Wishbone Design / Front brake disc sourcing questions! [Archive]

Steve-Imperial

01-27-2004, 01:07 PM

Hi, Im from Imperial college in London England. This year we started work on our second car, using a carbon monocoque/spaceframe layout. We are trying to use the minimum of two bulkheads in the front end which results in the pickup points for the front suspension being a fair distance apart. This means that when I put the upright well into the wheel for a small kingpin angle and offset I cant get the steering lock (24 degrees toe in, toe out for 100% ackerman is OK) I want because the wishbones hit the wheel rims! What I was wondering is whether anyone offset one of their wishbone tubes away from the upright pivot to improve this clearance. What I had in mind is sketched in the following diagram:
http://www.geocities.com/stevesridingsite/images/wishbonesketch
(the link doesnt work straight away - go to the parent directory and then to wishbonesketch)
What do you think?! Will this fall apart straight away?

Im also wondering whether to use 1/2" or 3/4" tubes (about 1mm wall thickness) in mild steel or T45 (similar to 4130) for the wishbone tubes. Either will be strong enough in bucking and tension but does the decreased stiffness of 1/2" tubes cause problems? Or are there other problems associated with narrower tubes?

Finally I would like to buy some front brake discs like this one:
http://aod.no-ip.com/images/fsae/img%20043.jpg
Anyone know if you can get discs like that off the shelf (motorcycle rear disc) or do you get them custom made? My requirements are:
about 5mm thick
about 220mm diameter
4 bolt fixing on a BCD of about 90mm

Thanks in advance for any useful replies http://fsae.com/groupee_common/emoticons/icon_smile.gif

Steve-Imperial

01-27-2004, 01:07 PM

Ben Beacock

01-27-2004, 02:30 PM

Your sketch looks like it puts some nasty bending into the front tube. We've run into the same problem a few weeks ago and are making arms that are like an A, but the top is a sharp angle and the legs kick out. We should be able to get 32 deg either way with 1 deg KPI and 10mm scrub radius.

5/8" tubing seems to be common for suspension arms

Ben Beacock
Co-Manager
2004 Gryphon Racing - University of Guelph (http://www.soe.uoguelph.ca/uogracing)

ben

01-27-2004, 02:41 PM

We use 5/8" 20swg mild steel for wishbones with a nitrotec coating.

As for the wishbone drawing, it might work but it looks ugly. I would look at doing something different with the chassis. The chassis is there to absorb the loads from the suspension, it's always better to design the suspension and then design a structure to deal with the loads.

Our monocoque design has no internal bulkheads other than the mandatory roll hoops and the impact structure bulkhead. You can do clever things with the floor and nose to feed the loads into the skins.

BTW are Imperial doing a new Class 1 this year?

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

Steve-Imperial

01-27-2004, 03:26 PM

Hi Ben Bristol,

No the car im working on is for class 2 entry this year and class one next year hopefully. This year we're entering class 200 with our current car, with a few weight saving mods (chop off the sidepods and wings!).

I think I meant 5/8ths not 3/4" in my initial post, so I'l probably go with them. Incidentally whats a nitrotec coating, a type of anodise finish?

The lack of internal bulkheads is what we are aiming for, I think I need to have words with the monocoque designer about a compromise regarding pickup positioning!

Do Bristol design and make a new car each year for class 1?

Hi Ben Beacock,

Your "A" arm design sounds interesting, am I understanding you correctly if I said that the wishbone arms bend outwards and at the bends a cross piece is welded?
I think you're right about my idea it would bend...

ben

01-27-2004, 11:32 PM

I'm originally from Bristol but at Uni in Birmingham, I'm Tech Director of the Birmingham team.

Nitrotec is a nitriding process (http://www.nitrotec.co.uk) it increases the compressive strength and also gives a nice shiny black coating. This is useful because we can visually inspect our wishbones all the time for cracks because they're unpainted.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

PatClarke

01-28-2004, 12:58 AM

Ben, 10mm is probably not enough scrub. Are you calculating from the wheel centreline or from the tyre centre of contact patch, which will be inboard of the c/l? If calculating from the c/l, you probably have negative scrub radius in reality.
On a light car (50kg front wheel weights) a bit more scrub is advantageous as it gives the driver something to work against. I would look at doubling your current number and adding a bit, all measured fron the centre of pressure.
Good news is this will give you more room for the wishbones =]
PDR

I'd give my right arm to be ambidextrous!

