15 replies to this topic

[Ninja2]

I read in Peter Wrights F1 Technology book about the Chassis Frequency of F1 cars, but I dont understand what chassis frequency means.... can anyone shed any light?

Thanks

Cb

[random]

Hit something with a hammer and you can measure it's frequency or modulation. You can clearly see this in things like tuning forks.

Chassis have these sorts of modulations as well. I gather that's what he's referring to.

[desmo]

Good explanation! I was once asked how one might ascertain the frequency of a crankshaft. I flipply responded, "drop it on the floor and see what note the resulting 'clang' is. That's essentially it.

[BRIAN GLOVER]

Frequency is also an indication of stiffness for steel frames. New passenger cars such as the MB E class and Olds Aurora vibrate at 27 heartz. The C5 Corvette at 25 htz and the new MB SL 500 is 20 and a M3 is 32.

Originally posted by Ninja2
I read in Peter Wrights F1 Technology book about the Chassis Frequency of F1 cars, but I dont understand what chassis frequency means.... can anyone shed any light?

Thanks

Cb

[alexbiker]

A mate of mine's dad was working fleet purchasing for a large UK engineering company. They were big enough to really screw down Vauxhall for a fleet of Vectras, so they got a special. Fortunately for him, he asked for some samples of the special before putting in the order for 500+ of them.

Turned out Vauxhall had slapped in an old engine they had lying around. The vibration of the engine was close enough to the chassis frequency to cause some serious resonance (an engineer can tell you if it would have been a fundamental or a harmonic of the chassis frequency) and those things fell apart in 500 miles - chassis welds were giving way.

I remember Patrick Head talking about the change to pump fuel, which happened mid season in the nineties (93/94ish I think, but someone else will know better). That subtle change created a vibration that resonated something else, and suddenly they had all sorts of problems. Temperamental thoroughbred? Too right.

Alex

[biercemountain]

Originally posted by desmo
Good explanation! I was once asked how one might ascertain the frequency of a crankshaft. I flipply responded, "drop it on the floor and see what note the resulting 'clang' is. That's essentially it.

Let me get this straight. If I want to find the chassis frequency of the Ferrari 2003 GA all I need to do is drop it on the floor and see what note it makes? I'll call Ross now and make the drop test arrangements!

I'll stop by the engine shop too and see about that crank shaft.

Can you imagine?!!!!

[Pong]

Interesting thread!

In addition to chassis frequency there is the frequencies induced by the engine and the track itself. This makes for a number of interesting combinations where the chassis, the engine and the circuit will work together, neutralize each other or work against each other.

Example: the human body is the chassis, the muscles in your thighs your engine and the ground you are walking on is the circuit. The best moment for the muscles to react is when the potential energy (spring load of muscles) of the is the highest - that's when your legs are spread out. The worst is when they are relaxed or in neutral position. (All human walking is based on this principle). Now imagine if the ground you were walking on was such that you hit a pothole for each step - it would be very difficult to walk.

To switch to F1: I have suspected the the Honda Athena 2000 program tried to do something here but they didn't get anywhere.

[Ninja2]

Ok, so now I understand that the chassis frequency means the natural frequency of the chassis. But what pysical effect does this have on anything? If I have a chassis with a freq of 5Hz, and a tyre with a freq of 10 Hz, rolling on a road with a freq of 5Hz, how will these interact?

Also, he mentions natural Yawing frequencies and natural roll frequencies. Presumably these cannot mean the same thing as the chassis frequency? Are they to do with the rate of change of the yaw and roll angles????

Thanks,

Cb

[JwS]

Chassis frequency is used as a measure of it's stiffness (primarily). The stiffer it is the higher it's resonant frequency.
It is checked by instrumenting the chassis with accelerometers in appropriate places and then exciting vibration in the chassis (hit it with a hammer, but a very special hammer) and measuring the acceleration of the various parts. This is also a standard non destructive way to test aircraft wings for cracks and damage, these will change the harmonics of the wing.
JwS

[YKTS]

Originally posted by alexbiker
A mate of mine's dad was working fleet purchasing for a large UK engineering company. They were big enough to really screw down Vauxhall for a fleet of Vectras, so they got a special. Fortunately for him, he asked for some samples of the special before putting in the order for 500+ of them.

Turned out Vauxhall had slapped in an old engine they had lying around. The vibration of the engine was close enough to the chassis frequency to cause some serious resonance (an engineer can tell you if it would have been a fundamental or a harmonic of the chassis frequency) and those things fell apart in 500 miles - chassis welds were giving way.

