Mark Ortiz Chassis Newsletter

[gt1guy]

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.


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


MAINTAINING BELT TENSION ON BELT DRIVEN LIVE AXLE

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.








ASYMMETRICAL BEVEL GEAR DIFF FOR UNEQUAL TORQUE SPLIT

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.

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.

 

[gt1guy]

This is the July 2020 news letter



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

TRIUMPH SWING SPRING

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.


=============================================================================================================================

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:
[url]https://www.youtube.com/watch?v=XiUUIJBL9nU

[/URL]



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).

 

[Weasel]

Neat newsletter. I've been getting Race Car Engineering and enjoy reading his stuff although much of it anymore I beyond me. Still enjoy trying to understand it.

 

[gt1guy]

Neat newsletter. I've been getting Race Car Engineering and enjoy reading his stuff although much of it anymore I beyond me. Still enjoy trying to understand it.

Ya, I just thought I'd post them up. Every now and then there's something we can gleam as usefull...............................last months newsletter not so much

I may go back over to the other site and copy the one over when he answered a question I had asked.

 

[gt1guy]

Found it. It was from 2015


I wrote Mark Ortiz a while back with some questions about designing a link front suspension on my Jeep. For those who have never heard of him, he runs a chassis consulting service, has a monthly column in Racecar Engineering and puts out the monthly Chassis Newsletter. This months news letter is on the questions I had, so I thought I'd share. The bolded part in quotes is my questions to Mark.

July 2015

From me

I was wondering if you could shed some light on designing a 4-link suspension for the front of a 4 wheel drive vehicle? Vehicle in question is a new style 4 door Jeep JK. I'm designing a custom long arm 4-link front and rear.

The stock geometry is four longitudinal links and a Panhard bar. What I'm doing is a double triangulated with no Panhard bar. Uppers converging at the axle and the lowers converging at the crossmember for the trans. Steering will be full hydro w/double ended ram. No mechanical steering box or drag link (so no bump steer). Coilover shocks mounted outboard on the axles.

I will be running this set up front and rear (rear, minus the steering of course). It's actually a pretty standard design in the 4x4/rock crawling world.

As I said, I'm kind of at a loss as to how the front reacts to the forces.

I've attached the Excel 4link calculator with my design on it.

Main question has to do with anti-squat. On acceleration there is weight transfer off the front end, so would designed in A/S actually end up being pro-lift? I'm thinking it would only act as A/S during braking. I'm kind of at a loss as to how to look at things with regards to the front end.

There is a large offroad community that would be very interested in your thoughts on this.
​

I am a relative newcomer to offroad chassis. I haven’t had clients in offroad motorsports. However, I have new neighbors here at the shop who do quite a bit of fabrication for offroad vehicles and also run offroad vehicles of their own. I find the whole field very interesting, but I should emphasize that I’m still in the steep part of my own learning curve and my thinking is still rapidly evolving.

I have at least gotten wise to this: there is a huge diversity of “offroad” vehicles and activities. If anything, there is considerably more variety than there is “on road”. And there isn’t any single set of desired properties for an offroad suspension system, any more than there is a single set of desired properties for all vehicles operating on pavement.

Returning to the original question, how do we understand longitudinal “anti” effects in a vehicle where all four wheels are driven, especially at the front, and what properties do we want in this regard? Taking the last part of this first, there is not a single answer for all applications. It depends on what we’re doing with the vehicle.

Usually, we do not speak of anti-squat when referring to the front wheels. Anti-squat means a tendency of the rear suspension to jack up under power, countering the tendency for the rear suspension to compress due to rearward load transfer. The corresponding property at the front is anti-lift: a tendency to jack down under power, countering the tendency for the suspension to extend. Under braking, we can have anti-lift at the rear. The corresponding upward jacking tendency in braking at the front is called anti-dive. All of these can be considered forms of anti-pitch.

Negative anti-lift is pro-lift; negative anti-dive is pro-dive – and so on.

100% anti-squat is the amount of anti-squat that will make the rear suspension neither extend nor compress in forward acceleration. That doesn’t mean the car won’t pitch. It just means it will pitch entirely by rising at the front; the rear won’t go down.

