Monoshock handling...

[Tom Duncan]

I am considering getting a monoshock FF, partially because everyone is telling me not to. Just what differences between a twin shock and monoshock car are there in terms of on-track behavior? What does it feel like compared to a monoshock car? It seems like design philosophy is much different in that roll handling is contolled by rear shocks primarily, and what passes for roll control at the front is by some crude washer/spring arrangement with no damping. Is that correct?

I like to stand a car on its nose and drag the tail around after it. Will a monoshock lend itself to that style?

Any and all illumination is much appreciated.

Tom Duncan

[carnut169]

Tom, lots of debating this topic... do a search under monoshock.

I think bottom line is that it can work very well if set up and maintained. It works MUCH better on smooth surfaces.
The shuttle does utilize beville washers that can be stacked in various ways and preloaded... the heavier the preload the more resistance to sway. Crude? I guess you could call almost any sway bar crude if you really look at it.

Sometimes, on a FC car, the mono has a bit of push that takes some time to get rid of.

Could you stand a car on its nose and drag the tail around? Sure, with plenty of rake and a stiff rear bar....

[lenhart05]

Most (if not all) 1600 Van diemen monoshocks used the die springs instead of the belleville washer stacks. The preload on the die springs is critical. Since you have opposing preloaded springs, the rate is effectively twice the rated rate of the spring. (the shuttle still moves, just at the higher rate) But, when you "shuttle" past the preload on one of the springs, the rate drops instantly. This can be a great tuning tool with testing, testing, and more testing. I ran enough preload to never rock out of the preloaded range to avoid this spike in the rate. The thought of the rate shifting as I drive through the kink at road america didn't sit too well with me.

Having driven both the van diemen monoshock and other four shock cars, the monoshock does have a great sweet spot. It is just hard to find the right combination of springs, preload, shocks, geometry. Once it is found, the car is a dream to drive with unbelievable turn-in. And, for some unknown reason, seems to have great grip in the wet. I would not hesitate to drive another monoshock Van Diemen. The only problem is the aero compaired to the Swifts. The '93 Van Diemen is really wide because it was the first car designed around the zetec. The '92 is better with the smaller engine cover.

Tad

[Rennie Clayton]

Tad,

A preloaded spring does not change rate - it simply does not deflect until the preload amount is reached. If you were running enough preload on your anti-roll springs to ensure that you never could overcome the preload, then effectively you were using an infinitely stiff front bar. You might as well have stuck in a solid steel puck, the effect would have been the same.


Cheers,
Rennie

[DaveW]

Tad,

A preloaded spring does not change rate - it simply does not deflect until the preload amount is reached. If you were running enough preload on your anti-roll springs to ensure that you never could overcome the preload, then effectively you were using an infinitely stiff front bar. You might as well have stuck in a solid steel puck, the effect would have been the same.


Cheers,
Rennie

Rennie, that is true if the preload is against a solid stop - if the two springs are loaded against each other, as in this case, the effect is as Tad stated.

Dave

[Rennie Clayton]

Dave,

How exactly are the springs mounted? Maybe a diagram would help.

All I am picturing in my head is two springs mounted in series, which will certainly not increase in rate with preload.


Cheers,
Rennie

[DaveW]

Dave,

How exactly are the springs mounted? Maybe a diagram would help.

All I am picturing in my head is two springs mounted in series, which will certainly not increase in rate with preload.


Cheers,
Rennie

The springs are mounted (schematically in parallel) on a slider ass'y. As the swaybar link(s) move they move the slider such that it unloads one spring while adding load to the other - the deflection of the springs is equal but opposite - one is compressed while the other extends the same amount. When the displacement is large enough that the extending spring is at zero load, its rate is no longer additive, and the spring rate of the total slider ass'y is cut in half.

It's obvious if you see a picture of the setup, but I don't have one.

Dave

[Rennie Clayton]

Dave,

A quick google image search turned up these two:





In which case, I assume that there is a spring / spring stack on either side of the rocker. Car rolls one way, the rocker presses against the spring on one side. Car rolls the other way, it presses against the spring on the other side right?

Question: the rocker rotates on a shaft that extends out quite a ways, and is centered on that shaft by the springs right? Under what circumstances are both springs being compressed at the same time, other than under preload?


Cheers,
Rennie

[Jim Garry]

Grainy photo but it works:

[Rennie Clayton]

Yeah. In those photos, the springs act to center the rocker, and any preload is additive to the system as a whole. I.e., if you add preload to one side, it preloads the entire assembly. But there is still no additive rate in the assembly.


Cheers,
Rennie

[DaveW]

2 identical springs moving in parallel have double the rate of one. As you offset the slider with both springs in contact it adds load to one side and subtracts it from the other. Each spring adds/subtracts load proportional to its rate, giving double the rate of one spring. When one spring is no longer in contact, then only the remaining spring's rate is in effect.

Dave

[lenhart05]

Example (rates are just for example):

each die spring is 100 lbs/in
each die spring is preloaded 400 lbs.

