View Full Version : PN Tower Bar Caster Trick

2010.08.10, 04:48 AM
Like several others have noted down before, now that the MR-03 has a proper front suspension system, the need for more caster becomes apparent. I was previously running the full PN aluminum front end (tower bar + camber arms + lower arm) but somehow still wasn't getting exactly the results (corner speed, feel) I wanted, so tonight I tried something new:


I just reversed the tower bar. In doing so, you will lose the ability to run the rocker-arm springs (forcing you to stick to a coil-on-kingpin setup), but in return it seems that you can get what looks to be 4-5 glorious degrees of caster:


Having driven the new setup just a short while, I don't notice any reduction in steering response/turn-in. The overall amount of steering is up, and the car feels easier to drive and more forgiving than before. I have yet to do lap time comparisons. Currently my roll center is a tad lower than stock, I didn't play with it tonight (had no time). Static camber is about 0.5deg with cutoff stock MR-03 suspension arms; static toe is about 0.5deg toe-out. These came about by pure luck, but they're a pretty good place to start out with. Looking forward to see if anyone has done this "mod" and what they think of it. :)

Note for those wondering: the current "test track" is a dining hall floored in linoleum and coated with a fine layer of beer (woo college! :rolleyes:). Traction with foam tires is just a little less than RCP (actually, about the same as Inside Line Racing's RCP) on a good day. I will have my own DIY test track soon so I can remove the track conditions from the list of variables I have to watch out for.

2010.08.10, 05:00 AM
Nice find!
I have modded the arms to get roughly the same result on the stock front. The feel of the car is absolutely fantastic when running this much caster.

2010.08.10, 01:54 PM
Cool tip color0. Thanks for the heads-up on this. From your pics it looks really good!:D

Old Crow
2010.08.13, 10:20 AM
I have the PN Alum. arms and carefully filed down the plastic top part of the chassis, where the pin for the arms goes in at the front and created 2.7 caster(up from 1.8 caster)by being able to add another shim for a total of 3. There is enough room and does not create any suspension or fitment issues that I've been able to see. You could possibly go one more shim, there appears to be room, have not tried it yet.

Good idea though, turning the bar around for those with the Reflex setup.

2010.08.13, 12:01 PM
Old Crow, issit possible to post a picture of your front end setup?

I have the PN Alum. arms and carefully filed down the plastic top part of the chassis, where the pin for the arms goes in at the front and created 2.7 caster(up from 1.8 caster)by being able to add another shim for a total of 3. There is enough room and does not create any suspension or fitment issues that I've been able to see. You could possibly go one more shim, there appears to be room, have not tried it yet.

Good idea though, turning the bar around for those with the Reflex setup.

Old Crow
2010.08.13, 12:19 PM
I have my car completely disassembled at the moment, it might be some days before it's together again, sorry.

I have the PN Aluminum knuckles, Arms, lower Alum. setup and spring holder plate. The only parts that are stock in my front end are the toe/steering bar(plastic 0deg.) and the top that covers the servo area, that's the part I modified, where the pins for the arms go through. So if I could post a picture you would only see that there are three washers. It's really quite easy, you only need to file 0.2mm off it. Hey, you could do the same thing with the PN Alum. upper bar, if you had it. You should be able to go one more washer(4 washers, 3.6 total caster) if wanted, haven't tried it yet though.

2010.08.13, 05:45 PM
The PN bar does have less material though, I'd be wary of filing down even .2mm from the front... also, if you file it too much the mounting point for the arm might get too short and the arm will be sloppy (not just sliding back and forth, but also bending back and forth in the pivot).

Yeah, i'm not sure how to get between 2-3 degrees of caster with this setup (not that I personally need it, I'm still loving the feel of 4-5). I don't think you can push the rocker arm springs too far though, eventually the arms will start scraping the front of the servo casing.

2010.08.13, 08:25 PM
Hi there
I tried to do this with the reversed kingpin solution and will testdrive it this weekend.

Then some video:

2010.08.17, 08:10 AM
I know this has been tried by a couple people, but can anyone verify the exact caster in degrees? Any math majors out there who can determine this accurately for us?:)

I know color0 says it seems to be 4-5 degrees but I can't help but think that after looking at it some more it might be more than that like on the order of 5-10 degrees caster... Of course I'm just guessing, but I am thinking that maybe it would be best if we could get the exact caster amount in degrees if someone could do the math on it... Since steering is so important:D

Definitely a cool find color0, not knocking this at all my man.:D Just wondering if anyone knows the exact amount.:)

2010.08.17, 10:50 AM
the only question i have with this set up is if the knuckle pin binds in the steering linkage?it looks to me that the agle of the knuckle pin is to great for the straight up and down hole of the steering link...

