Suspension strain gauge loads vs. time

[Tommy Cookers]

Most cars now days are equipped with strain gauges, especially on pushrod suspensions, but the problem you will have for trying to derive something like that is the signal to noise ratio on a car in motion on track is pretty bad. Of course you can filter it and do all the normal tricks and you can try and make something out of it, but IMO it wont be the right way to do the job. Don't get me wrong though, it can be very useful for something more aimed at trends, downforce, total load transfers and model validation... but seeing something as fine as system responses I would look elsewhere. Try to get on Dave W's rig my $.02

I was assuming that 'strain 'gauges' was just engineer's short jargon referring to force transducing functionality integrated into dedicated suspension parts for test purposes (with strain gauges at the heart of that functionality)
(eg since pushrods have end fittings and are connected to other parts, such functionality could readily be incorporated there)
this requires is some internal structure dedicated to force transducing and internal signal conditioning etc for high S/N ratio

or are people actually eg bonding actual strain gauges onto actual ordinary pushrods ??
(that would be a problem farm)

[DaveW]

I was assuming that 'strain 'gauges' was just engineer's short jargon referring to force transducing functionality integrated into dedicated suspension parts for test purposes (with strain gauges at the heart of that functionality)
(eg since pushrods have end fittings and are connected to other parts, such functionality could readily be incorporated there)
this requires is some internal structure dedicated to force transducing and internal signal conditioning etc for high S/N ratio

You are correct in the first part. For the second part, it is necessary to record the six (at least) components of suspension load in order to resolve the six components of load acting at the wheels. The resolution depends upon link geometry, which generally varies with wheel position. Hence, whilst push rod load by itself can be resolved into vertical load, the result will be "contaminated" by loads reacted by other suspension arms and wheel position. A vehicle model can help to resolve vertical loads, but the result will be in error of the model is in error.

[Tim.Wright]

Most cars now days are equipped with strain gauges, especially on pushrod suspensions, but the problem you will have for trying to derive something like that is the signal to noise ratio on a car in motion on track is pretty bad. Of course you can filter it and do all the normal tricks and you can try and make something out of it, but IMO it wont be the right way to do the job. Don't get me wrong though, it can be very useful for something more aimed at trends, downforce, total load transfers and model validation... but seeing something as fine as system responses I would look elsewhere. Try to get on Dave W's rig my $.02

I was assuming that 'strain 'gauges' was just engineer's short jargon referring to force transducing functionality integrated into dedicated suspension parts for test purposes (with strain gauges at the heart of that functionality)
(eg since pushrods have end fittings and are connected to other parts, such functionality could readily be incorporated there)
this requires is some internal structure dedicated to force transducing and internal signal conditioning etc for high S/N ratio

or are people actually eg bonding actual strain gauges onto actual ordinary pushrods ??
(that would be a problem farm)

In F1 they bond strain gauges onto the titanium inserts at the top of the rods. Its quite easy to come across used push rods for sale on the internet with strain gauge leads hanging off.

[Tommy Cookers]

In F1 they bond strain gauges onto the titanium inserts at the top of the rods. Its quite easy to come across used push rods for sale on the internet with strain gauge leads hanging off.

I'm thinking that these inserts should in principle take the whole pushrod load (ie form a transducing structure effectively in series with the pushrod)
or be thin and flimsy so that they are driven by the pushrod loads to the same strain as the pushrod ie the inserts are slaved
(such straingauges with thin metal backings are manufactured to allow use in this way

can you tell which ?

[gixxer_drew]

The actual loads are noisy, I'm sure electrical can be difficult as well, though not my department.

[gixxer_drew]

There are strain gauge wheels out there as well, pricey though.

[Tim.Wright]

In F1 they bond strain gauges onto the titanium inserts at the top of the rods. Its quite easy to come across used push rods for sale on the internet with strain gauge leads hanging off.

I'm thinking that these inserts should in principle take the whole pushrod load (ie form a transducing structure effectively in series with the pushrod)
or be thin and flimsy so that they are driven by the pushrod loads to the same strain as the pushrod ie the inserts are slaved
(such straingauges with thin metal backings are manufactured to allow use in this way

can you tell which ?

I can't find any pictures now but your first description is what it appears they are doing. The last thing you want in the suspension is any more built in elasticity. I assume they calibrate on an Instron machine or something similar. A reasonably simple task since the push rod only transmits tensile/comoressive forces.

