steering leverage

[Omer]

hi all
we are a first year team so no experience.
my question is what is your distance between the outboard tie rod on the upright to the wheel center(horizental distance only)?

I designed it to be 55 mm(2.16 in) but have a feeling it's not enough and that it might be difficult to steer the car.

our steering wheel will be 230 mm dia, pinion 26mm dia,track width 1300 and mass around 300 kg.

[Omer]

hi all
we are a first year team so no experience.
my question is what is your distance between the outboard tie rod on the upright to the wheel center(horizental distance only)?

I designed it to be 55 mm(2.16 in) but have a feeling it's not enough and that it might be difficult to steer the car.

our steering wheel will be 230 mm dia, pinion 26mm dia,track width 1300 and mass around 300 kg.

[Denny Trimble]

Well your steering ratio is 4.23:1 (steering wheel degrees : wheel degrees), which is fairly quick. Most FSAE cars have ratios from 3:1 to 8:1.

Another thing to consider is "toe stiffness", i.e. how stiff is your steering system (from the tire contact patch to the steering wheel). Increasing your steering arm length (the 55mm dimension) will reduce the forces on your steering rack and rodends, and it will slow down your steering ratio.

[Omer]

thanks denny
but i didn't understand how you calculated my steering ratio without me giving you the angle of the wheels and the angle of my steering wheel that creates it?but you are close we figuerd 100/25=4 steering ratio.

this toe stiffness is what i'm afraid of i want my drivers to steer the car without putting too much effort,and only after dealing with the driver i can buy stiffer rodends and build it stronger.

is 55 mm considering the other dimensions sounds ok?

[terra_dactile]

omer,
if you are really worried about your driver having to input as little effort as possible, i would recommend also looking to reduce your scrub radius as much as possible, the difference bettween the steering effort of our 2004 to 2005 car was huge becuase the scrub radius was reduces substatially.

Jude Berthault
ETS Formula SAE
Steering director (2004-present)

[SnailRacer]

Omer-
Don't forget about your caster trail, that will have a substantial effect on steering effort when cornering.

Jude-
How much did your scrub radius change? I was under the impression that a large scrub radius, causing the wheel to rotate about its free axis (~horizontal axis) when steered, wouldn't have a big effect on steer effort... unless your braking I suppose. Am I missing something?

[Denny Trimble]

I calculated the steering ratio from your pinion diameter (I assumed 26mm was the pitch diameter, but that may not be correct).

Steering Ratio = steering arm length / pinion radius = 55mm / 13mm = 4.23.

Assuming the rack isn't very far off (fore/aft, up/down), and you don't have too much ackerman built in either way. It's all nonlinear anyway.

I'd recommend you keep your mechanical trail (or "caster trail") less than 1", even less than 1/2", for low steering wheel effort. But there are several other tradeoffs to be made there.

Z has kindly shared his articles from Racecar Engineering with us:
http://students.washington.edu/dennyt/fsae/zapletal/