 2020 issue of Racecar Engineering and one thing about it struck my interest. It was that they were using a cog belt drive. Cog belt drives are 7 to 10% more efficient than a chain drive so using one at first appears to be a pretty good way to improve power without adding more engine stress. I have done some looking into belt drive for the small Bonneville lakester that my son and I race at the salt, that uses a motorcycle engine with chain drive. To have belt drive that will work you have to provide a fairly high amount of tension to the belt to insure it stays engaged. I contacted Gates application engineering with my specifications and drive requirements and they were recommending a belt tension of over 700 pounds! This level of tension almost surely requires that some sort of swing arm suspension be used at least on the drive side of the engine as you need, as seen in the photo of the DSR suspension, some way that ensures that the belt is maintained under a constant tension (and alignment) regardless of suspension travel. I have a design for a system that will work but I would not be able to fit it into the narrow confinements of our small car.

    I think that the belt drive is probably the main reason that the illustrated DSR is using the trailing arm suspension and not some sort of more sophisticated suspension design.

    Actually, the same thing can be accomplished with four trailing links. Each pair just need to both be the same length as the pulley center to center distance, and either be parallel to each other and also parallel to a line connecting the pulley centers, or non-parallel with an instant center somewhere on that line. To create no wedge change in braking with a single brake, the links need to be parallel, but not necessarily horizontal. To create zero bump steer, they need to be horizontal. So there may be a conflict between belt drive geometry and rear steer properties, but it is possible to completely eliminate wedge change in braking with a single brake.

    The thing that I was concerned about is that when using, say, a pair of equal length trailing links, their length has to be exactly the same length as the belt pulley centers; even a slight difference could change the belt tension considerably as the belt has to be tight and they are pretty stiff in tension. A very substantial idler pulley would be an absolute requirement to compensate for any possible change in pulley center distance due to suspension travel and of course you will need some sort of lateral location control that keeps he pulleys vertically in line to keep the belt flat across them.

    Well, yes, but the same applies to a single arm.

    At some penalty in weight, cost, and complexity, it is possible to use a tensioner. One appealing design is used for the belt drives in wheel balancers. It can be used for any kind of belt or chain, provided that we don’t need to maintain precise timing as with a cam drive. The system has two idlers, one on the top run of the belt or chain and one on the bottom run, each mounted to the frame on its own arm. The arms are free to swing with respect to the frame, but are connected to each other with a tensioning spring.

    When no torque is being transmitted, both runs are pinched toward each other. When torque is applied, the tension run straightens and the slack run bends more. The geometry is such that this stretches the tensioning spring and tension increases with torque, especially on the tension run. This effect can be tuned with the geometry and spring rate.

    The mechanism also cushions abrupt variations in torque, and works equally well on decel.

    It is common with toothed belts to provide guide flanges on just the drive pulley, and make all the other pulleys wider than the belt to allow some lateral (axial) float.


    The German idealist philosophers were right: anything is possible. You just have to say the magic words: “Nobody would ever do that.” Look what Doug Milliken sent me in response to my recent articles on differentials.

    In Jan-Feb 2020 you wrote:
    “And theoretically at least, we could make a bevel gear diff with unequal torque split too. This would involve having different size side gears, and pinion gears at an oblique angle. This is not a very attractive approach and I don’t expect to see anybody do it, but it’s theoretically possible.”

    See attached photos, taken in the lobby of the Segrave truck plant in Clintonville, WI (formerly the Four Wheel Drive Company). There was no signage and no one I met (on a weekend) could give details, but it looks to me like a heavy duty transfer case opened up for sales/training, with fixed-

    ratio torque split (and also a clutch pack limited slip). I didn't try turning the crank--don't think our guide would have appreciated that.

    We were in the lobby to see the AJB Special aka "Butterball" that Bill/Dad raced in the 1950s.

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    Photos courtesy of Doug Milliken

    It definitely is a transfer case, for a heavy all wheel drive vehicle. The U-joint yoke at the top with the crank is the input, presumably from a front engine and transmission, and the smaller lower yoke at the lower left is to drive the front axle. Looking at the close-up of the diff in the lower picture, it looks like the smaller side gear has about half the pitch diameter of the larger one. That would mean that the front drive shaft would get about a third of the torque, or about half as much as the rear drive shaft.

    I expect that for this application, the idea is not to facilitate throttle steering or allow for rearward load transfer. More likely, the vehicle will have two rear axles and the idea is just to have similar torque to each of the three axles. Generally, the front axle of a truck’s rear tandem will have an inter-axle diff and its own diff, often with a driver-controlled lock for the inter-axle diff. This vehicle would have five differentials!

    This is a two-speed transfer case. In the picture, it’s in high range. The shaft at the top with the rod end on it works the range selector shifter fork. Interestingly, the teeth of the low range input gear are used as engagement dogs to select high range. I take it this is not intended as a shift-on-the-fly system.


    This is the July 2020 news letter


    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: Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

    Thank you for your interesting article on the Corvair, (Racecar Engineering, August 2020, p.51 Corvair Vindication?) and the swing axle design that so attracted Nader's ire. As you say, that design and its shortcomings were well known in the 60s. You listed several manufacturers, so attracted by this cheap and simple suspension as to use it in their cars. But you omitted the car whose designers evolved the swing axle more successfully and cheaply than most, Standard Triumph. They chose the swing axle for their 'small-chassis' series of models, the Herald, Spitfire, GT6 and Vitesse. Motoring journalists knew as well as designers how to get such an axle to 'jack-up' and did so immediately, giving the cars a poor reputation. Triumph replied for the more powerful GT6 and Vitesse by adding a lower wishbone, but the design was heavy and expensive. As you say in your article, "what works best for a swing axle is stiff springing in ride… and soft springing in roll" but you only gave Formula Vee as the example. Triumph came up with the "swing spring" that did just that, by allowing the transverse spring to pivot in the centre, to reduce the roll resistance of later cars by 75% !

