deciding the steering rack position

[dhingra]

i am fom defianz racing,india.this time we are planning to place our rack on the upper tub mount in the nose.kindly guide me for the correct position of the tie rods relative to the uprights.also guide me how to considerabely reduce the steering effort.urgent.....

[dhingra]

i am fom defianz racing,india.this time we are planning to place our rack on the upper tub mount in the nose.kindly guide me for the correct position of the tie rods relative to the uprights.also guide me how to considerabely reduce the steering effort.urgent.....

[Fyhr]

the correct position is for you as an engineer to figure out, location will define your ackermann progression ratio as well as bumpsteer. To minimize bumpsteer align the tie rods with the ones from the a-arms. As for ackermann, make a 2d model in matlab, figure out what progression ratio you are looking for, try a few different placements and iterate until you are close enough to your desired geometry.

As for reducing steering effort, its a bit of a balancing act to maintain feedback. Things that affect your steering effort are scrub, caster, kpi and gearing. Again, start out with desired steering force on the steeringwheel, diameter of said wheel, pinion effective diameter, travel of the rack and length of the steering lever on the upright. This together with lifting effort from caster and turning moment of the tires at given scrub should provide sufficient data for your calculations.

[Christopher Catto]

Yes, align the steering tie rod with the upper wishbone for zero bump steer. In fact, bump steer does not have to be zero but for your first design it will simplify your design and calculation.

Also, on the steering bracket on the upright, have the rod-end of the steering arm clamped between many washers. This way you can shuffle washers above and beneath the rod-end to tune the bump steer. This is common in many racing series where they actually use bump steer in order to have different tyre slip angles for different tracks. You can have the bump steer asymmetrical if you want to take advantage of the track if some corners are more advantageous than others.

As for Ackerman, you can try different brackets to mount on the upright. Aim for say 100% ackermann for your first design and then see which way you want to go (more or less) depending on the track and the driver style. Of course there is a theoretical amount related to the minimum corner radius in FSAE but in fact you can use Ackermann values that your tyres will happily take.

Tyre overheating is generally only an issue in the endurance. So for example you can change rod-end mounting hole between sprint (to get higher slip angles and hence more heat into the tyre to get a good lap time) and enduro (less slip angle to preserve the tyres), providing the judges do not object to this as a change in car spec.

As for steering effort, there is a book by Donald Bastow I think that has a chapter on this. To do with steering rod lengths and steering arm length on the upright etc. Sorry, but you may not have time for Matlab models. You will just have to make many assumptions if you have little time. Its life.

[kbuckert]

honestly guys...have you done any steering research???

the rack can be placed anywhere you want...the key to eliminate bump steer is to have the tie rods run through the instant center of the suspension. aka, when you decide where the rack is mounted, determine where the IC is of the suspension, draw a line between the IC of the suspension and the connection point on the steering rack and the intersection point with the upright is where the mount points should be.

It takes some research...but that is the key...be a good engineer and design it before building!

Kris Buckert
'07 Bradley University Team Captain
Steering/Electrical/Overall coordination

[Christopher Catto]

sure, in simple terms yes. but doing 2d sketched models and placing the steering column anywhere as long as it satisfies your condition is not necessarily going to give you what you want. i have a ton of average books and student reports that read just the same. all you do is this or this.... same as making a cake by just adding the ingredients and not following a more detailed method.

I am not saying it is not a good tip, just it assumes a few too many things. a car is a 3-d object in a 3-d world. 2d is a good start but go and see some good cars and ask the team why they did not place the rack just anywhere else and just figure out the IC position. their answer will be that there are some nice places to place the rack, where it works quite well (it meets several requirements and design targets) and other places where it may work at some ride-height or suspension motion but may not be ideal in other conditions.

same thing goes for the IC. ask yourself why the BMW F1 cars have drooping wishbones and why the Ferrari dont. Are they doing their IC calcs in 2-d or are they looking at the problem from different perspectives and seeing what works in a reliable manner in the configurations run on their cars?