I remember Patrick Head talking about the change to pump fuel, which happened mid season in the nineties (93/94ish I think, but someone else will know better). That subtle change created a vibration that resonated something else, and suddenly they had all sorts of problems. Temperamental thoroughbred? Too right.

Alex

sorry to take th thread off at a bit of a tangent, but this quote got me remembering about what a lecturer of mine told me in the first year when we were doing about SHM.

it seems that when renault bought out the new shape clio for the same time, through some miscommunication the windowipers ran at the same frequency as the front supsension, making the car shake violently from side to side when the car was stopped with the window wipers on! cue everyone in class running to the local renault garage to find - unsuccesfully - a said clio!

[dosco]

Originally posted by Ninja2
Also, he mentions natural Yawing frequencies and natural roll frequencies. Presumably these cannot mean the same thing as the chassis frequency? Are they to do with the rate of change of the yaw and roll angles????

Thanks,

Cb

When he talks about yawing freq, he's discussing the stability and control of the car in yaw (turning). If the car is stable and understeers or oversteers, it will do so when disturbed and return to straight line driving at a particular frequency. If this frequency is faster than the driver's frequency (according to Wright humans max out at like 5Hz), the driver will lose control of the car. If the frequency is less than the driver's, the driver can recover from an upset.

So.....if M. Schuey is driving his Ferrari and begins to oversteer for some reason, the Ferrari engineers must ensure Schuey can return the car to straight line driving....so M. Schuey loses is for a split second and recovers without incedent - we see it all the time. If the designers had somehow had a higher yaw frequency, Mr. M. Schuey would be screwed because he'd lose control of the car.

The roll frequency is a stability issue that is governed by suspension design. When the car is disturbed, the suspension (springs and dampers) will roll and then (presumably) return the car to "straight line" motion. The spring rate of the springs and the damping action of the shocks will dictate how the suspension responds to said disturbance.

[dosco]

Originally posted by JwS
Chassis frequency is used as a measure of it's stiffness (primarily). The stiffer it is the higher it's resonant frequency.
It is checked by instrumenting the chassis with accelerometers in appropriate places and then exciting vibration in the chassis (hit it with a hammer, but a very special hammer) and measuring the acceleration of the various parts. This is also a standard non destructive way to test aircraft wings for cracks and damage, these will change the harmonics of the wing.
JwS

I assume you're referring to ultrasonic inspection?

[nir]

Originally posted by alexbiker
I remember Patrick Head talking about the change to pump fuel, which happened mid season in the nineties (93/94ish I think, but someone else will know better). That subtle change created a vibration that resonated something else, and suddenly they had all sorts of problems. Temperamental thoroughbred? Too right.

Alex

I remember it too ! It was the second part of 92. As you might recall williams and mansell domianted this season. both cars retired because of the reason you described at Monza.

[Greg Locock]

ninja2 wrote "Ok, so now I understand that the chassis frequency means the natural frequency of the chassis. But what pysical effect does this have on anything? If I have a chassis with a freq of 5Hz, and a tyre with a freq of 10 Hz, rolling on a road with a freq of 5Hz, how will these interact?"

I don't know how important all this modal alignment stuff is on race cars, but it is of vital importance with road cars, and has certainly caused problems for all sorts of sports cars as well.

However, the problem tends to be refinement (specifically shake, and squeaks and rattles), rather than anything more fundamental.

When the frequencies line up then all hell breaks loose. Chassis torsion and bending are poorly damped, typically, and the excitation forces available from tyres are huge, compared with most refinement forces outside of the engine, so if you have a lightly damped resonance lining up with a big force, then everything shakes.

The basic problem, poor chassis stiffness, can and does cause handling problems, specifically it compromises the maximum stiffness of the spring/shock system, since after a certain point the chassis bends rather than allowing the strut to operate properly. I've seen this even on quite refined cars, admittedly in the bad old days of 5-10 000 ft lb/deg chassis.

[KenC]

Interestingly, or maybe not, if you don't play, golf shaft stiffness is also measured by their resonant frequency, for both steel and graphite shafts.

And, didn't we see at Montreal, a situation where Patrick Head explained that adding on-board cameras to the mirrors of their cars, changed their resonant frequency just the right amount that the glue holding the mirrors on, was no longer sufficient, and thus the flopping mirrors.

[JwS]

No, it is not the same as ultrasonic testing.
Basically an intact wing has a certain natural frequency, denoting a certain stiffness. If there is any structural damage, the stiffness changes and the frequency changes, so you can wack it with a hammer and listen to the note to see if the internals are ok. Same with the flywheel on a car, or a grinding wheel in the shop, you can often detect a crack by tapping them and seeing if they 'ring' correctly, when they are cracked they will sound 'dead'
JwS