70 Bug Mid Engine

I need some input on those of you who have experimented with scrub radius. I am shooting for 0° on my current setup. I have heard pros and cons about going slightly positive or negative. What do you guys set your cars up with? By moving it around say 2°, is it going to mess with bump steer? Any input would be great.

camarokid91

https://en.wikipedia.org/wiki/Scrub_radius

Zero is bad you either want above or below zero. At zero is when squirm occurs. I was convinced my camaro had zero or really bad ackerman and set out to fix the issue and was able to. Trial and error and planning is how I got my car to behave better. The link below should be a decent starting point.

http://www.racingaspirations.com/app...try-calculator

70 Bug Mid Engine

Thank you for the one and only reply in a week! I guess not that many people get in to this kind of detail when setting up a car. I have used the racing aspirations app to design my suspension already, mostly for bump steer, but it does not give scrub radius. It was easy enough to figure out on paper though. With the wheels I have on the car now, I am at 2° positive scrub.

I guess my main question was, for those of you who have tried both positive and negative, what did you end up with and why did you choose to go that route?

camarokid91

I am by no means an expert I wouldn't want to miss lead you incase something I say is incorrect. I would talk to some more experienced chassis builders like Terry Fair/ Alan Blaine or Filip.

"When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock."

[Quote from h&r article below.]



http://www.blainefab.com/
http://www.hrsprings.com/technical/scrub_radius

70 Bug Mid Engine

Thanks for the articles. Looks like I will start on the positive side and see how things go. Might need to buy some new front wheels but good thing they are cheap! Hopefully I can get by without it though.

camarokid91

http://blainefab.com/wheelspacers.html

LS1-450

iTrader: (33)

IDK what to say here. Have never read articles regarding pros/cons related to pos or neg scrub. Always paid attention to scrub on the best handling cars & then following suit.

Two best in stock form that we have driven are BMW & Miata. Each have scrub close to zero. Keep in mind that you can mess up the rest of the alignment & then blame a resulting issue on pos/neg scrub. So, I'd advise not to over think it.

The Munster has very close to a zero scrub (slight toward positive) & has always performed best there.

70 Bug Mid Engine

LS1-450

Seeing that you are running a Miata, you will probably be my best help. I have the spindles off a NA6 (1993) and are pretty sure they are the same as yours (minus caliper mounts). What are your front wheel dimensions? What do you run camber and caster at? Obviously, your car is going to be quite a bit different in weight, weight distribution, CG height. blah blah blah, but should give me a decent starting point.

On another note, what brakes are you running? I was looking at the Flying Miata stuff and it looks good for the price.

LS1-450

iTrader: (33)

Maybe 30% remaining is Miata. Car is 70% GM, Ford, custom made parts. Your spindles aren't the same, cuz' mine are not stock Miata spindles.

Best advice that I can offer is to get the scub radius (where kingpin inclination angle crosses center line of wheel on pavement) as close to zero as possible favoring positive. Caster should be max, IMO. 2.5* camber on front is a good starting point. Make sure that your car is corner weight balanced (LR/FR, RR/FL). This is more important than is front to rear weight equality.

Sorry, can't go into much detail regarding wheels & brakes. V8 kit companies have a tendency to borrow good designs without permission. So, we keep details off of the internet. Keep in mind that your brakes are as important as the suspension set up, on a road course. Going fast means having the braking advantage.

70 Bug Mid Engine

Thank you for the info you could share. I can add as much caster as needed. Probably up to 20°. So max caster won't be my friend. Keep in mind, I have no power steering, so thinking of starting around 7° caster. A lot of people have said 2.5° camber is good for a starting point, so 2.5° it is.

LS1-450

iTrader: (33)

Never heard of ability to adjust to 20* caster. So, yes 7* is good.

70 Bug Mid Engine

That's the glory of designing your own suspension. I can do 10° of camber and adjust width up to 2" overall. Wheel base is also adjustable with 1" per wheel. A track car needs to be adjustable.

Supercharged111

iTrader: (1)

I measured my Camaro at -1/4" of scrub with Z06 wheels, no spacers. Thanks for reminding me, my track width is over right now so something for me to keep in mind is to not go below 3/8" of spacer. Here's an article (one of a series) that mentions 3/4"-3" being a decent range for a RWD car.

http://www.motoiq.com/MagazineArticl...and-Scrub.aspx

LS1-450

iTrader: (33)

IDK how anyone could consider 3". Personally, I wouldn't even go to 3/4". Am not sure where these numbers come from, but, also haven't read much on the subject. Would say that maybe it's our short wheel base. Then again, a BMW has a much longer wheel base & they are designed close to zero, as well. Sounds like we'd just be chasing our tail by reading a lot on the subject. I guess sometimes ignorance is bliss; in our case anyway.

70 Bug Mid Engine

Well as I was looking for more information on this subject, I found a great article. Lets just say that scrub radius is the least of my concerns now. Now its caster vs camber gain vs KPI and the effects it has when steering. The joys of designing suspension.... Glad I have a lot of room to make adjustments but I should be close with my base settings.

http://www.pro-touring.com/threads/1...ck-Performance

Supercharged111

iTrader: (1)

Pretty easy to bitch about the shortcomings of an existing suspension, but a whole nuther ballgame when an unstarted project is staring you in the face isn't it? Might be easier to just build something that sucks and then redesign it.

70 Bug Mid Engine

Not too sure what you mean by all that. I am not using an existing setup. I have miata spindles, that's all i have off of an "existing" setup. The only thing common with my suspension and a miata is that the kpi is same.

I would rather ask questions and make sure I do things right the first time even if it takes more time, requires doing it twice or even costs more to make it right. I would rather do that then make something that sucks from the start.

Supercharged111

iTrader: (1)

Didn't choose my words very good. I meant there's likely going to be a point where you have an A, B, and possibly C option with all 3 theoretically giving desirable results differently. Maybe less spring and more antidive, deciding on caster/kpi ratios, camber/caster gain curves etc. If you obsess over it, I imagine you'll pull the trigger and build it one way, then once it's dialed in wonder if option B would have been better because of X, Y, and Z and redo it accordingly. Or maybe you dream up option D based on feedback and performance and go with that 2nd instead. Point being the whole theory vs real world aspect.

70 Bug Mid Engine

Now you are speaking my language! I agree with what you say and that's exactly what is going on right now. Like you said, I am trying to figure out if a b or c is the best starting route. Just trying to get somewhat in the ballpark for starting off. For all I know, I might trash the whole suspension once it hits the track but at the same time, that is what I am trying to avoid. I have built enough adjustment in to everything so that I should not have to start from scratch if my base setup does not work out. The only thing I really can't adjust is the anti dive. That requires cutting stuff. I have slight adjustment for camber gain and that's always a plus. One day I will get to take it to the track and then the real fun starts