Slip angle (pain in the ass)

[Wolverine_1987]

Hello guys... i've been working lately on the steering geometry and when it came to the determination of the slip angle in order to estimate the true tie rod length.. i started looking in books like fundamental of vehicle dynamics, race car vehicle dynamics and automotive chassis engineering principles. They all mentioned simple equations for slip angle which i highly doubt it to be true cause it's never taken into consideration the road conditions, understeer and overtseer conditions. When i searched the forum i found alot of experiments using sensors and stuff but that's our first car we are not optimizing a ready made one plus our schedule is tight... I read that the slip angle can't go further than the steering angle so, does that mean that i can take the steering angles to be the maximum slip angles to design a tie rod ??.... or may be not.... please help

[Wolverine_1987]

Hello guys... i've been working lately on the steering geometry and when it came to the determination of the slip angle in order to estimate the true tie rod length.. i started looking in books like fundamental of vehicle dynamics, race car vehicle dynamics and automotive chassis engineering principles. They all mentioned simple equations for slip angle which i highly doubt it to be true cause it's never taken into consideration the road conditions, understeer and overtseer conditions. When i searched the forum i found alot of experiments using sensors and stuff but that's our first car we are not optimizing a ready made one plus our schedule is tight... I read that the slip angle can't go further than the steering angle so, does that mean that i can take the steering angles to be the maximum slip angles to design a tie rod ??.... or may be not.... please help

[exFSAE]

I'm not seeing why you necessarily need slip angle information to design a tie rod...

The equations ARE simple anyway.. assuming all things being rigid.. you can start with front slip angle being a function of steering, yaw rate, speed, and body sideslip angle. It's laid out in RCVD.

[RollingCamel]

For the thousand time, your steering geometry will determine your "geometry". Why are you going through forces, slip angles and stuff now?

So as i told you please focus on the steering geometry in "Race Car Vehicle Dynamics", "Automotive Chassis Engineering principles" and "Tune to Win".

[Wolverine_1987]

Using virtual centers method to determine the tie rod length requires the slip angle (alpha) to be already known

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wolverine_1987:
Using virtual centers method to determine the tie rod length requires the slip angle (alpha) to be already known </div></BLOCKQUOTE>

...wat?

[BillCobb]

The method you have mentioned is for closed loop control of front sideslip gain. Are you planning o using active front steer ? If so, then you need to define/specify the steering wheel gain as a function of speed and sideforce that you want (or your driver can accomodate). If not, then define an understeer vs g-level function you can live with and compute the slip angle from it. This function is also best obtained from an experiment to measure your driver's ability to operate the car in closed loop control at its maximum g capability. So, lets say that he/she needs .10 g/100 degSWA gain to sustain closed loop control, that translates to an understeer function given your wheelbase and the limiting speed and the turn radius you have, thus the total roadwheel steer angle required. This angle translated to left and right wheels via the tire curves for maximum force is used to define the distance between the steer arm and the rack balls or pitman arm (which ever you are using). The steer arm length and pinion radius need to be chosen such that the maximum driver input angle at the steering wheel is not much more that 90 -100 degrees or else sprained wrists will be a problem. Steer arm length to pinion radius ratio is sometimes referred to as the overall steering ratio.

The best way to start this out is with a real or virtual constant radius test on a track using the constant speed method (as opposed to the constant acceleration method). A good driver reveals themselves to you in this manner. But, it takes a car to do this. I posted this method as a good way to sort out or train a driver, but it seemed to be lost, as a retort that "seat time was what it takes". Well, bad seat time but lots of it is what loosing teams use to develop a car. Good teams measure the driver and pick the best closed loop driver and give them the pretty shoes.

BTW: Really good drivers operate well below the .1g/100degSWA gain derivative (like at .02 - .05). That's why they look so smooth.

[RollingCamel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wolverine_1987:
Using virtual centers method to determine the tie rod length requires the slip angle (alpha) to be already known </div></BLOCKQUOTE>

...wat? </div></BLOCKQUOTE>

I second that.

[Adambomb]

Re: BillCobb:
Don't worry, you're insight isn't totally lost here. In your posts I sense a whole different league of experience than the norm here. I generally don't reply because I really don't have anything to contribute beyond that. I'm planning on doing my darndest to implement your suggestions regarding driver training...being able to put a metric on it beyond SCCA lap times has been a dream of mine for a couple years (and I also have a "to read" list that's about a mile long, and subject to time available after grad research and getting the car done).

Until then, like many teams, we still fight with the just taking care of the basics (and acknowledge that fact). Active steering on a first year car? Yeah...

[Mikey Antonakakis]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adambomb:
Re: BillCobb:
Don't worry, you're insight isn't totally lost here. In your posts I sense a whole different league of experience than the norm here. I generally don't reply because I really don't have anything to contribute beyond that. I'm planning on doing my darndest to implement your suggestions regarding driver training...being able to put a metric on it beyond SCCA lap times has been a dream of mine for a couple years (and I also have a "to read" list that's about a mile long, and subject to time available after grad research and getting the car done).

Until then, like many teams, we still fight with the just taking care of the basics (and acknowledge that fact). Active steering on a first year car? Yeah... </div></BLOCKQUOTE>
what he said