[Jersey Tom]

One challenge in putting an axial load cell literally in series with a push-rod or other similar tension/compression element is alignment. If everything isn't perfectly aligned on the same axis I believe you will run into bending loads on your load cell. Probably not a good thing! I believe I have seen push rod load cells though, somewhere up near one of the eyelets IIRC. Can be done, just needs to be done right.

Or, depending on what level of resolution you need... you could calibrate some wheel load channels based on damper travels.

[DaveW]

I can't find any pictures now but your first description is what it appears they are doing.

Apologies, Tim, but I think the second description is probably a better fit. Normally, 4 strain gauges are fitted, 2 "active" gauges are mounted with their sensitive axis aligned along the direction of the load, placed diametrically opposite (to cancel out beam bending) & 2 "passive" gauges are mounted with their sensitive axis across the direction of the load. The 4 gauges are wired to form a full bridge Type 3, with the active gauges diagonally opposite.

The arrangement works well if the strains in the host structure are constant over the gauged area (i.e. away from regions of stress concentration). That is probably something of an issue in the present application.

[Tommy Cookers]

..... Normally, 4 are fitted, 2 "active" gauges are mounted with their sensitive axis aligned along the direction of the load, placed diametrically opposite (to cancel out beam bending) & 2 "passive" gauges are mounted with their sensitive axis across the direction of the load. The 4 gauges are wired to form a full bridge Type 3, with the active gauges diagonally opposite.
The arrangement works well if the strains in the host structure are constant over the gauged area (i.e. away from regions of stress concentration). That is probably something of an issue in the present application.

I can relate to that, having designed and made application-integrated force transducers

if the pushrod was metal the nominally dummy gauges would (usefully) be experiencing a 'Poisson' strain around -30% of the strain on the active gauges (the same effect as eg if a rubber block is compressed heightwise, it bulges widthwise)
a CF pushrod is presumably non-isotropic so the 'Poisson' strain might be anomalous, also CF is less than ideally elastic ?
but there should be a usefully large strain (several hundred ppm), so it seems workable without integral signal conditioning etc

strain gauges for stress analysis purposes have mostly been replaced by numerical/FEA/CAD methods
but (straingauge-based) force transducers/load cells/balances have apparently not been so replaced
the pushrod application here measures one force
the wind tunnel balance measures aggregated forces and moments at one point (although the aero surfaces are many)
between these extremes are aero (and other ?) points of significance where forces and moments can readily be measured
by substitute mounting etc structures with designed-in force transducing functionality, for tunnel and track use
what's not to like ?

PS other than pushrods, where else are forces currently measured on car ?

[speedsense]

Yes, pushrod end Heims can be used for strain gauges, however there are problems associated with it. In F1 the pushrods are carbon fiber, stains don't work on them, so they have to use the ends. When strains are used on the adjuster ends (for mounting purposes and wiring) the adjuster end becomes a source of problems. Repeatability is the the most important aspect to strain gauge measurement on a pushrod. When mounted to a heim end, the threads of the heim cause repeatable issues for measurement. The strain CANNOT be located over or fractionally close to the threads of the heim. If it is the calibration of the sensor becomes useless. And the strain must be located on a "flat" of the heim. Furthermore, straining two sides (180 deg. opposite on the same heim)

The standard practice of strain (without large amounts of money) currently is to strain the pushrods for repeatable results. The major difference (when done by someone with lot's of experience at adapting pushrod straining) from a load cell vs strain gauges is that the metal itself is not controlled in it's content (not by you), as it is in high grade load cells. This being a built in error, if you will, and uncontrolled when applying a strain to a pushrod that was formed in an unknown way.

For instance, BERU builds highly accurate torque sensors, but won't do so unless they also form the axle shaft, to remove any errors that the metallurgy may cause.
When it comes to existing designs, strain gauges work perfectly especially for tuning purposes or backing up calculation. But in a design routine, has to be controlled to right down to the metal choice. IMHO

[speedsense]

..... Normally, 4 are fitted, 2 "active" gauges are mounted with their sensitive axis aligned along the direction of the load, placed diametrically opposite (to cancel out beam bending) & 2 "passive" gauges are mounted with their sensitive axis across the direction of the load. The 4 gauges are wired to form a full bridge Type 3, with the active gauges diagonally opposite.
The arrangement works well if the strains in the host structure are constant over the gauged area (i.e. away from regions of stress concentration). That is probably something of an issue in the present application.

I can relate to that, having designed and made application-integrated force transducers

if the pushrod was metal the nominally dummy gauges would (usefully) be experiencing a 'Poisson' strain around -30% of the strain on the active gauges (the same effect as eg if a rubber block is compressed heightwise, it bulges widthwise)
a CF pushrod is presumably non-isotropic so the 'Poisson' strain might be anomalous, also CF is less than ideally elastic ?
but there should be a usefully large strain (several hundred ppm), so it seems workable without integral signal conditioning etc

strain gauges for stress analysis purposes have mostly been replaced by numerical/FEA/CAD methods
but (straingauge-based) force transducers/load cells/balances have apparently not been so replaced
the pushrod application here measures one force
the wind tunnel balance measures aggregated forces and moments at one point (although the aero surfaces are many)
between these extremes are aero (and other ?) points of significance where forces and moments can readily be measured
by substitute mounting etc structures with designed-in force transducing functionality, for tunnel and track use
what's not to like ?

PS other than pushrods, where else are forces currently measured on car ?

"strain gauges for stress analysis purposes have mostly been replaced by numerical/FEA/CAD methods"
The only problem with this, is someone formed it, cut it, welded it or put it together, FEA/CAD are perfect, the rest of this sentence isn't.

PS other than pushrods, where else are forces currently measured on car
Concerning strain measurement-
Wing mounts, shifters (Seq. Transmission, IE Porsche GT3 cup cars, though used for rpm dips to shift), steering arms, racks or columns (understeer measurement based on steering load), assorted structures in design stages, IE uprights, a-arms, chassis braces, etc.
Pressure sensors-all fluid pressure, aero sensors are strain

The list would get rather long if you meant "all forces", all depends on money. In F1, very few forces are unmeasured.. in general racing reality data acq., lot's of forces are measurement, most are calculated..

[speedsense]

Thanks, JT. Your reply made me think a little harder about why I thought the data would be helpful. My emphasis was on "vs. time". My understanding is at the level you just described, which I feel is rather quasi-static. I can resolve forces at the contact patch into compression/tension in the upper and lower links, pushrod, and tie rod. I can sum forces and moments iteratively from a car running in a straight line to a car reaching maximum roll angle at maximum steady state lateral acceleration.

But as far as I know, this isn't real vehicle dynamics. This is Gillespie, or Puhn, or Staniforth, etc. It is sufficient to roll a new race car off the trailer for the first time, but that's about it, and only if the competition isn't any smarter.

If I take that narrow quasi-static view and switch it to the plan view, I start thinking about the yaw moment required to enter and exit a corner. The simple, 2-D front view taught me that rise of roll angle was always going to lag the rise of lateral acceleration, if for no reason other than moment of inertia of the sprung mass and damper forces. The plan view implies that the lateral acceleration will lag the force build up. Since the steering axle has to generate force at the contact patch/in the links first, or else there is no yaw moment, no yaw acceleration, then there will be tension/compression in all the front end links, including the pushrods, before significant lateral acceleration has been developed, which means not much rolling moment.

So let's just say all of the above amounts to my first question: how do roll angle and lateral acceleration lag yaw moment?

Then my next two questions would be:

How can I apply this knowledge to transient cornering? (My expectation is that the time constants associated with initial yaw acceleration are too small to be important for anything other than a transient scenario.)

How can I apply this knowledge to roll angle overall? (The quasi static front view iteration didn't reveal that when and how rapidly a control moment is applied would determine the roll rate.)

Please tell me if I'm on the wrong track!

I know what your looking for. I found your answer out in 1996. With strain gauges. However, I will suggest to you, a graph of suspension sensors which you probably have now.

Separate your roll measurement to front and rear (instead of one). (Push rod strains in use will make the following picture clearer when they are applied in the same manner mathematically)

Now, take and build a graphed signal of LF-RR (Lf +Rr) suspension and RF-LR (Rf-lr) suspension ( calculated movement at wheel is best), as this shows "in" roll effects and de-roll effects on the chassis.

Take one of the signals (depending on the track) and applying negative numbers so it opposes the other (instead of following each other). Add a comparison of front and rear rolls, and obviously along with driver inputs.
A little further advance would be to add a derivative (accelerations) to the wheel movement (individually) signals and plot along side. Bingo, there's your lag measurement!! Only thing next is figuring out, if the car rotation is driver induced or car induced and of course track induced, though there ain't a sensor that will tell you which one, only a combination of sensors will.

Should help answer your post #1 question, at least it did for me when I discovered the way to see it. Hope it helps you out.

[gixxer_drew]

Just a thought, why not do this with accelerometers?

[Tim.Wright]

Because measuring acceleration is not going to give you push rod force