calculating the load to be moved by steering system

[hinduja]

hi
i have come across this term called 'module' that has a direct relation with rack travel also module is 25.4/diametrical pitch.
it also looks like ,to decide the module of a rack and pinion we ought to know the load it is supposed to move . in terms of steering how to calculate this load . i believe this load is inclusive of the weight of the wheels and a lot more external factors depending on the terrain.kindly throw sum light on this concept. pls reply i am in urgent need of help........

[hinduja]

hi
i have come across this term called 'module' that has a direct relation with rack travel also module is 25.4/diametrical pitch.
it also looks like ,to decide the module of a rack and pinion we ought to know the load it is supposed to move . in terms of steering how to calculate this load . i believe this load is inclusive of the weight of the wheels and a lot more external factors depending on the terrain.kindly throw sum light on this concept. pls reply i am in urgent need of help........

[The_Man]

The steering forces are very difficult to determine to a very good extent without any physical data from data acquisition systems or data about tyres. So your best bet here is to slightly overestimate them.

The forces you will have at your steering are mainly caused by the tyres' interaction with the road. The steering force that you will encounter is due to the realigning torque that is produced by the wheels more than the weight of the wheels, uprights and the other unsprung mass. The realigning torque is dependent on you mechanical and pneumatic trails on the tyres which in turn depend on the caster and steering axis offset. The rest is determined by the tyre characteristics and camber which is the hard part. Try getting a range for approximate cornering stiffness and pneumatic trail values of FSAE tyres and then a quick calculation should get you a good estimate for the forces on the steering.

To see how the forces are transmitted to the rack and pinon I suggest using ADAMS or something similar. Shouldn't be too difficult to do by hand too.

I hope this helps.

[hinduja]

thanx mahek.......so u mean the forces depend largely on the type of tires we use....hmm.......we are planning to do the analysis on ANSYS will it work? and one more thing is there a relation betwen the no.of turns lock to lock of the steering wheel and the steering ratio we set? if not then y do people find it difficult 2 set steering ratios less than 8:1

[The_Man]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by hinduja:
so u mean the forces depend largely on the type of tires we use </div></BLOCKQUOTE>

No, I mean the forces originate from the tyres you use but the magnitude is also a function of your steering arm length, caster, steering axis offset and steering geometry among many other things. Very roughly you need the moment that acts on the type about the steering axis.Then the force that acts on the steering tie rod is this moment * the steering arm. Then rest is just forces through linkages.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by hinduja:
we are planning to do the analysis on ANSYS will it work? </div></BLOCKQUOTE>

ANSYS is mainly for Finite Element Analysis. So if you are designing the gears in the steering gear box and want to be sure your gears are not going to fail then ANSYS is the way to go. If you just need to know the forces acting on the gears to determine the steering wheel size, steering forces, etc. the I do not see ANSYS helping you out there.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by hinduja:
and one more thing is there a relation betwen the no.of turns lock to lock of the steering wheel and the steering ratio we set? if not then y do people find it difficult 2 set steering ratios less than 8:1 </div></BLOCKQUOTE>

Yes of course, isn't it for every rack and pinion? Infact that is the very definition of the steering ratio. Just imagine once with a mechanics point of view. See how much force your driver is comfortable applying on the steering wheel, minimize the steering rotation from lock to lock for this steering force making sure there still at least +-90deg rotation. This is when you can choose your steering ratio.

Mahek Mody
IIT Bombay Racing

[hinduja]

thanx for the explaination mahek.......one more thing......which one is better .....to directly link the steering wheel with pinion through the steering column or placing 2 uinversal joints at intermediate locations? i found a few excel sheets on the net which take in a few inputs like track width, mass of the car etc.....and gives the output in terms of pinon and rack pitch addendum etc........r these sheets reliable? kindly post some links which have such useful calculation excel sheets.......

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by hinduja:
thanx for the explaination mahek.......one more thing......which one is better .....to directly link the steering wheel with pinion through the steering column or placing 2 uinversal joints at intermediate locations? i found a few excel sheets on the net which take in a few inputs like track width, mass of the car etc.....and gives the output in terms of pinon and rack pitch addendum etc........r these sheets reliable? kindly post some links which have such useful calculation excel sheets....... </div></BLOCKQUOTE>

Or you could just do the work yourself instead of asking for a hand out.

I'd want as little compliance in my steering system as possible though... so no u-joints ideally.

[The_Man]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by hinduja:
thanx for the explaination mahek.......one more thing......which one is better .....to directly link the steering wheel with pinion through the steering column or placing 2 uinversal joints at intermediate locations? i found a few excel sheets on the net which take in a few inputs like track width, mass of the car etc.....and gives the output in terms of pinon and rack pitch addendum etc........r these sheets reliable? kindly post some links which have such useful calculation excel sheets....... </div></BLOCKQUOTE>

It really depends on your design. If you can do the direct link well and good you should avoid the universal joints. See if can do with one universal if your manufacturing is accurate enough. No one is going to tell you which one is the way to go, weigh the pros and cons.

About the excel sheets, there are loads of the available on the internet. I suggest you do not use them as black boxes. You can check the calculations, see if the make sense to you and judge for yourself whether they are reliable or not.

Mahek Mody
IIT Bombay Racing