2010.08.17, 03:58 PM
I agree with Felix! I think that is a lot more caster than the predicted... I would guess even more than 10... Do you have the exact distance that the king pin holes goes backwards inverting the arm holder?

2010.08.17, 04:58 PM
The arm mounting location gets shifted back about 1mm by reversing the bar, which is why I guessed about 5 degrees (for MR-02, 0.25mm change was roughly equal to 1 degree of camber or caster). I overlaid a protractor onto the original image, it looks actually to hit just above 5 degrees. Of course this isn't entirely accurate, I can't guarantee the lines are perfect, etc., but it's definitely not 10 degrees. ;)


2010.08.18, 06:52 AM
wow... sorry for the wrong shoot... The image is very clear!

If someone needs more caster (I don't think someone will...), it can be mixed with the PN Aluminum arms with more 1,8...

But I'm very tempted to try this trick with my 03.

2010.08.18, 04:24 PM
Definitely try it out, Doug! I'm playing around on a tile floor the last couple of days but I can get my 03 to drift fairly controllably without gyro assistance -- the caster makes the car very easy to drive! Of course I'm also curious what the laptimes on RCP will be (:o) but that has to wait till I can take some time off from work. :)

2010.08.18, 09:35 PM
color01... great tip and great shot (re: protractor overlay)... now if i can only get my hands on one... :D

2010.08.19, 09:24 PM
color0 - Thanks for the excellent pic of the protractor overlaying the MR03's FS. But...;)... I can't help but think, and you said it also, that this isn't 100% accurate. The pic isn't at a dead right angle with the chassis, so would you maybe think that the actual caster gain could be anywhere from 5-8 degrees? I am no expert so that is why I am not adamant about my guess being correct:o. I am just wondering, that if the actual caster-gain for this mod is for arguments-sake on the order of 7-8 degrees, would that be too much caster? I know that as of recent more caster has been on everyone's modification list.:) I can totally see how 4 degrees can be good for high-speed, large tracks. But is there a point of negative returns, and if so, what would you and others say the max-cutoff for caster gain should be?

Thanks again for all the info.:D

2010.08.20, 02:39 AM
I would argue that this caster mod cannot go over 6 degrees -- I trust Cristian's measurements on the new Reflex front end, and that tower bar goes between 2 and 4 degrees by reversing the pieces. The PN tower bar, which retains stock geometry, therefore must go between 0 and 6. However, because of the geometry of the suspension, the farther back you move the suspension arm, the less caster you actually gain per mm of relocation. So the upper limit of caster with this mod, assuming the Reflex front end provides accurate numbers should be 6 degrees, and probably less. :)

As for the theoretical discussion, too much caster would definitely be a bad thing. The way the physics works, Mini-Z's don't roll nearly as much as larger cars, so slow turns don't really cause much chassis roll. This means that too much caster would create too much camber when the wheels are turned, and you'll lose grip unnecessarily. Most Mini-Z tracks I know of have several slow turns and one or two high-speed ones; only the high-speed turns should create enough body roll to necessitate anything like 7-8 degrees of caster, and the car would suffer through the low-speed technical section where you need less caster for the best turning response and maximal tire contact patch. I don't have a whole lot of larger-scale experience so perhaps other racers could help me out here.

In any case: if, when you test the car, it gains speed in sweepers and loses speed in technical sections, then we know that it's too much caster. It's not entirely that simple, but almost. :)

2010.08.20, 10:43 AM
Love the explanation, Brian... and agree with the calculation too!
I'm tempted to try out the Reflex top with the PN inverted kinkg pin front...

2010.08.21, 02:43 AM
color0 - Thanks for the break-down of everything for me. I will have to try this reverse-PN caster bar trick and see how it does. Thanks again, always like reading your technical info.

2010.08.29, 05:43 PM
Alright, so I finally got a chance to test out the increased caster at Kenon. To sum it up in one sentence, I'd say that this tower bar arrangement works, however, only for very fast, flowing tracks and definitely not for technical sections.

Numerical results: I tied Friday's 70t lap record of 8.3xx despite being short on power (old batts).

Subjective results: For racing, I don't like it. The 5-6 degrees of caster gave the car entirely too much steering at high speeds, forcing me to either watch my steering angle very carefully or craft a rear spoiler for my 911. I, being lazy, stuck with the former option. I also feel that the high caster gave up some entry steering in the infield. I could not turn into corners hard enough without lifting completely off the throttle, which is NOT the usual case for me -- the way I drive, corner speed comes from keeping the revs up despite slowing and turning in. On top of that, corner exits became scary, the car would slip sideways a few degrees almost without fail if I drove over a certain tile on a certain turn. In conclusion, too little entry steering, too much exit steering: sounds like too much caster for me.

