Z10 Radius Arms
  -joe_r,   6-15-01

Soon to be a common hybrid setup:

EG hatch
ITR cams
Skunk2 manifold
chipped P28
AGX with GCs 400F/300R
Z10 Radius Arms
GSR rear disks
15" Aguzze with 195/50/15

Seems normal enough…. But wait, Z10 Radius Arms? WTF are those? And who is Z10?

Z10 is a new company out of New Jersey releasing some very interesting parts for Hondas. Hard core race parts, with a heavy emphasis on form-follows-function design. There is a complete and total lack of style parts in their catalog. It would be pretty easy for them to create a billet oil cap or shifter knob, especially with 4 CNC machines at their disposal. So it's not everyday we find good quality parts designed by racers for racers.

But what are radius arms?

Take a look:


In the most simple view, the Z10 Radius arms connect the lower control arms to the front subframe. The effect is total elimination of wheel hop by preventing the lower control arms from deflecting under the load of acceleration, even on stock worn out bushings. How effective? More effective than any upgraded bushings you could buy or make. In fact, at the time of this writing the fastest Z10 equipped car is an EK hatch that runs 9.7s on a built turbo b20/Vtec power plant.

Okay so they work on a drag car, but what about my street/strip warrior?

The radius arms pictured here fit EGs, EKs, and DCs. EF/DA units are almost ready. I have been rolling a set on my EG for more than 3500+ miles. The radius arms (the silver bars) are adjustable, so you can dial in a little or a lot of preload. This allows them to work with all the different tire/shock/spring/bushing set ups out there. However, just to get familiar with them, I initially installed them with no preload. I wanted to see if they affected the up and down motion of the car, especially over uneven bumps, driveways, and at speed around corners. I was happy to feel nothing during normal driving. Absolutely no difference in the ride, handling or steering. It was as if they were not there. However, my ears let me know, because with no preload there was always a mild rattling during clutch work and on low speed bumpy roads. I added some more aggressive quick starts, and immediately learned something about my car. Apparently my lower control arms deflect away from the front subframe with even just a mildly aggressive take off. I hesitate to even call it a launch. Normal, I-am-kinda-in-a-hurry driving. When driving like this, I could feel and hear the bars tighten and slacken. That was interesting. I suddenly had a much better understanding of why the EG and EK chassis can suffer from wheel hop. With no preload, the tightening and slackening of the bars was kind of like an accidental mechanical telemetry setup up that gave me audio feed back and a slight feel through the steering wheel whenever my take off was aggressive enough to flex the suspension.

For the next stage of my testing, I tightened both arms 1.5 turns and drove around at slow speeds. At sub 40mph speeds and with a calm driving style they were still simply transparent. It was immediately noticeable that the rattling was gone. I headed towards the freeway. As I merged on, I instinctively gripped the wheel anticipating the usually catch-a-groove-and-track-it trick as I crossed the rough asphalt to concrete transition. There was a tiny twitch but that was it. Perking my interest, I shoved my right foot down harder and crossed another lane accelerating to the flow of traffic 70 mph. In my mind I was thinking OMG, I can feel it. The Momo was rock solid on my hands. I smoothly flicked the wheel left and right in my lane. The effect was immediate and accurate, which certainly was not what I was used to! I never really thought of my bushings as shot or soft, but my perspective instantly changed! I couldn't help but think BMW. Curious, I pushed up to 90. Now I said "Damn" outloud. I wasn't imagining it. I didn't realize how much improvement was possible in my EG. It was as if the front end had gained 500 lbs of rock solid stability. The tendency for the little bumps and patches in the freeway to knock my car off track was greatly reduced. I was experiencing 2 finger stability at 90+ on notoriously rough LA concrete. Later during one of many 350+ mile LA to San Jose trips, I sampled this stability at speeds 10 to 15 mph faster with exactly the same feeling of confidence. Mark me down for 100% convinced.

So then it was time to test the wheel hop elimination. I had some great fun around the abandoned industrial parks in my area. I tried slips and dumps at a variety of rpms. The car felt good, but I could generate a tiny amount of hop. Nothing serious, and certainly less than before.

So I added another 1.5 turns of preload to 3 total. The result? EG hooks up gooooood. Sweet, straight line dumps with instant heavy 2nd gear spin and a solid third gear chirp. And I swear there is less torque steer.

How hard is it to install?

Easy as pie. Easier than changing your shocks. So easy I am not even going to bother with a detailed explanation. Unbolt toe hooks, bolt up Z10 bar, remove lower shock bolt, slide C shaped brackets over bolt, put bolt back on (give it some extra torque, mine slipped when I gave it Helm spec torque), and then attach the radius arms using the supplied hardware. Then comes the only part that might cause you to hesitate. The directions require a hole to be drilled through the subframe to secure the center tang. You can get away temporarily with using the existing threaded hole, but when the tiny 10mm bolt breaks you will cuss and swear and wish you had done it right the first time. Anyone one want to guess why I know this? (#&$@@ *&#$@%) So don't be a wuss and grab your Makita.


One easy simple hole. No whining!

Some install tricks from my experience. The cross bar is very strong. One of my tow hooks was bent from a serious off track excursion. Thus the cross bar did not line up perfectly. I left the bolts loose and just forced it into place. Viola. But then the center hole did not line up perfect. Easy solution was to ignore it, and install the radius arms. Then by adjusting the radius arms, I was able to get the holes lined up. Yes, the stock bushing allow THAT much movement.


Line up the center mount hole.

Once installed, adjusting them is as easy as twisting the radius arms by hand and tightening the lock nuts. I was able to just get 3 turns by hand. Z10 has machined flats in one end of the radius arm that makes adding more turns effortless with a box or crescent wrench. According to Z10, by adjusting the bars individually, you can adjust for any tendency to pull to one side you might have at the end of the strip. With high 130 mph trap speeds, the 9s EK found this to be a very attractive feature.


Turn the arms to adjust preload.

So any downsides?

The lowest part of the radius arm setup is same as the lowest part of your tow hooks. So if you do not scrape your tow hooks, you won't scrape these. On my car at full lock I can just touch the tire to the radius arm. With my fat 205s on skinny 6" rims its no big deal. When I was using some 195s on 7" rims, then i had to be more careful because I could get a light metal to metal scrape. But that is only at full lock. At 98% of full lock there is no contact. And Z10 reports that mine is the first car they know of that can touch the bars with the tires. WOOT, lucky me.

Also keep in mind that there is a slight toe change when adjusting. Z10 reports each turn is about .125 degrees toe out. I checked my toe with 3 turns and it looked within spec, but if you need more than 3-4 turns you should probably check your toe afterwards.

I'm not done testing them, but I am already really pleased. I'll take the EG to the drag strip soon, probably on some slicks and report back. But I am so pleased with these that I felt I had to get the word out. I like them that much.

Z10 Engineering
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