Steve-Imperial

01-28-2004, 03:20 AM

Sorry Ben about the Bristol/Birmingham thing it was late at night when I replied...

I would agree about the 10mm scrub radius being too little, I read somewhere on this forum that tyre deflection can lose about half an inch of that. I currently have 40mm though which is too much....

ben

01-28-2004, 03:45 AM

I wouldn't say that 40mm was too much to be honest. Looking at some of the lateral tyre deflection at FS last year I would say they were in inches.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

Ben Beacock

01-28-2004, 05:46 AM

Time to defend my design I guess http://fsae.com/groupee_common/emoticons/icon_smile.gif

The low scrub radius is to reduce the sensation of braking and traction forces which I feel is important due to the AWD. There will be mechanical trail to work against and the caster is fairly steep(and I'd like these forces to dominate). The 10mm is from the centreline but I've accounted for the tire patch location (and dynamic movement) to make it essentially zero. Its a tradeoff, but if it doesn't work out then a wheel spacer will fix the problem easily. If I design for more scrub, it would be hard to work backwards.

Ben Beacock
Co-Manager
2004 Gryphon Racing - University of Guelph (http://www.soe.uoguelph.ca/uogracing)

Frank

01-28-2004, 07:56 AM

dunno 'bout FS rules..

but for FSAE i'd be trying to get 26-27.5 deg on the outside wheel, and 100% ackerman on the inside wheel

use long steering arms, so you don't lose ackerman through deflection in tight corners

id be going for bout 40mm scrub (no damn kingpin either)

IMO kingpin is the only suspension parameter that you cant justify, other than "we needed it because we have bad / lazy packaging"

8 deg castor, 25mm trail

it is the combination of castor angle and scrub (and of course steering) that give you diagonal weight jacking (which you want)

that wishbone sketch is just plain scary.. don't do it

make the chassis fit the suspension http://fsae.com/groupee_common/emoticons/icon_smile.gif

as for disks, yeah there should be heaps of places where you can get disks made custom, either 4mm plain or 5mm drilled / slotted .......perfect

if your using wilwood calipers (easy and cheap option for 1st year teams). use polymatrix "B" pads

Frank
http://fsae.com/groupee_common/emoticons/icon_wink.gif

[This message was edited by Frank on January 28, 2004 at 11:22 AM.]

ben

01-28-2004, 10:09 AM

Aren't wheel spacers illegal?

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

Steve-Imperial

01-28-2004, 12:12 PM

Cheers for the useful reply Frank.
Unfortunately my steering arm is currently underneath the lower wishbone (rearward of axle)to ease steering rack packaging, so its current 70mm length thats about as long as I can get it! Is that really bad?!

Frank

01-28-2004, 06:02 PM

70 mm is not bad...

But when you put it co-planar with an A-arm (on front suspension). 70 mm is not much

I would try to put it as close to central between the kingpin nodes (upper and lower sphericals)

((Yes you can do this and have no bump steer, if you know how to place it))

People get away with putting the steering arm co-planar with an A-arm on the rear WHEN they use lots of castor (neg or pos). Think about it as defining three corners of a triangle, and the plane of the triangle is in bending

Back to the front suspension. Think about Ackerman for a minute. The difference in steering angles is caused by a difference in the moment arm that the steering acts about the kingpin

Therefore the INSIDE wheel has a small moment arm and is the compliant one. Hence, like I said. You "lose" your Ackerman.

Another thing to remember is that the more steering you have, the less Ackerman you can use, before the linkage inverts (obviously as you come close to inversion you get more compliance)

Personally, I believe most of these cars need at least 26 deg of steering at the outside wheel and CAN NOT POSSIBLY use a lot more than 100% Ackerman at full lock.

Kind regards

Frank

[This message was edited by Frank on January 28, 2004 at 09:16 PM.]

Kevin Hayward

01-28-2004, 07:02 PM

Frank,

Just a comment on this:

> IMO kingpin is the only suspension parameter that you cant justify, other than "we needed it because we have bad / lazy packaging"

I have to disagree with you on that one. We added a small amount of KPI in order to make our hubright package work. This allowed us to save building quite a few parts which also cut down on weight. While people may call it bad packaging (free opinions and all) it was certainly not the result of being lazy.

However we did only use the minimum we could to get what we wanted out of our steering system.

I would also hazard a guess that you would probably need to use KPI if you were running a 10" wheel setup - but this is only my uneducated guess.