    This most successful modification is, I believe , unique, and I feel that Triumph's ingenuity should have been recognised!

    The "swing spring" is one of many ways to skin this cat. The oldest I know of is Mercedes' third coil spring.

    For those unfamiliar with it, the swing spring is a transverse leaf spring similar to the one that the Spitfire/Herald suspension already had, only rubber mounted. This allows the spring to swing as the suspension displaces in roll, but the rubber resists this a bit. The spring itself is also made stiffer than the original. This allows the system to act like the original system with a camber compensator added, or maybe a bit better, with fewer parts. Also, although I have not investigated the patent

    situation surrounding this, I would surmise that the swing spring probably would be a patentable invention.

    The swing spring reduces the elastic roll resistance by about 75%, compared to (I guess) an identical leaf spring rigidly mounted. The system still has a lot of geometric roll resistance, and it still jacks quite a bit. Indeed, Triumph could have just mounted the spring so it could swing freely, eliminating rear elastic roll resistance entirely. This would make the system functionally equivalent to a "zero roll" setup on a Formula Vee. The system would still produce substantial geometric roll resistance, and accordingly considerable load transfer and jacking when cornering. However, I doubt that it would have been patentable like that.

    Another way to reduce the jacking and limit oversteer is to add front anti-roll bar stiffness. The less load transfer the rear has (meaning the more the front has), the less the rear swing axle system will jack.

    Depending on tires, road surface, and rear toe setting, swing axles can produce judder on the inside wheel when we reduce rear load transfer. I have seen this with Mk. 3 Spitfires, on the street, on street radials. This is sort of the Y-axis analog of rear wheel hop in braking with a lot of rear brake and anti-lift. Swing axles can also produce judder on the outside wheel.

    There is no way to make a swing axle suspension as good as more complex forms of independent suspension. The Triumph swing spring does not produce handling equal to the earlier GT 6 suspension. However, it is somewhat lighter, definitely cheaper, and a significant improvement over the earlier swing axle setup. And it achieves this essentially for free - really no added parts at all. So to that extent, it is indeed a clever solution.

    In theory, GM could likewise have used just a swinging leaf spring on the Corvair - basically just a multi-leaf camber compensator - and dispensed with the coil springs, although there might be structural reasons that wouldn't work, at least not without re-engineering the engine and transaxle mountings. Those mounts would then be holding the back of the car up, and there would also be significant bending loads applied to the transaxle and engine.

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    Hi Mark-wanted to thank you for the chassis newsletters...I reference them all the time when setting up both our cars. This photo is from a race a while back at Portland International Raceway where we won the big-bore vintage race in this former Mark Martin Winston Cup car when Mike Laughlin "drop snout" perimeter chassis were supplied to Roush Racing and others. Steve Hmiel, Mark's crew chief said in an interview I saw recently they used the Laughlin chassis at Roush for "flat tracks" including road courses. Steve said they built a total of 46 cars from the late 1980's through 1994 (the year the interview was videotaped for broadcast).

    Steve was crew chief for Mark Martin there through 1996 and says they used one of three drop snout versions of the Laughlin chassis during their emergence as a winning team in early to mid-1990's on flat tracks with the cars.

    Can you clarify specifics why the evolution of drop-snout cars geometry there and later the use of it on Hendrick chassis cars so improved the geometry overall?

    Here’s a link to one of several YouTube editions of that interview:

    For those unfamiliar with these cars, they have a perimeter ladder frame made of 2” x 3” rectangular tubing, with a lot of added round tubes providing crash protection and triangulation. The rectangular rails are out in the rocker box region at the cockpit. Forward of the firewall they run inward and upward to a point about midway along the engine, and then horizontally straight forward again. This portion is called the snout. This plus the front cage structure is called the front clip.

    The upper control arms are a tubular a-arm with a solid cylindrical-bushed cross shaft. The cross shaft bolts to a plate bracket welded onto the top of the frame rail. This means that the frame rail constrains the height of the upper control arm pivot axis. Lowering the rail lets you lower that pivot axis. That, in turn, permits more camber recovery in roll for a given roll center height, or a lower roll center height for a given amount of camber recovery.

    When considering NASCAR vehicles, especially from this period, it’s important to remember that many of their design features are about working the rules. There were rules about static ground clearance, and rules about spring rates. There were even rules about the proportions of the lower control arm, which controlled spring to ball joint motion ratio.

    To get the car’s front valance as low as possible on a flat track, while still passing inspection, it was desirable to get as low a wheel rate as possible, and if possible, make the front end jack down in cornering. One way to reduce the wheel rate when the rules lock you in on rate at the ball joint is to use a very short front view swing arm length. To get that and a low roll center at the same time, you need the upper control arms to slope downward toward the frame quite steeply.

    As related in the video, Laughlin offered three standard snouts: regular, dropped (down an inch and a half, or about 38 mm), and half-drop (down ¾ of an inch, or about 19 mm).