Long story: Went to Kenon with Philip at 12 noon, started driving at about 1PM. About an hour into the testing day (~2:00PM), I noticed that my front suspension was binding like crazy! Turns out that it was a combination of worn-out Reflex kingpins and the stock Kyosho arms not playing nice at all with the PN caster bar (keep that in mind if you have this setup on!). I was driving around with this setup still, though, hitting 8.6, 8.7xx laptimes and getting one lucky 8.5. Mid-day I swapped out the Reflex kingpins and MR-02 springs for PN long kingpins and AWD Pro II springs (very short stack height). This helped a bit, I started to hit the 8.5's more often but my absolute fastest lap was no faster.

The next change after that was rear spring rate, I had it set too soft so I wasn't putting down [my rather meager] power. I use the PN dual-spring shock, so I swapped from Red main spring to Green to Purple and finally settled at White. Again, no faster fast lap, but more consistency.

I got curious about the 911 vs F430, so I threw on the F430 for a bit and threw down 8.6xx as the absolute fastest lap; so for me, the 911 is faster. However, the F430 is worlds better than the 911 off the walls, it just glances off rather than dig itself in like the 911 prefers to do. So my laptimes with the F430 were actually more consistent, if 1-2 tenths slower per lap. I also did not experience any traction roll with the F430 that day, despite the grip levels coming way up. Chalk it up to a raised roll center and limited suspension travel due to A-arm binding. :D

After switching back to the 911, at some point in time I just didn't have rear grip anymore on my decimated rear PN 8's, so I mounted a scrubbed-in set on some +3W wheels. BAM, a couple laps later I dropped an 8.3, and a couple laps later I repeated, and then so long as I didn't touch the wall, I got 8.4xx and 8.5xx with pretty good regularity.

By 9PM the car started to chatter, and my fast laps stayed at 8.5, 8.6 no matter how clean I drove. I took the car in and realized that I had literally run all of the grease out of my tri-shock setup despite a rebuild the day before. Oh well, I got 8 straight hours of driving out of the car before it happened, and it was getting late anyway, so I said "screw it" and continued to flog my chattering MR03. :D At this point the car was feeling pretty crappy, my back and legs were feeling pretty crappy, but I was just didn't feel like quitting, lol... I ended the day running over 1400 laps on the counter and yeah, I'm pretty certain I have too much caster. :o

Ideally, what should come out of this is that I'd love for Philip to release a 2deg caster tower bar that I can flip to get 4deg, but we'll see. :) I think somewhere between 2 and 4 degrees caster should be perfect for my driving style and the PN layout, so for now I'll go back to the ~2 degrees afforded by the PN aluminum A-arms and see how that fares.

I stole the Kyosho MR02 kingpins from my MRCG and stuck them through the PN Pro Lower Arm for MR03 -- this arrangement is as smooth as the reverse-kingpin setup but I prefer the coil-over spring setup for ease of adjustment. Playing with caster will have to wait till PN makes a new tower bar or I pick up a Reflex one. :)


2010.08.31, 01:38 PM
Hi Brian, Great info. Are you running your 911 at 94? Also, were your really runing +3s on the rear?

2010.08.31, 04:21 PM
Yes and yes. I would run the 911 at 90mm but the PN 90MM motor mount isn't quite low enough for a high-grip track like Kenon. As for the +3W rims -- hey, the lap times weren't hurt. :) I usually try out different tires or different wheels/offsets before touching my suspension setup, just a habit.

I plan to go again to Kenon this coming Friday, but with the F430 replacing the 911 and new wheels (+1N/+1.5W). I'm so out of practice, I need to get off the walls faster. :o

2010.08.31, 07:40 PM
Brian - as far as the 02 kingpin in the PN arm....how do you like it? I had it that way awhile ago but never tried it. I guess i thought the weight of kingpin (since its not clipped at bottom), would be a little heavy?...if you know what im trying to say.

2010.09.01, 12:53 AM
As long as you don't run excessive amounts of front droop, it will be okay -- the kingpins are sprung after all so they won't fall through the lower arm. :) I start off my setups with zero front droop, counting body and battery weight... haven't had a problem yet (shakedown/bashing in the dorms). I have considered gluing the kingpins to the Delrin balls, but I'd like to be able to non-violently remove the balls for maintenance/testing on other lower arms (stock Kyosho, for instance).