Kev

Toy Constructor
UWA Motorsport

Frank

01-28-2004, 09:29 PM

oh yeah, these angles castor and kingpin

well they have effects that are sins of angles

so 2 deg of kingpin is more than twice as bad as 1 deg of kingpin etc etc etc

1-2 deg kingpin dosen't bother me, it's the sinful 4 deg outrageous 8 deg and ludicrous 12 deg that causes me pain

kind regards

Frank

Frank

01-28-2004, 09:33 PM

atually i just changed my mind again

any kingpin is *&$*^#

Brian Smith

01-28-2004, 09:46 PM

I agree with frank. KPI is for sissy girls.

Brian Smith

01-28-2004, 09:49 PM

oh yeah... Steve dont feel bad about that a-arm design. I saw the exact same idea on a mazda kudzu. (alms profesionals) with plate welded in the open triangle. (not that its a good idea....)

Frank

01-28-2004, 10:07 PM

or you could say..

KPI is like dating an ugly chick

you might DO it... but you DONT brag about it

Kevin Hayward

01-28-2004, 10:45 PM

Frank,

I once believed that nothing would make me put KPI in a car. You only have to look at our 2001 and 2002 entries which both have none.

When designing our 2003 vehicle it became clear that through the addition of a little KPI (what I now consider "acceptable" KPI) we were able to achieve quite a few mechanical design breakthroughs (well at least we see them as breakthroughs). Knowing the sins of KPI and the horrible evil we were about to unleash on the world we still went ahead and put in some.

To this day I am afraid of being struck by lightning for my crime. However it will probably be repeated this year (finally my last year - if the PhD can find a way to write itself).

So in advance I ask you for your forgiveness.

With shame,

Kev

KPI Sinner
UWA Motorsport

jack

01-28-2004, 11:44 PM

just wondering if anyone actually measured lateral tire deflection? or "saw" it, or guessed? what kind of tires (construction) were deflecting? and as long as we are talking about tire deflection screwing with "feel", shouldn't we also consider pneumatic trail, which also influences handling?

remember, designing a racecar is all about compromises. i have also found that the most challenging part of race car design (which in theory should be trivial) to be packaging. on our car, we made a compromise in our geometry, and hopefully, design judges will realize that it was the best option available, and therefore optimally designed...

jack @ WWU
http://www.etec.wwu.edu/

PatClarke

01-29-2004, 12:49 AM

Hi all,
I belong to the 'I hate KPI' club too, but.
Were I building a car, I would put the axle c/l at 90 degrees to the steering axis, and live with whatever positive KPI I ended up with when I dialled in some negative camber.
The angle between the steering axis and the centreline of the tread is referred to as the "Included angle", so on my design, the included angle would be zero, not the KPI. KPI and Camber would be equal.
I would do it this way for manufacturing reasons and for interchangeability.

Oh, and Ben, ....Okay http://fsae.com/groupee_common/emoticons/icon_smile.gif
PDR

I'd give my right arm to be ambidextrous!

ben

01-29-2004, 12:54 AM

I'm in a similar position to Kevin at UWA. I've added a little bit of KPI to make the upright simpler to manufacture. I think being very rigid about things like this is not always the best approach.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

01-29-2004, 03:15 AM

Just one short point backing up Ben and Kevin. Our car at Monash had an 'outrageous' 11 degrees of KPI because of packaging reasons. We won autocross by a long way from teams with zero KPI, even with that number. Zero KPI is not going to make or break your suspension characteristics.

For us the lack of resources available to manufacture new uprights (they were carry over from 2002) meant it was more practical for us to continue with that number rather than lose testing time waiting for new uprights.

We will be much, much lower than that this year because KPI isnt good, but if the packaging is made easier, then practicality may be worth more than pushing hard for zero.

This is no way justifies KPI obviously and our 11 degrees is, no doubt, one of the reasons for our placing in design (11th after all, nice symmetry).

ben

01-29-2004, 05:59 AM

My thought process was to design for around 3 degrees static negative camber (radial tyres BTW) and then make the spin axis perpendicular to the steer axis as Pat has mentioned for ease of manufacture of the upright. Thus I came out with 3 degrees KPI I don't reckon that's enough to make a difference.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

Frank

01-29-2004, 07:14 AM

PMSL @ Kevin http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

"KPI and Camber equal" what i meant to say

it's unlikely you'll use more than 3 deg static neg camber

a good first approximation might be that lateral and vertical tyre compliance is the same

Frank

Charlie

01-29-2004, 07:29 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by jack:
just wondering if anyone actually measured lateral tire deflection? or "saw" it, or guessed? what kind of tires (construction) were deflecting? <HR></BLOCKQUOTE>

It can be significant, of course there are lots of variables.

