SUSPENSION FREQUENCY & MOVEMENT AT BONNEVILLE

[rickyracer]

I'm trying to finalise the front and rear end designs for my streamliner having solid tyreless front wheels and rear driven tyres regarding solid-mount vs compliant vs suspension.

Everybody comments that at Bonneville over 200 mph the vibration smooths out but can someone provide data regarding front or rear suspension oscillation frequency and movement range with speed, on tires or solid tireless, preferably on a light to medium weight streamliner capable of 300 mph plus? Mine will be about 1800 pounds with approx. 600/1200 pounds front/rear.

Also, vibration data from a car or bike with no suspension.

I appreciate variations in salt condition, vehicle weight, suspension design and damping are major factors so that information is also relevant.

Any advice appreciated.

[revolutionary]

Ricky,

You will want to first determine the natural frequency of your chassis. there are a few very good books on race char chassis design that will help you with that.

The racing surface is constantly changing, like a dirt road but more variable.  If you imagine that on one dirt road there are humps every 1 foot and on the next road there are humps every 1.5 foot and the amplitude of the humps varies by 1 inch as well. Going from one course to the next or even the same course throughout a day can offer similar variations, though perhaps not to the magnitude in the example. As your vehicle changes speed, the frequency of the road bed humps remains the same but the response frequency of your vehicle is constantly changing.

The point is, you need to first work on developing a suspension that will work with the natural frequency of your chassis, regardless of minor variations in the road bed.

[Stainless1]

I don't think any of Jack's liners have suspension... Send TurboRick a PM, sounds like that is the direction you are headed.

[manta22]

It would be interesting to see some on-board data if anyone has ever recorded it on a run at Bonneville.

Cars running "no suspension" still have some spring provided by their pneumatic tires but at super high inflation pressures, it is pretty solid. At higher frequency less motion is transmitted to the chassis due to the mass of the unsprung weight; that's why things smooth out at higher speed.

The amount of vibration that will be transmitted to the chassis is also determined by the ratio of sprung to unsprung weight. A light car needs to have very low unsprung weight (tires & wheels, brakes, uprights, and about half of the weight of the suspension arms and springs & shocks) to minimize vibration. A low spring rate gives a "softer" ride but it is easily compressed by downforce. It's always a compromise.

Regards, Neil  Tucson, AZ

[Bratfink]

It would be interesting to see some on-board data if anyone has ever recorded it on a run at Bonneville.

Cars running "no suspension" still have some spring provided by their pneumatic tires but at super high inflation pressures, it is pretty solid. At higher frequency less motion is transmitted to the chassis due to the mass of the unsprung weight; that's why things smooth out at higher speed.

The amount of vibration that will be transmitted to the chassis is also determined by the ratio of sprung to unsprung weight. A light car needs to have very low unsprung weight (tires & wheels, brakes, uprights, and about half of the weight of the suspension arms and springs & shocks) to minimize vibration. A low spring rate gives a "softer" ride but it is easily compressed by downforce. It's always a compromise.

Regards, Neil  Tucson, AZ

What he said.

The 200mph smoothing phenomenon is likely the transition of the frequency response from mass to frequency dependent. Doesn't necessarily mean you are in any better control of the vehicle. The trouble here is this type of vehicle is not well understood by the usual vehicle dynamics models. Dragsters are a poor fit since their chassis are designed as effectively a large leaf spring and streamliners are generally designed to be as stiff as possible.

You'd really want to baseline the characteristics of the chassis before committing to suspension development, which presents a catch 22 problem since the suspension will be intrinsic to the chassis design! Thus a best guess approach is required. Make the chassis as stiff as possible and give it some suspension rather than solid mounts, not so much for travel as for high speed bump and rebound damping. That'd be my starting point.

If you have access to a K&C rig you'd be able to learn more about the chassis characteristics to be able to pre select spring and damper packages before going on to doing a 4 post program. But try and find a 4 post with a long enough wheel base and narrow enough track! (if you do, let me know!).

Sorry it's doesn't really answer the question, but hopefully it'll give you somewhere to start looking.

[kiwi belly tank]

If you're going to do suspension in a liner you will need to eliminate body roll. Narrow track liners have a high CG that exagerate the roll on suspension & that will induce a yaw situation. Your CG will be too far back in the car making the front end light. Add in the solid front wheels with less bite than a rubber tire & you're likely to be chasing this thing all over the salt or it's just going to want to go backwards.
Don't strive to build as light as you can, this isn't drag racing. You can use a water tank in the nose to pull the CG forward, that way you don't have to travel with the weight, just fill it up when you get there.
I have 20 plus years experience in liners, I don't like suspension. MHO.
  Sid.
 

[Bratfink]

Oh yeah, I forgot to mention that I also think the CG is too far rearwards.

Don't misinterpret my suggestion of installing suspension as actually having a meaningful suspension system (like a normal car), I am suggesting it as a minimal travel stiff system to aid frequency response of the wheels on the track and as a way of tuning vehicle attitude to aid in aero balance.

[kustombrad]

I went with suspension in my 'liner after actually checking out how the course is constantly changing and watching the cars with even minimal travel and how they were consistently faster than those at the mercy of the track. The front wheels are in really close track to each other and the rear is a swingarm so I'm not worried about body roll. I have access to a shock dyno so I can fine tune my setup.