If a side load of 100 lbs is added to the shuttle. The shuttle will only move 1/2" because one side will load to 450 lbs and the other side will drop to 350 lbs. This will give you the 100 lbs difference. Thus, an effective rate of 200 lbs./in.

(This doesn't account for the friction as the shuttle tries to slide across needle bearings! Not the best design.)

Tad

[Rennie Clayton]

Hmm, interesting. Yes, I scoured my books and looked up the equation for this scenario last night, and you are absolutely correct. My bad. My question in that case, if you were using enough preload that you were always using the doubled rate under normal roll travel, is why not just use a stiffer spring?


Cheers,
Rennie

[DaveW]

(This doesn't account for the friction as the shuttle tries to slide across needle bearings! Not the best design.)

Tad

The friction factor is one of the main reasons this system fell out of favor. The other is the inability to individually control damping at each corner.

Theoretically, if friction is not taken into account, all roll stiffness is controlled by the shuttle stiffness alone, improving the ability to tune handling. Unfortunately, the above factors complicate the issue.

Dave

[DaveW]

Hmm, interesting. Yes, I scoured my books and looked up the equation for this scenario last night, and you are absolutely correct. My bad. My question in that case, if you were using enough preload that you were always using the doubled rate under normal roll travel, is why not just use a stiffer spring?


Cheers,
Rennie

Since, with one spring, you would not equally oppose the loads (they are equally opposed using 2 springs), the corner weights would be radically changed by preloading only one spring.

Dave

[Rennie Clayton]

Dave,

Sorry I wasn't explicit - the proper question should have been: why not just use a stiffer spring set?

In other words, if you are now using (for example) 100lb/in springs, but are at the point that you are preloading the assembly such that the entire normal range of roll is occurring in the preloaded 200lb/in range, why not simply use 200lb/in springs with no preload? I would think that the whole point of preload is to enable you to have higher nose pressure for better transient response, while retaining the lower rate in the normal range of motion for better mechanical grip.


Cheers,
Rennie

[DaveW]

AHAH. Now I understand...

IIRC, the reason for not using zero preload is the difficulty in achieving a smooth transition during zero roll - i.e., the transition from one spring to the other without either (1)momentary reduction to zero rate (minute amount of play) in the stiffness curve, or (2) a spike to double the rate (minutely preloaded) during the transition. Also, at zero preload, spring rates near the beginning of compression are not very consistent, so you wouldn't know what rate you actually had near the transition.

I know from personal experience that #1 is VERY discomforting - I've had slight play in the front swaybar linkage, and due to the lack of roll stiffness just off of straight ahead, the transitions were very disconcerting - I just couldn't predict what the car was going to do. Theoretically, #2 would cause a lack of grip at turn-in (just like excessive friction or steep nose angle), and if the transition were just a bit off center, or the road camber was changing, it would react differently depending on the condition.

[Rennie Clayton]

Understood, it is to avoid overcenter behaviour inconsistencies. So the SOP for this style of monoshock is to pick half of the anti-roll spring rate you really need, then preload the living bejeebus out of it. Makes one wonder why they never implemented a torsion bar in roll to avoid these problems, but that's probably another story.


Cheers,
Rennie

[DaveW]

Rennie,

IMO, the preload is what causes most of the friction, since, as you know, no spring is entire concentric in its loading characteristics. The more preload, the more side load, and the more friction. This was probably the cheaper solution, compared to a torsion bar.

[Rennie Clayton]

IMO, the preload is what causes most of the friction, since, as you know, no spring is entire concentric in its loading characteristics. The more preload, the more side load, and the more friction. This was probably the cheaper solution, compared to a torsion bar.

Ironic, given the mechanical nature of what an anti-roll bar is, which they abandoned the monoshock in favor of...


Cheers,
Rennie

[R. Pare]

Rennie:

A simple way to visualize the rate of a preloaded spring setup like this is (if you ever have to explain it to someone else!):

Take 2 1000 lb rate springs and compress them against each other 1.00". Each spring is producing a reactive force of 1000#. Now move the contact point between them .100 inch. One spring is now developing 1100# of force, the other 900#. 1100# minus 900# = 200 pounds for that .100 inch movement, hence a combined effective rate of 2000#/inch.

The friction in these systems is a combination of the friction produced by the lateral loading that the springs produce against the slider bearing, plus the side load friction that the force from the pushrods produces against that same bearing. In both cases, it can be changed, but not fixed, by the choice of springs (bellevilles don't generally produce side loads, but when stacked in series in direct surface contact they produce a lot of friction from one spring surface rubbing against another), and the types of bearing used and it's surface Cf. We measured about 200# of contact patch load change on an IPS car before the shuttle would move at all with the original roller bearings, and then decreased the CPL shift to less than 100# with a change in bearing type. Pretty significant change in the front end grip!

I've seen a picture of a design that does not utilize a sideways-sliding shuttle bar, but instead used a series of links to get a somewhat traditional sway bar to twist. From what I could tell from the picture, it would be heads above the shuttle system in not producing friction.

I don't know who was the first to try a mono system, but in general it has been replaced by the "old fashioned" 2-shock setup. For street courses, the mono system was extremely effective - instantaneous response at initial turn-in can be a big plus on that type of course. On ovals, they will work well, but the final level of performance always ends up being just where in the corner you want the inevitable push to start, and just how bad it gets. On high-downforce cars, the extra beating that the tires get is not as big a detriment to grip as it is on cars with little to no downforce. Many F3 teams replace the mono with a twin-shock setup for the majority of courses when their testing indicates that having a higher overall level of grip is more important than the instantaneous turn-in response.

[carnut169]

Rennie,

IMO, the preload is what causes most of the friction, since, as you know, no spring is entire concentric in its loading characteristics. The more preload, the more side load, and the more friction. This was probably the cheaper solution, compared to a torsion bar.

Wouldn't stacked beville washers prevent this side load?

[DaveW]

Wouldn't stacked beville washers prevent this side load?

Yes, to a large degree, but if the stack were not perfectly concentric (perfect alignment is pretty much impossible due to the need to allow clearance so the washers can move), the slight offset still would cause some side load to be present.

Richard is probably correct, though, that the main cause of side load is the pushrod loading.

[Swift17]

Hey Tom ....
Aren't you glad you asked the question .... ?

I actually loved the discussion .... good theory mixed with engineering analysis and tuning experience ... I am sure an FormulaSAE team would love it given the nature of their competition and the points system given to creative systems as well as the design of the competition course upon which they compete.

JAMES LEE RACING in Miami has made a MonoShock Van Dieman work very well
With Cole Morgan's dad Jim driving last week qualified 5th and did well (2nd) at the Sebring Nat'l -- Call James next week (At Homestead Nat'l this weekend) - this is the same car he used at the Runnoffs '06 and it is for sale 1.305.815.6439

Personally, having driven both systems - here is my take ...
4 is easier if not better than 3 !

[Scott Hanba]

Sean,
While the bellevilles will reduce the side load compared to coil springs, a stacked set of bellevilles will have a lot of sliding friction, which is also bad. As the bellevilles deflect, the ID and OD contact points move, and when stacked, must slide across one another. This is very similar to the sliding between leafs in a multi-leaf spring suspension.

[DaveW]

Sean,
While the bellevilles will reduce the side load compared to coil springs, a stacked set of bellevilles will have a lot of sliding friction, which is also bad. As the bellevilles deflect, the ID and OD contact points move, and when stacked, must slide across one another. This is very similar to the sliding between leafs in a multi-leaf spring suspension.

That is true if they are stacked with all of them oriented the same way (like spoons in drawer). If they are stacked alternating their direction, ID to ID, and OD to OD, the "leaf-spring" friction doesn't occur.

I would assume, though, to get enough rate, the "spoon" orientation is the one most often (always?) used. One could reduce this source of friction by alternating the washers with Teflon discs, or some other similar friction-reducing devices.

[carnut169]

Ok, really been doing some head scratching on this and need a few things clarified.

1st- Why would a spring rate double? If I take the two 1000lb springs and load them against each other wouldn't the spring on the same side as the bump effectively "help" compress the springs on the other side? Seems like the effort to compress the spring would be the same with 10,000lbs of preload or 100... in the case of 10,000lbs I'd have the other side pushing w/ 10,000lbs of force already... seems like it would cut the rate in half. I may just be thick.

2nd- With the car jacked up and no bevevilles in the assembly you can easily move the shuttle from side to side by lifting a wheel. What is interesting is that the other wheel moves in the opposite direction- a bump on the right wheel would cause the left wheel to move down. I'm assuming that this is why a monoshock can give a good initial turn in- the car leans and the wheel that would be unloaded in a regular 4 shock set up gets pushed down to the track- better grip, right? Now things start to get complicated... there is a 3rd spring attached to the shock. Now that bump can cause the shock to compress and the whole front end goes down, the mono to slide and the opposite side goes up or a combination of the two... the opposite wheel stays put? How on earth would I go about determining the right rate for the monoshock assembly to work properly with the spring rate of the shock? I would imagine that it should work really well if they are matched...

Changing the beveville stack is super easy. What would be a good testing procedure or is there a formula that gets me close?

[JIM McQUAIG]

check the fc section . there was recently a lot of monoshock discussion. and some diagrams

[carnut169]

Thanks. I posted those!

[rfvd_nut]

This is an old thread, though maybe somebody can help. I have always been interested in monoshock setups.

I would like to acquire one, just for the curiosity of having one and studying it. It may sound strange but that's what it is.

There seems to be systems in early RF VD that were incorporated as part of the frame (the 93 for instance). I am not interested in this. I am interested in the full front setup that came I believe on the RF96, shuttle, Belleville washers, lever assembly.

Please PM me if you have anything of interest.

[MotorCade]

In terms of practical setup - keep in mind that the monoshock setup (I'm assuming a 96VD here, as it's what I have) will essentially limit your front suspension to (mostly) managing heave. So you spend some time getting the roll control right with belleville/pre-load on the shuttle by tuning around steady state cornering first, then work the rear suspension to properly control dynamic motion. Some iteration through that process is useful. Get the front close, then work the rear to get turn-in and track-out weight transfer right. Then refine the front again, and refine the rear.