For front wheel drive, 100% anti-lift is the amount that will cause the front suspension to neither extend nor compress in forward acceleration. Again, the car will still pitch, but it will pitch entirely by squatting at the rear; the front won’t come up.

Likewise, in braking 100% anti-dive or anti-lift is the amount that will result in zero displacement at the end in question when braking.

Although linguistic evolution has given us four different terms for these effects, they are all fundamentally the same thing: jacking effects resulting from longitudinal ground plane forces.

In all cases, including also jacking resulting from lateral ground plane forces, jacking force equals ground plane force times jacking coefficient. Referring to the graphic in the main page of the attached spreadsheet from the questioner, the jacking coefficient corresponds to the slope of the force line, the green line lowermost in the frame. The slope of this line equates to the ratio between jacking force induced by the suspension linkage and the ground plane force applied to the system.

The slope of this line is also the inverse of the instantaneous slope of the path that the contact patch center follows as the suspension moves, when the wheel is locked in a manner appropriate to the situation being considered (i.e. braking or propulsion). In the case shown in the spreadsheet, the force line has about a 1 in 4 slope. This means that for every pound of longitudinal ground plane force, the suspension induces a jacking force of about a quarter of a pound. In this case, when the force is forward (propulsion), the jacking force is downward (anti-lift).

The spreadsheet is evidently designed with rear wheel drive in mind. The 100% anti-squat line shown is correct, assuming that the other wheel pair doesn’t contribute to propulsion. In that situation, 100% anti (in this case anti-lift) happens when the force line intercepts the opposite axle plane at sprung mass c.g. height (light blue horizontal line). In that situation, it is also impossible to get any jacking effect at the opposite axle at all, because there is no ground plane force there.

That of course is not the case with four wheel drive, except maybe if drive to the rear is disabled. When both front and rear wheels contribute longitudinal force, as in braking with most vehicles and with all wheels driven, we need a steeper force line slope to get 100% anti at a given axle. However, we can get jacking forces at both axles.

The procedure when solving graphically is to lay in what I call a resolution line at a location corresponding to the ground plane force distribution between the two axles, and compare the heights of the intercepts of the front and rear force lines and that resolution line to the height of the sprung mass c.g. If, for example, the front wheels make 60% of the ground plane force, the resolution line is 60% of the wheelbase from the front axle. If the front wheels make all the ground plane force, as in the spreadsheet, the resolution line is 100% of the wheelbase from the front axle, as shown.

If the vehicle has a center differential, we have a known ground plane force distribution, at least until some locking is imposed on the center differential. However, when we have a locked transfer case, we do not have a known torque distribution. We have a 1:1 driveshaft speed distribution, and a highly variable torque distribution and ground plane force distribution.

If the vehicle is running straight and there is similar traction at both ends, we will have close to 50/50 ground plane force distribution. However, if one end has more traction than the other, there will be more torque to that axle and more ground plane force from that wheel pair. When traction is good at both ends and the vehicle is turning, often the torques and ground plane forces are not only unequal but opposite in direction. The front wheels will follow a longer path and consequently need to turn faster than the rears, but be unable to. The rear wheels will then drive and the front wheels will drag. There will be reverse torque on the front drive shaft and extra torque on the rear drive shaft to counter that. The ground will exert rearward force on the front contact patches and forward force on the rear contact patches. In the questioner’s vehicle, the front will try to lift under these conditions. When it’s propelling the vehicle, its jacking forces will try to hold it down instead.

So there’s considerable uncertainty about what the induced jacking forces are going to be, because of the extreme variability of the ground plane force distribution. Do we at least know what we want the jacking forces to be?

Sort of, but that varies with what we’re doing with the vehicle. For an application such as offroad racing or Global Rallycross, we want the jacking forces to fight pitch, but not too much. If we get too greedy with our antis, we will get wheel hop on pavement or other high-traction surfaces.

For mud, things are different. There, we want both ends to jack up under power, vigorously. Why? Because when we’re stuck, sometimes the momentary tire load increase when we goose the throttle and the suspension pushes up against the frame will get us moving. It doesn’t always work, but in a useful percentage of cases it will.

And for crawling? I’m not sure it matters a whole lot, since the speeds and accelerations are so modest. I think probably the most important thing for a crawling suspension is to have huge travel, and a combination of stiffness in roll and softness in warp.

---