Scrub Radius - How does it feel?

[D J Yates]

I'm just about to finalise the front suspension geometry for this years car. I've curently got zero king pin angle, which is a good thing. The resulting scrub radius is about 35mm. I've done my research and am well aware of the effects of scrub radius, but i'm just trying to get an idea for how it feels to the driver.

Do any of you guys run such ammounts of scrub radius? How heavy does this make the steering feel?

I could do some calculations for the moments etc, but it doesn't mean a lot. I just want an idea of how it feels. An hour in a twin engine pro kart - the standard sort of thing you'd pay £15 to thrash around for 30mins - on a large outdoor track with lots of fast corners is a good workout. I'm sure you've all done plenty of karting, so how's the steering compare to an FSAE with 35mm+ plus scrub radius?

My instinct tells me that 35mm is ok, but is it worth adding 1 degree king pin for the sake of 4x£50 gym memberships for the drivers.

[D J Yates]

I'm just about to finalise the front suspension geometry for this years car. I've curently got zero king pin angle, which is a good thing. The resulting scrub radius is about 35mm. I've done my research and am well aware of the effects of scrub radius, but i'm just trying to get an idea for how it feels to the driver.

Do any of you guys run such ammounts of scrub radius? How heavy does this make the steering feel?

I could do some calculations for the moments etc, but it doesn't mean a lot. I just want an idea of how it feels. An hour in a twin engine pro kart - the standard sort of thing you'd pay £15 to thrash around for 30mins - on a large outdoor track with lots of fast corners is a good workout. I'm sure you've all done plenty of karting, so how's the steering compare to an FSAE with 35mm+ plus scrub radius?

My instinct tells me that 35mm is ok, but is it worth adding 1 degree king pin for the sake of 4x£50 gym memberships for the drivers.

[Colin]

from our experience i'd say 35mm is pretty big, have you looked at how that value is loading up your hubs?

[Kevin Hayward]

I'd say 35mm is close to the upper range of scrub. I wouldn't say it is too large. Our '03 car (04 US) had about 30mm scrub. Steering was not a problem. You need to consider the caster angle and steering ratio along with the scrub but I think 35mm wouldn't be too big an issue.

Colin is right on about looking at the hubs ... but a nice wide bearing base is not a bad idea anyway.

Kev

[Frank]

"corner weight jacking in steer"
is proportional to
sin (steering angle) * sin (castor) * scrub radius

we've had a few different lengths, 30mm, 40mm, 50mm

i'd be sticking around 45mm with 6-8 deg castor

135 degress of total steering wheel angle available

with (app 100% ackerman) 35 deg at the inner wheel, 25 at the outer wheel at full lock

this is based on a 1575 WB 1150 front track

[Brian Smith]

i would say our 02 car set the bar for the upper range of scrub. It looks like its about 4 inches with no kpi http://fsae.com/groupee_common/emoticons/icon_smile.gif. It made the driver work!

[Kevin Hayward]

Frank's numbers look pretty good to me.

I take it that the Queensland monster still has a locked differential and not much more than 50% on the rear.

The weight balance of the vehicle and the differential setup will affect what scrub / caster combination you should run. Also remembering that your weight balance will affect the choice of differential settings.

Also worth considering the effect of the tyres you are using. On a purely surface analysis the Goodyears are a little more sensitive to camber changes than the Hoosiers. This can make running high caster angles a little bit of a problem. Hence if you are looking for a little extra jacking it becomes an issue.

I'd love to see a bit more posting on this topic as I think the steering geometry of these cars has a lot to do with the overall performance. Unfortunately some of this stuff is harder to build in as adjustable features for testing purposes.

Just so Frank's not the only number poster here's the ranges we have used (pretty similar to Queensland):

Scrub: 30-50 mm
Caster: 5-8 deg
Pretty much same steering angles as the Queensland car
Ackermann: 50-120% (First car had adjustable ackermann ... haven't thought it necessary to build it in as adjustable since. Run about 100% now.)

We run more weight on the rear now and our wheelbase is 1750. 1200 Front track.

