Project Mazda3: Part 3 - Improving our Contact Patch

by Per Schroeder

Now that Spring has sprung here in the Mid-World, it’s time to get our 2014 Mazda3 ready for some autocross fun.  We are planning on competing in SCCA’s Street Touring F category against MINI Coopers, Acura RSXs and Mazda2s, as well as other Mazda3s.

This car still sees regular commuter duty and there are very few aftermarket parts available—but we’ll be tearing into it anyway as we want to at least get the ball rolling towards our goal.


Last year, we ran the car in H Street, with just the addition of some hand-me-down Hankook RS3s on older Mazda3 alloy wheels.  Fun, but not thrilling.  

The big issue at the moment is that there are no aftermarket struts or shock absorbers available for the Skyactiv chassis.  Koni is currently working on them but they are at least six months out.  That’s most of the 2015 season, so we’re just going to forge ahead and see what we can do with the stock dampers.


The stock wheel and tire package is pretty tame: MPG-friendly Yokohama 205/60R16 all-season tires on 16x6.5-inch alloys that weigh 17 pounds a piece.  

We believe that the trick to making a car work for autocross is to maximize the contact patch under the rule set, both statically and dynamically. By statically, we just mean making sure we have the best tires and widest tires on a wheel that meets the letter of the STF ruleset.  By dynamically, the we want to limit body roll (and resulting camber change) so that the keeps gripping mid-way through the corner.    

This installment will take care of the static contact patch and get us going on the dynamic portion of things.


Our next step along the wheel and tire idiocy that consumes most autocrossers was a different set of wheels and tires for our street duty.  A friend of ours had gotten rid of his MX-5 and had some Kosei T1S wheels that still had some decent Dunlop Direzzas on them. 
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Tuesday, May 05, 2015 5:15 AM
Dear Original Poster, please check AutoEXE for suspension parts, true they're in Japan but they have all sorts already available
Tuesday, May 05, 2015 8:57 AM
So no impressions of any sort? Not even a seat of the pants feeler?
Per Schroeder
Per Schroederlink
Tuesday, May 05, 2015 9:05 AM
Actually, it's quite good, but I didn't want to say "OMG11!!! it is teh awesome" until I had some data. So far it feels very reminiscent of the Mazdaspeed3 prototype that I drove a few years ago (ok, a lot of years ago) with a coilover package. albeit trading spring rate for bar rate.
Per Schroeder
Per Schroederlink
Tuesday, May 05, 2015 9:15 AM
Ninja edit: Added some impressions that round out the update for now.
Adam T
Adam Tlink
Tuesday, May 05, 2015 12:47 PM
Have you contacted Jeff at ProParts? I bet they can get some Konis working for you, be them 2812s or just Sports.
Wednesday, May 06, 2015 4:32 PM
Good work, I guess. I too was hoping for impressions. Looking to do a similar sleeper suspension build on my 8th gen SOHC Civic. Shame they don't let you go bigger on wheels. 225/45R17 is about perfect for cars of this weight and horsepower.
Per Schroeder
Per Schroederlink
Thursday, May 07, 2015 6:07 AM
I've added some impressions to the seventh page of the article--but am reserving final thought until I've done more than a few spins around a Piggly Wiggly parking lot--I'm waiting for autocross impressions as just casual corners here and there aren't telling me much.

I can certainly use a 225/45R17--but I'm interested in lowering the top speed of second gear from 65mph to just under 60mph, and the shorter 225/45R16 will do just that, as well as lowering the car.
Tuesday, June 09, 2015 11:34 AM
Doing a very similar build on my 12 Focus NA for STF. Curious to see what you do for coilovers when the time comes. I'm not sure if I want to go with something decent and cheap like K Sport or BC, or shell out the big bucks for the KWs... Any thoughts? I do use mine as DD as well.

I actually saw your car this sunday in toledo. You beat me by 3 1/2 seconds or something...ha

Additionally, I am a novice so I'm not too concerned with times quite yet since you obviously know what you're doing lol. Idk how much you like to give away your secrets, but I was wondering if I could get some advice for where to go with my build. Trust me, I know I need to become a better driver first, but I am always upgrading my car because it's what I love to do. I just wanna make sure I'm headed in the right direction at least.

Great series though. Definitely following.
Monday, July 06, 2015 6:07 PM
Mr Shroeder, when you estimated the rear sway bar rate at the wheel as being 50% of your calculated rate, this was based on the link being attached half way out on the control arm (lever arm). Consider the bar bolted down with the bushing/brackets on a test bench, pinned at one end and loaded at the opposite end. This is the confiuration of your calculated "bench rate". Say it tested at 400 lb/in. Now mount the actual lower control arm at the load end, with the endlink attached half way out. It only takes 1/2 the force at the ball joint to load the bar end at 400 lbs, with 1 " deflection there. But the ball joint has moved up 2", so the rate at the wheel is 200 lbs / 2" = 100 lb/in. This is the Motion Ratio (MR) effect. The rate at the wheel is the bar end rate x MR^2 .

But this calculated bar rate assumes a bench test condition (Fred Puhn developed that formula), which reflects just one wheel rising up alone, as if that wheel ran over a brick. When you talk about a steady cornering condition, the bar bench rate must be doubled. If it just took 400 lbs to raise one end 1", it would take an additional 400 lbs to push the other "pinned" end down 1", so you get 800 lb/in in roll for the bar rate.

Then reality steps in. The bench formula assumes no deflection at the two support bushings. I was recently involved in a bench test of an oem front bar. Our measured rates were about 50% of the calculated rate. The error was due to flex in the poly bushings. When the span between the holes for the endlinks approaches twice the span between the mounted bushing centers, the bench rate will drop about 50%. I encourage you to do similar testing.

Thursday, July 16, 2015 12:04 PM
Apparently too wordy, so:
Well presented article, with some good tips, But ...

1) You were WRONG on page 5 when you stated this, regarding your calculated rear sway bar rate:
"Note: the effect at the wheels is much less than this, as the bar attaches to the suspension arms about mid way down their length, making the wheel rate approximately half that amount."
Ask Mike Kojima, the rate at the wheel = bar rate x MR ^2, so it's not "half", but a quarter od the bar rate in this case.

2) The bar rate you calculated was most likely based on some form of the equation by Fred Puhn, which is really assumes a bench test, with only one end moving up and down, like reacting to a brick in the road. For the cornering mode, this rate MUST be doubled. Check CircleTrack articles. Not sure if you did this.

3) If you really want to know the wheel rate for the bar, to be used with the spring rate at the wheel, you MUST consider the bar to frame bushing material, and the span between bushings, vs the distance between the two endlink holes used. The ratio of (bushing span)/(hole span) determines how much the poly bushings will soften the rate. For this rear bar, the ratio is ~close to one, so it's about a 10% decrease. If the ratio is around 50%, like up front, the rate for endlink loads up to 300 lbs is reduced by 50%. You will not find this in a text book, It is based on physical testing.

Per Schroeder
Per Schroederlink
Thursday, July 16, 2015 12:26 PM
Gotcha. Thanks for your feedback.
Sunday, August 28, 2016 11:36 AM
I enjoyed this piece and found it useful. As I've begun to consider building a 2015 Mazda 3 s Touring street/track toy, I'm wondering if you have any updates to report?
Friday, March 02, 2018 12:17 PM
I too am curious how the car is faring. I have just purchased a 2018 model and was eyeballing the suspension combo you specced out. I’d be very interested to hear how the shocks have held up to the increased spring rate....and any further insights for that matter.
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