Quite a bit-
http://www.eng.auburn.edu/organizations/SAE/AUFSAE/2003endurancefinish.jpg
More like static-
http://www.eng.auburn.edu/organizations/SAE/AUFSAE/2003mattendurance4.jpg

-Charlie Ping

I just need enough to tide me over until I need more.

ed_pratt

06-14-2007, 07:54 AM

Hi,

Sorry to bring this old topic up from the depths, I'm having trouble understanding why kpi is SO bad? Surely as long as it is limited to a "reasonable" amount and there is sufficient caster angle to provide the opposing negative camber things should be ok (I hope! http://fsae.com/groupee_common/emoticons/icon_confused.gif )

We're (Uni of Glasgow) entering class 3 this year and at the moment ive found that 4-5 degrees of kpi is helping keep my vertical RC movement in roll (its about .5mm for 4 degrees) really low. Im using 7 degrees of static caster and -1 degree of static camber currently.

Im finding this extremely confusing - should I be concentrating on getting rid of my kpi, or should I not worry so much about it and be happy that the RC isn't moving about too much?!

Cheers

Ed

flavorPacket

06-14-2007, 08:59 AM

the camber effects from KPI with steer should easily be counteracted by your caster. What you want to look out for is your spindle length.

Some people talk about KPI changing wheel center height with steer, but when you run the numbers I think you'll see that you should spend development time elsewhere.

Tris

08-29-2011, 10:46 AM

Alright, well it's pretty clear that you all find kpi to be evil and its use to be punishable by death, but why? I know that it makes the steering heavy, but I was under the impression that it aided steering feel, helped with centering, and certainly makes packaging easier. What am I missing? I'm a first year suspension guy, trying to make informed decisions.

whiltebeitel

08-29-2011, 11:08 AM

KPI adds camber with steering, like caster, but the camber gain is non-linear with steering. the less linear, the less predicatable the car behavies over a range of inputs. I don't think a couple of degrees of KPI is bad if you need the packaging space, but it's something to be kept to a relative minimum.

Caster is the preferred way to get the self-centering effect, but note the jacking effect it can cause.

Tris

08-29-2011, 12:14 PM

thanks for the reply. I have another question regarding kpi and caster in the rear of the car. Since the rear should have no steering (hopefully), kpi and caster shouldn't have any affect on the kinetics of the suspension. What am I missing?
Also, what affect if any does scrub radius have on the rear? Thanks.

whiltebeitel

08-29-2011, 01:47 PM

Well, consider the tire will compress with increasing load, so really excessive caster and KPI could make the scrub and trail move with vertical load, but they'd have to be pretty huge. As far as scrub radius, the lower, the better, since as the car accelerates, the longitudinal force creates a moment about the steering axis with the scrub as the moment arm, leading to lesser effective toe stiffness.

Tris

08-29-2011, 03:39 PM

Good stuff. Thanks.

Adambomb

08-31-2011, 01:56 PM

IMHO KPI isn't such a bad thing. Total camber gain in steer can easily be calculated using caster and KPI. Just look at the effect adding each adds and it's not hard to come up with a good combo of KPI and caster that doesn't get too wonky. There are combinations that give quite linear response over the range an FSAE car will experience.

It will add more jacking forces to the front, but again this is a function of scrub radius, so naturally you'll likely want to do more of a tradeoff between KPI and scrub radius to get the right balance of steering and brake feel. And at least the jacking forces are in the "right" direction. But reducing the scrub radius is cake when you've got a bit of KPI...

As for the rear, assuming your car has little to no bump or roll steer, I don't see it as a bad thing at all, and will give you more a-arm clearance while making it easier to get the scrub radius close to zero. Especially if you are running phat 8 in. rims (which are just plain cool in my non-engineering opinion http://fsae.com/groupee_common/emoticons/icon_razz.gif )

We've run 5 deg. KPI front and rear and our overall package felt fantastic. However I can't attach any top figures from competition to that (due to other factors unrelated to KPI), but thought I'd through that out there since there a lot of names I respect saying that KPI is evil (albeit from 7 years ago...). Anything I'm missing here?