Anyone else run a Monoshock?

[nick roberts]

We are having issues with our '05 setup which uses a front Monoshock and i was wondering if anyone has any experience with them.

-nick roberts

[Kurt Bilinski]

Ya know, I've thought about doing this on and off for years. Assuming I'm thinking of the same physical layout you are, the only problem I see is if you hit a one-wheel bump, the shock will move X. If you hit a two-wheel bump, it'll move 2X. I haven't thought it through but that could be an issue. Other then that, separating bump and roll seems link a great way to simplify things, saves buying one expensive shock, and makes things a bit lighter.

[jack]

sounds like there is a reason no one uses them...

Z, care to comment? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Z]

Originally posted by jack:
sounds like there is a reason no one uses them...

Z, care to comment? http://fsae.com/groupee_common/emoticons/icon_wink.gif
Jack, Dallara have and continue to use them with great success (eg. front suspension in F3).

Monoshocks aren't much different to a conventional "spring-at-each-corner + 2 ARB's" type setup. Ie from a 4-wheel-mode perspective they have equal bounce and pitch mode stiffness (set by the monoshock "axle-bounce" springs) and equal roll and twist mode stiffness (set by the "axle-roll" springs). However, the monoshock allows greatly different spring rates for these pairs of modes. So you can having agressive rising rate for the monoshock "axle-bounce" spring (good for body aero control, and absorbing big bumps), and linear rate "axle-roll" springs (which are softer over single wheel bumps). They can also save the weight of one damper at each end of the car (although monoshock-type suspension can have two shocks).

With only one shock at each end, the car is completely undamped in roll and twist. Low damping is good, but no damping can be worse. Hence Dallara typically only has the monoshock at the front. This gives good aero control for the low mounted front wing, and the two shocks at the rear help damp roll. The two rear shocks also loosen the car (add O/S) during corner entry, and tighten it (add U/S) at corner exit, which is usually helpful. And I'm sure there are many other little details...


Nick, What are the "issues"?

Z

[Storbeck]

I was under the impression that most monoshock formula cars are setup for zero droop, so they actually are damped in roll.

Z- is this incorrect?

[nick roberts]

Lots and lots of inside wheel spin. Weve actually got videos of the car lifting a wheel around almost every cone in a slalom. We had initially thought it was only happening under hard braking and conering but it looks to be happening more often then that.

Anyone else had to deal with this before? We just increased our front spring rate in an attempt to keep the nose a little higher but im not sure if this is going to completely solve the problem...

-nick

[Z]

Storbeck,

If the monoshock is droop limited (or even locked solid) then the two wheels can still move in roll - this is the point of separating the "axle-bounce" and "axle-roll" modes via the monoshock linkage. Think of the system as being like a beam axle with the monoshock sitting vertically on the middle of the beam, and then another system (say, a normal ARB) controlling roll. So with a rigid monoshock the beam axle would behave a bit like the front axle of a farm tractor - it could "articulate" (ie. move in roll) over single wheel bumps, but the nose of the car would always be at the same height above ground (good for a front wing). Like this there would be no damping from the monoshock during body roll.

If the wheels themselves are droop limited, say at the rockers (?), then yes, any body roll will result in some compression of the monoshock and hence some damping.


Nick,

If you are actually lifting the inside rear wheel in corners, then it sounds like the front roll-mode spring is too soft (and/or the rear springs are relatively too stiff). You should try stiffening the roll-mode spring(s) (NOT the monoshock spring), or softening the rears. A photo of the front suspension would help...


Z

[Akos]

Nick,

I had a fair bit of experience running monoshocks on both formula BMW and Renault.

From you post though, it sounds you have another problem. Lifting inside rear wheel always comes from 2 things. Too much rear roll stiffness (either from ARB or spring), or most likely on an fsae car you are running out of droop travel on your shock.

If the car was running fine before the problem is usually a cracked front ARB, or in the case of the monoshock a busted shuttle bar springs.

Cheers,

Akos

P.S. I hope you aren't running a monoshock in the rear.

[Storbeck]

Z, I was refering to when the wheels are droop limited thus rolling would compress the monoshock and give you some damping in roll. Do monoshock cars regularly have the monoshock droop limited but not the wheels, vice versa, or neither? I'm asking because I haven't spent a lot of time around full sized formula cars, and it seems like you may have.

It would make more sense to me to droop limit the wheels.

Nick, what is your roll stiffness distribution? Weight distribution?

Also, if you don't mind me asking, why did you choose a monoshock?

[Z]

Originally posted by Storbeck:
Do monoshock cars regularly have the monoshock droop limited but not the wheels, vice versa, or neither? ...
It would make more sense to me to droop limit the wheels.
Storbeck, First I should point out that these days (and last ~20 years) I spend very little time around formula cars. Any time I do spend is usually in the background (as uninvited rubbernecker) muttering "that doesn't look right..." while the guys in the foreground are scratching their heads (boys don't like to be told how to play with their toys...). http://fsae.com/groupee_common/emoticons/icon_smile.gif

I don't know if there is a "normal" setup for monoshocks. They are still a minority suspension type, and a lot of them end up getting converted to a 3-spring type layout (actually 4 springs including the roll mode spring) - supposedly because it gives more tuning options, but maybe out of desperation?

Anyway, the idea of droop limiting the monoshock itself is (possibly?) to make sure the front wing doesn't lift too high off the ground and lose downforce, while still allowing the two wheels to move a greater distance in roll (one up, the other down) for a softer ride over kerbs, etc. Likewise, the monoshock bump stop prevents the front wing getting too close to the ground and scrapping, stalling, or generating too much downforce and upsetting aero balance. Again, picture it as a beam axle with a short travel spring at its centre (to maintain constant nose height), and with more movement in axle-roll (to cope with bumps and twist in the road).


Nick, As Akos suggested, a broken roll-mode spring(s) could be your problem.

Z