Bus Chassis

[triumph993]

Folks,

I'm having trouble interpreting a Modal Analysis I did on a bus chassis. This is for my own interest only - I'm somewhat a busnut.

I'm modeling a bus chassis from one of the major tour bus companies. It's 102" wide, 12.5 ft tall, and 45 ft in length. The chassis is a space frame. The model is composed of frame elements, no shells.

The bus is plenty stiff in static analysis and stress levels are modest.

Here are the lowest six modes produced by Lisa FEA software.

Mode 1: 5.40 Hz, Torsional
Mode 2: 8.59 Hz, Lateral Bending
Mode 3: 9.70 Hz, Torsional
Mode 4: 10.60 Hz, Torsional
Mode 5: 12.46 Hz, Lateral Bending
Mode 6: 12.96 Hz, Vertical Bending

I'm thinking that steady-state road excitation coupled through to the sprung mass through the suspension is typically in the range of 0 Hz to 10 Hz.

The bus uses a reasonably sophisticated multi-link suspension at the drive and tag axle. The drive and tag are stick axles while the steer uses an independent suspension. All axles have 2 shocks except the drive which has 4. The air bags are spaced wide approximately the width of the chassis.

I would expect the bus to resonate considerably on virtually any road. How is it that this bus has such a fine reputation for handling, ride, and sturdiness?

Thanks,
Triumph993

 

[MikeHalloran]

Maybe your model needs to include the skin.



Mike Halloran
Pembroke Pines, FL, USA

 

[triumph993]

Hi Mike,

Thanks for the reply. The side walls are fiberglass and the roof is a single piece of stainless steel. Also, the windows are structural glass, if there is indeed such a thing. If I account for the stiffness of these components, what change would you expect in the frequency response? I gather from your answer that as things stand, this thing should rattle to bits???

Thanks,
Triumph993

 

[MikeHalloran]

I'd expect the fiberglass to add to the stiffness, and I'd expect its fasteners to add quite a lot of damping, if modeled accurately. You may need a bigger computer for that.



Mike Halloran
Pembroke Pines, FL, USA

 

[triumph993]

Thanks Mike,

The skin is glued on as are the windows.

I'm new at this stuff, but it seems that the first fundamental needs to be above the road excitation frequency range. Perhaps their suspension is so good that it filters and dampens better than most. I'll go back and add some elements to model the skin, but I expect the change in frequency response to be incremental, but it may be enough. It seems that a vehicle this size will have the first six modes in the 10 Hz to 20 Hz range. Thanks for the confirmation

Triumph993

 

[triumph993]

Okay,

I added some skin and I added the fiberglass front and rear clips. The clips significantly brought up the first fundamental frequency. here are the new numbers:

Mode 1: 11.20 Hz, Torsional
Mode 2: 12.42 Hz, Torsional
Mode 3: 15.36 Hz, Torsional
Mode 4: 17.93 Hz, Vertical Bending
Mode 5: 19.76 Hz, Lateral Bending
Mode 6: 20.71 Hz, Lateral Bending

Looks like the chassis fundamentals are slightly above the rough road steady state excitation of 0 to 10Hz. Are these natural frequencies reasonable for a 45' long touring bus?

Thanks,
Triumph993

 

[MikeHalloran]

I really can't answer that question. ... but you've said the bus market did.

Rules of thumb that I sometimes remember:
Structures designed to a deflection limit of span/360 will resonate at around 4 Hz.
Airplanes have (aileron, I think) flutter responses around 7 Hz.



Mike Halloran
Pembroke Pines, FL, USA

 

[bradleyelwood]

To really look at the responce of a bus, you need to also model the tires as spring elements, the unsprung mass (axles and wheels) and the air spring-suspension system. Firestone and Goodyear have pretty good writeups on suspension frequencies etc of their products for vehicles.

I would think that the natural frequencies of the suspension are low (vertical, roll, lateral) and that the suspension isolates a lot of the roads from the vehicle.

 

[BrianPetersen]

Ordinarily the natural frequencies at the suspension are so much lower (1 - 2 Hz range) than the natural frequencies of the bodyshell/chassis that it basically doesn't matter what the chassis does; substantially all of the movement will be taken up at the suspension. If there is something that imposes a deflection or oscillation in the chassis, it causes motions at the suspension that are quickly damped out. The suspension design obviously has an enormous influence on the ride quality.

Although trucks and buses share a common heritage, their suspension designs have diverged considerably. Dump trucks often still have extremely stiff rubber-block "suspension" or really heavy-duty leaf spring suspensions with extremely high spring rates. You can often see and hear those rattling every time they go over bumps or potholes. That's an example where the natural frequency at the suspension is very high and could be comparable to what the chassis does. Trucks with air suspension usually use a linkage design that gives soft vertical compliance but extremely stiff compliance in roll - because a compliant body-roll situation could result in the truck turning right over. This gives a low frequency in straight-up-and-down motion but a high frequency in roll, so even with air suspension, they still have a jarring ride (on one-wheel bumps).

Buses have proper linkage-located axles which sacrifice some roll stiffness but the result is a lower frequency in roll, and much better ride. They can get away with this because the cargo isn't necessarily as high up and as heavy as a tractor-trailer could have.