Cheers,

Kev

[Cement Legs]

Is reverse Ackerman popular for these cars. It seems to me that with Ackerman steering you are going to be dragging your inside tire around a lot of turns, especially if your lateral forces are over 1g.

Anonther question; our current design for our spindles is down to zero scrub (almost) with a 2 degree kingpin angle and about 7 degrees of caster with 50% reverse Ackerman. Does anyone see any problems with that setup?

[Jarrod]

we run about 25mm of scrub, but i've designed to allow experimentation on this years car. We ended up at around 6 degrees of caster on both of the last two cars. Is your zero scrub allowing for the fact that the center of the contact patch is not at the center of the tyre due to camber? You could quite easily end up with negative scrub. The best theory i have come across for reverse ackerman is that the peak force occurrs at higher slip angles with higher vertical load, so the outer tire has to turn further, and generate a larger slip angle. This works with high cornering forces and low steer angles. (ie high speed large radius corner like a circuit) Anyone else have an opinion on this? We have always run adjustable ackerman, and found about 120% worked for us.

[clausen]

What tyres are you guys using pro ackerman with? They slip angle vs lateral acceleration curves for the goodyears (that we're using this year) clearly show that they should love anti ackerman.

To add some more numbers to the thread, our design setup is 25mm scrub, 0KPI, 10deg Caster (not much camber change through bump) and the aforementioned anti ackerman, the percentage of which im not certain.

We also have adjustable trail, which I'm looking forward to playing with.

Only a couple more days till we get to find out how it all feels!!

[Kevin Hayward]

Clausen,

I would be a little careful with the goodyear data if it is either of the commonly available sources (i.e. the posted graphs or the ADAMS files). First of all the graphs are for an R095 compound (if I remember correctly) and do not match the coefficients in the ADAMS files.

If the coefficients in the ADAMS file are correct you may be interested to note that the trends for peak grip versus slip angle reverse at a low load ... as in the sorts of loads that the inside tyres of these cars would see.

However it is likely that the coefficients generated were curve fitted to values obtained at significantly higher loads (similar to the graphs) and the curves have been fit to the data. Hence they probably do not hold as being too accurate in the loads we are interested in.

If the graphs are correct then there are a few more inconsistencies.

If you have obtained the data yourself with your own test program then please excuse my post. Actually you are more than welcome to send any such test results on to us http://fsae.com/groupee_common/emoticons/icon_smile.gif

Kev

[Jarrod]

we use Dunlop wet compound slicks. We have some hoosiers to test with this year though. The data we have is not great, only about four points at two vertical loadings. I have seen radial tyre data that definitely suggests anti-ackerman, but thats irrelevant here. With 100% or less ackermann we were having really bad turn in response, increasing it helped a lot. I've never tried anti-ackerman, so i don't know whether it may be better or not, but the testing we did suggested it wouldn't work. I may be wrong and probably am. What kind of slip angles are you trying to get? I will definitely be watching with interest to see how it works, may have do some experiments over summer.

[Mick_P]

At Swinny this year we are running 12mm scrub, but this is mainly due to packaging. We didnt exactly aim at a number, we knew we wanted it less than 25mm from previous experience and looking at other FSAE cars and after the wash 11.6mm came out. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

Jesus Kevin, what did you have to go and tell me that for? http://fsae.com/groupee_common/emoticons/icon_frown.gif

We were just using the data from the Goodyear site.

I just hope we get enough time to mill up some clevises with different ackerman angles to try.

[Charlie]

Ackerman for these cars seems to be driven more by its transient effects than steady state. Especially in the older tracks, fast sweepers were a lot less common than quick hairpins and slaloms, especially in autocross. Ackerman gives better turn-in, mostly from the fact that the inside wheel reaches a high-grip slip angle quicker. At least thats how I understand it.

I have not seen any successful FSAE cars with anti-ackerman. But different suspension types and of course different tires might want different things.

Definitely make this a parameter on your car that is easily adjustable.

[clausen]

Charlie, I'm pretty sure Wollongong ran with Anti-ackerman the year that they won.

[Charlie]

Found this pic of Stumpy...

http://students.washington.edu/dennyt/fsae/Comp_Pics/wollongong/pages/DSC00083.html

[jack]

it seems to me like weight transfer would dictate the decision to run anti or positive ackerman. its a bit like hearing about someones roll center, its just one part of the big picture.

either that of kev is way smarter than me...http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Denny Trimble]

Jack, were you sleeping during that part of Claude's seminar? It depends on the tire behavior. If you graph lateral force vs. slip angle at your range of normal loads, then draw a vertical-ish line through the peaks of the curves, the direction of that line will tell you to run ackerman or anti (or parallel). If the lighter-loaded tires reach their peak at smaller slip angles, you want less than ackerman. And vice versa.

Of course that's a steady-state thing, and as Charlie is hinting, transients are what it's all about in FSAE. I don't have any answers for ackerman in transients though. Seems toe-out can benefit turn-in as much or more than ackerman, which comes into play progressively as steering angle is applied.

[Eddie Martin]

I don't think any sae cars run anti-ackerman. If you were running anti-ackerman what kind of slip angle would the inside tyre have on a tight corner.

How much ackerman depends on what type of tyre you are running and other parameters in the car. You really have to have some adjustability to get a good setting. I wouldn't go off the data the companies provide it really needs to be tested on the track, it does make a difference.

Stumpy had adjustable ackerman and we changed it for different events. It ran from 130% to 200%, if we had more time we would have tried more settings. As with all suspension related stuff everything depends on everything else. Each car is different and it will behave differently.

[Kevin Hayward]

Jack,

My post was placed to mention some inconsistencies with the commonly available Goodyear data.

The line that Denny talks about is not linear according to the Pacejka constants in the ADAMS file. The other data is pretty much all out of our load range. Given that the models are an empirical fit to data I would suggest that the tyres need to be tested in the load regions they will be used.

In this respect Jack has a point about tyre loading. It is possible with a considerable rear weight bias that the inside front is not really doing too much at all ... except in the transient phase.

Eddie's (and a lot of other teams) approach to testing different ackermann angles is probably the best way to find out what works.

Cheers,

Kev

[clausen]

Eddie, are you talking about the car you took to the 2003 aussie event, or the 2002 aussie event? The one you took to the 2002 aussie event was the one that won detroit wasnt it?

I swear that Michael (cant remember his last name - the one that works for Claude Rouelle now) told me that you ran anti ackerman on the 2002 car? It looks like I am mistaken. Is that photo that Charlie posted of your 03 or 02 car?

Nice steel uprights!

Like I said, we're looking forward to doing some experimenting for ourselves

[Eddie Martin]

Yeah i understand if you were talking to naughty (michael) he is a space cadet at the best of times. He probably meant that the tyres liked anti ackerman. Although i'm not a suspension guy my understanding of this is that the type of course (eg. tight corners or open corners) is what determines the ackerman/toe you need to run and then the tyre characteristics will skew it one way, either more or less ackerman. I may be wrong on this though.

I was talking about the 02 car that won at carrum downs and in the states in 03. That is the 02 car in the pic that charlie attached.

Yeah the judges liked the uprights, steel is such a great material, thanks BlueScope Steel. (had to put in that gratuitous plug for our main sponsor. http://fsae.com/groupee_common/emoticons/icon_wink.gif)

When you do experiment you may find the car likes different ackerman/toe on the skid pan vs autocross. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Eddie Martin
UOW Racing Alumni
"don't let university get in the way of your education."

[clausen]

Yeah that makes sense Eddie http://fsae.com/groupee_common/emoticons/icon_smile.gif

and nice marketing http://fsae.com/groupee_common/emoticons/icon_smile.gif

I did have different settings in mind for the skidpan & autocross, cheers.

Not sure about the space cadet thing, but Mike's definitely one of the cleverest people I've ever met. I worked with him at the V8 team that they consult to.

[Eddie Martin]

Naughty is bloody clever which is one of the reasons why our 03 car sergio (his suspension) was so quick, once we got it sorted.

Keeping him focused on sae and away from females and other distractions was the problem. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[jack]

...yes, i was sleeping during parts of the seminar. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif