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Building the R-Geo "GVP" chassis


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#1 JimF

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Posted 27 October 2013 - 11:03 PM

I just received this from Rick at R-Geo to build the justly famous "GVP" chassis. In a case like this, with a proven winning design, I'm not going to reinvent the wheel (or the chassis). This won't be an exact copy; it will be an attempt to remain faithful to the race winning design of the original. Any deviations will be in the interest of strengthening.

 

Because of my work schedule, I couldn't finish this tonight. I'm going to break this into two segments for the complete job. I promise that I won't drag this out over the next few weeks showing one solder joint a day or somesuch. Off we go...

 

Here's what you get to start with. All brass is .032", bracket is .050". This is a nice builder's kit as opposed to some of the "Lego" kits out there.

 

e7f9c67b-e326-4df4-8874-35cde5f871cb_zps

 

First step is to square and prep the bracket. This is basic stuff. The bracket face is filed until flat then the legs are bent square to the bracket face. The bracket is cut out slightly to accommodate the tuning fork rails which will run along the inside of the bracket. To the side are two bits of 3/32" square tubing about 3/4" long. These are soldered together and then dressed flush and square. These will be the fork hinge tubes.

 

2f443a7f-a98f-41f5-bd06-91277e6af1bf_zps

 

Here is the nosepiece with the hinge tubes. Lines on the ears of the nosepiece show they need to be sanded square or at 90* to the chassis centerline.

 

b022586b-d483-450c-a121-976f675d2982_zps

 

Nosepiece squared up and the guide tongue reinforcer is notched to clear the hinge tubes.

 

60ea4b34-73a4-4a5a-aa35-44938302546d_zps

 

Here's the nosepiece with all parts tacked in place but not yet fully soldered.

 

d132e727-13c7-43ac-b51e-342b5f7175b7_zps

 

The nosepiece is dropped into your jig along with the motor bracket. The paper template underneath will be your guide for bending, placing, and adjusting the forks. The first bend in the fork will be 1" forward of the bracket face. Both bends in the forks will be 45*. Neither the distance forward from the bracket nor the angles are critical.

 

cfcd63ed-591b-40e2-b4b5-154fded178cf_zps

 

Take two pieces of .062 piano wire about 5" long, and place a sharp mark at about the 2.100" point. Then measure .495" forward of the first mark and make a second. These will be your two bend points.

 

bd90f10a-456a-4f0f-afaf-c0fcd22da62d_zps

 

Make your first 45* bend. A little hand drawn template is handy here to check your angle.

 

e228a210-3141-4c92-b4d6-650c74cd8c6a_zps

 

Make your second 45* at the forward mark you made on the wire. Again, the template comes in handy to check that the fore and aft legs are as close as possible to parallel. You'll be surprised how accurately you can judge this by eye. After your bends are complete, your angles matched as close as possible, and the legs are parallel, slap this piece on a block and bend the legs until the piece lays flat. Then go back to the template and check the angles and the parallel again Repeat this process for the second fork rail.

 

4b29f5eb-44a4-4e81-94f5-ab4db7087bb3_zps

 

When both fork rails are bent, tweaked, flattened, and aligned, slip the forward ends into the hinge tubes and the aft legs inside the bracket legs and see how you did. The template paper will show you how well you hit the angles necessary and how well matched they are.

 

In this case, my right rail was almost perfect, but my left was off a little at the two points indicated.

 

f58aeafd-1c60-4bc3-b4f8-b9dc72d0f8a1_zps

 

A little corrective bending got the left rail a lot closer as seen here. At this point, I could have just kept on bending and tweaking that rail but I decided to just bend another. The next one was spot on so I called it good and moved on.

 

eeabed54-6ece-4d5b-9eea-87abf91448f5_zps

 

That's the end of the first segment. Next up are the main frame rails, pans, some reinforcement, and finishing up.


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Jim Fowler




#2 SlotStox#53

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Posted 28 October 2013 - 12:31 PM

Very nice kit and a very good start to another informative and comprehensive build *how to* thread.

 

Good work, Jim.  :good: :)



#3 kvanpelt

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Posted 28 October 2013 - 01:50 PM

What would those "Lego kits" be?


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#4 James Grandi

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Posted 28 October 2013 - 02:07 PM

One question; is it a necessity to use square tubing for the fork rails to fit into, or would round tubing be fine?
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#5 Ramcatlarry

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Posted 28 October 2013 - 02:57 PM

You have a stronger solder joint with the square tubing and less corrosion/sticking on the inside of the tubing as well.


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#6 Greg VanPeenen

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Posted 28 October 2013 - 03:00 PM

James,

 

Not my thread but I will answer I have built many, many tuning fork chassis with the front hinge Some of the early ones with round tubing but most with square . Square gives the tuning fork rails a little extra wiggle room and that seems to work better for me. 


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#7 Danny Zona

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Posted 28 October 2013 - 03:16 PM

I'm watching this build like a hawk. My kits should be in next week then I'll give it a try.

 

Thanks for the how-to thread. I need it. LOLs.


Test, test, test and go test some more.
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#8 Dennis David

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Posted 28 October 2013 - 03:49 PM

Silly question, is the fork just slipped into the rear bracket or is it soldered?
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#9 JimF

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Posted 28 October 2013 - 04:21 PM

Just checking in at lunch (from work). Thanks to all for the comments and esp. to Greg (it IS his design after all.

 

For Dennis... yes, the rear legs of the forks are soldered solid to the bracket.

 

For James, I've done a lot of "forkers" too and while I don't think it matters tremendously, I like the square for this application because it's easier to work with. The rest should wrap up tonight. I've done a few things differently as an experiment but those can be reversed later after testing.

 

I may be a little tardy in finishing this up because I've got this to deal with the parking lot at my apartment. Good for business when you run a ski shop for your daily bread and butter.

 

5509cd4a-e9d2-48cc-b31a-3139fb07e69e_zps


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#10 bbr

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Posted 28 October 2013 - 04:30 PM

Ski! I need some good ski.
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#11 James Grandi

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Posted 28 October 2013 - 04:32 PM

Thanks for the answers, guys. I have one of these R-Geo kits waiting for me to build it, and I'll be watching this thread closely.

At the same time I'm actually about halfway through hacking the hell out of an original JK kit and building one that way. With all the cutting/filing it's taken to do, it makes one appreciate a well-made chassis kit even more.
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#12 JimF

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Posted 28 October 2013 - 04:40 PM

James:
 
The original JK kit is a great starting point for a lot of projects. It is just a basically good design that one can convert into almost anything. I use the JK kit a LOT as the starting point for experimental stuff that I do. This new kit from Rick just simplifies this particular design to a point that, if this is what you want to build, it's kinda silly to go elsewhere for your starting point.

In the second segment (tonight, I hope) I'll point out some slight differences and choices that the individual builder can make with this kit as the starting point.
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#13 MantaRay

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Posted 28 October 2013 - 06:15 PM

Why did the nosepiece ears need to be "trimmed"... 1/16" removed?
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#14 Half Fast

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Posted 28 October 2013 - 08:16 PM

How was the guide reinforcer notched? It looks like that would be a difficult cut in steel.
 
Cheers.

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#15 JimF

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Posted 28 October 2013 - 08:49 PM

OK... Just got home from work. Have more pics to take and bit more work but should finish tonight.
 

How was the guide reinforcer notched? It looks like that would be a difficult cut in steel.

 

why did the nose piece ears need to be "trimmed"... 1/16" removed?

 
Bill: The GTR was notched with a Dremel disc and finished out with a file. Pretty easy actually as the steel is not super hard. I held 'er in a vise grip and buzzed away at it... didn't even shatter a disc.
 
Ray: Didn't actually need to be trimmed but this bit of brass with all the cuts and shapes is pretty flexible and in the stamping/shearing process there was a bit of flat plane crescent shape to it. You can see in the top view that the front surfaces of the ears still have a bit of forward arc to them. Squaring the rear face up on the disc sander enabled me to use that face as a reference for squaring up the centerline of the tubes.
 
Not necessary... looks better.
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#16 Dennis David

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Posted 28 October 2013 - 09:59 PM

One more question. What's the dimension between the front uprights?


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#17 JimF

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Posted 28 October 2013 - 11:12 PM

2.150".
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#18 JimF

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Posted 28 October 2013 - 11:53 PM

OK... wrapping it up.
 
Paper template out of the way and main rails tacked in place. These are .062" wire about 4.750" in length to start. They'll run wild at the back of the bracket and get trimmed later. Everything is checked for jig wheels flat on the deck, main rails flat... etc. Then we slide the bracket brace in, douse each joint in flux, turn on the fan, and get serious about all the solder joints.
 
b501aa38-8411-4431-9789-2bf739ab79af_zps
 
Out of the jig and wash off the flux residue. Then onto the granite "flat block" to check for flat and examine each joint.
 
267efa0a-8f3e-48a8-9a23-8902f77c245e_zps
 
An additional bit of .055" wire goes on top of the main rail to increase the solder grip of the main rails to the bracket.
 
615a39e2-b03d-467e-aaac-cbed10d426eb_zps
 
Tubing bits in place for hinges and stops. Hinge tubes are 3/32" round about 3/4" long, up/down stop is 3/32" square about 1/4" long. Front face of the stop tubes will be beveled at about 45° to clear the front tires.
 
104d92ad-bfb8-4ffd-a215-c32517af3bea_zps
 
Setting up the hinges and stops. The hinge rods are .062" and the front stops are .055". All have a dogleg bent into the rod as indicated by the red arrows. There is also a small relief cut (blue arrows) into the pans to help in the hinge rod laying flat on the pan.
 
I'm going off the reservation a little bit here by running the pan windows to the rear. Two reasons for this: (1) the placement of the front pin is farther aft than my normal as is. If I reverse the pans, it'll be 3/8" further back yet. (2) As light as this nosepiece is, I'm considering the fore/aft weight balance. At the end, I checked it and I guessed right.
 
dc0e439e-bcf4-4781-b225-a2f05af4cc0a_zps
 
Here is a brace for the nosepiece/front axle carrier. One piece of .055" wire per side
 
b1932d88-9803-40aa-85fc-74a6ad283353_zps
 
Last things here. Two bits of .047" to reinforce the ears (blue arrows) and body pins. Front is a "floater" back tubes are fixed.
 
b9221892-32a2-4a68-93e6-ac8dfac8f56c_zps
 
She's done! Details...
 
Wheelbase = 4.0".
Guide lead = .850".
Weight as shown here = 107.4 grams with body.
Fore/aft balance point is 2.010" forward of the rear axle (.050" to .070" more rear bias than usual for me).
 
5bdb060a-90e2-4907-a7d9-8f30cf98d732_zps
 
This was an easy build as all the parts were cut correctly and there was very little straightening or other prep work to do. I know the design is a proven one so I'm excited to spin some laps. There are few things that may get experimented with over the course of time. One might be reversing the pans for even more rear weight bias. Another might be using .047" wire for the pan stops (gaining more pan movement) This is a pretty short guide lead for me so that'll be on the stove as well. For sure, it'll get tuned as is without changes to the build.
 
Edit to add... Forgot to mention... other versions of this frame have used a "limiter" in this area to limit twist flex. I may do this too but it will be very easy to add/remove on the first track day.
 
be2e3b9a-9fb3-47d5-9413-9f063b75ea14_zps
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#19 Danny Zona

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Posted 29 October 2013 - 08:06 AM

Sweet build. I love how you explained and showed your step-by-step process in building the car.
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Test, test, test and go test some more.
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#20 JimF

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Posted 29 October 2013 - 01:08 PM

Danny:
 
Thanks, I'm glad that you may have gleaned something even though this was not really directed at someone at your level. Rick has mentioned that things that just come without thought to some of us are a complete mystery to others. Thus, something that's a simple and intuitive kit to me may look like one of those 1,000 piece jigsaws of the Eiffel Tower to someone else. Hence these kit tutorials that again, are not really aimed at the accomplished builders out there.
 
Another important reason for my reply is to point out where some credit is due. I'd personally like to thank you for this tool which is nearly indispensable to me in my building efforts.
 
97cf7071-a6fb-4885-9758-339eccb8af1c_zps
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#21 Rick

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Posted 29 October 2013 - 01:48 PM

You summed it up well, there are those that this will help a lot for building their kit, and even varying on to their own ideas, but the basics are laid out there in easy steps to follow. Jim did this as a special favor for me, it is snowing and he makes his living in a ski shop and is slammed with his own work now.

Indebted to your amazing building skills. Thank you very much!!!...
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#22 Tex

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Posted 29 October 2013 - 02:08 PM

I enjoyed the "how-to" also. While I'm no stranger to scratchbuilding, I really struggle to see the different movements built into a chassis just by looking at a picture; holding one in my hands is worth a thousand words. But Jim's write-up and pics make it easy to understand.
 
OK, I do have one question. Would getting the "limiter" installed be as simple as unsoldering just those two mainrail joints on the nosepiece, slipping the limiter over the wishbone, and then soldering those mainrails to the nosepiece again? Of course, to do so I realize that one would have to get the pan hinges, downstops, and wishbone inserted into the proper pieces of tubing first.
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#23 raisin27

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Posted 29 October 2013 - 02:09 PM

Interesting that you reversed the pans. I am in the process of building my second R-Geo GVP kit and I too am reversing the pans for the same reason as you did. I like my front body pin a bit further forward. If rear weight is needed I was thinking I could fill in the holes with a brass piece.
 
Very nice clean build.

Raisin
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#24 raisin27

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Posted 29 October 2013 - 02:16 PM

I enjoyed the "how-to" also. While I'm no stranger to scratchbuilding, I really struggle to see the different movements built into a chassis just by looking at a picture; holding one in my hands is worth a thousand words. But Jim's write-up and pics make it easy to understand.

OK, I do have one question. Would getting the "limiter" installed be as simple as unsoldering just those two mainrail joints on the nosepiece, slipping the limiter over the wishbone, and then soldering those mainrails to the nosepiece again? Of course, to do so I realize that one would have to get the pan hinges, downstops, and wishbone inserted into the proper pieces of tubing first.

 
Not sure if this is quite what your thinking, but I installed a "limiter" on my fork rails simply by soldering two pieces of tubing together and slightly crushing them so they fit tight over the rails. By sliding it for or aft it will change the "flex".
 
Raisin
 
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#25 JimF

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Posted 29 October 2013 - 02:26 PM

Tex:
 
Mike nailed it very well but I will probably not use that device myself. I will probably just use a bit of ~ .039" wire and solder it at the crux point. Not sure if that is necessary or even advisable but it's a quick and easy test that can be done and then reversed while in the midst of a test session.
Jim Fowler

#26 Greg VanPeenen

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Posted 29 October 2013 - 03:09 PM

Jim,
 
Although you have done a very nice build and I look forward to seeing your test results.

Your chassis has much less wiggle then the proven winners have. Also we are at 60/40 weight distribution, 60 in the rear and still adding weight to the rear and going faster. The .039" hinge wires running into the 2mm square tubing have much more room to move then what you have done  

We have also now moved to two rails of .055" for the main rails and 2mm and .039" for the pan stops. This is also working better then the prototypes that finished first through fifth in the Can-Am A Main at the Sano. The very same chassis went 1, 2, 3 in the Coupe class with added weight. The RTR cars came in at just over 100 grams, letting the racers place the weight where it would do the most good. 
 
I know that there is more then one way to skin a cat. Best of luck with your interpretation.
 
Regards,

GVP

#27 Dennis David

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Posted 29 October 2013 - 03:23 PM

Another reason why I need to make a trip to SCR so I can see these chassis in person.

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#28 JimF

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Posted 29 October 2013 - 04:07 PM

Greg:
 
Thanks so very much for those clarifications. Since I didn't know those details at the outset, I couldn't include them in this build. It will be very easy for anyone following this thread to insert your formula into these guidelines. I would certainly encourage anyone to do that if they chose. Keeping in mind that metric tubing is not all that readily available here, I'd note that simply stepping down each wire size by one increment in the fork rails, pan hinge rods, and the pan stop rods would accomplish similar results.
 
As for the 60/40 weight distribution idea... I think we may be using different criteria to compile our results. Mine were based upon the simple balance point between the axles. If I were to try to achieve 60/40 (R-F) based upon that criteria, then even an additional 15 grams at the rear of the pans would not get there. OTH, if your figure is based upon distribution from R axle to guide center, then your balance point is indeed .010" to the rear of mine which calculates at 58/42 using that criteria. That would make some sense given the small amount of weight in the windows and the small change in distribution caused by reversing the position of those windows.
 
Thanks a ton for your contribution to this thread.
Jim Fowler

#29 Dennis David

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Posted 29 October 2013 - 04:13 PM

If were to do this chassis I would need to go with smaller wire because of the shorter wishbone length.

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#30 JimF

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Posted 29 October 2013 - 04:14 PM

Another reason why I need to make a trip to SCR so I can see these chassis in person.

 
You will be able to see them much sooner via a visit to FTH for our next Retro meet there on Nov. 20.
 
Just sayin'...
Jim Fowler

#31 JimF

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Posted 29 October 2013 - 04:17 PM

If were to do this chassis I would need to go with smaller wire because of the.shorter wishbone length.

 
If you are talking 1/32 then yes... probably an accurate assumption. I have yet to build a 1/32 with tuning forks but for sure the 3" wheelbase cars are stiffer than 4" wheelbase cars using the same components.
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#32 Tex

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Posted 29 October 2013 - 04:27 PM

Not sure if this is quite what your thinking, but I installed a "limiter" on my fork rails simply by soldering two pieces of tubing together and slightly crushing them so they fit tight over the rails. By sliding it for or aft it will change the "flex"

 
Yes, this is what I was envisioning when I asked my question. But those had to be slipped over the wishbones BEFORE soldering it all together, right? I was trying to figure out how Jim would put those pieces of tubing over the wishbones AFTER having soldered the chassis together.
 

Mike nailed it very well but I will probably not use that device myself. I will probably just use a bit of ~ .039" wire and solder it at the crux point. Not sure if that is necessary or even advisable but it's a quick and easy test that can be done and then reversed while in the midst of a test session.

 
But, as you mention here, you had no intention of using the tubing "limiter" as shown in Raisin's pic. Your method is easy to do/undo after the initial build is complete. It's funny how the mind works; I'd SEEN the tube "limiters" on a number of chassis and naturally ASSUMED that yours would be the same. Talk about "limiting"... monkey see and monkey can ONLY do what he's seen... instead of thinking for himself (me!). LOL
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#33 JimF

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Posted 29 October 2013 - 05:20 PM

Yes, this is what I was envisioning when I asked my question. But those had to be slipped over the wishbones BEFORE soldering it all together, right? I was trying to figure out how Jim would put those pieces of tubing over the wishbones AFTER having soldered the chassis together.
 
But, as you mention here, you had no intention of using the tubing "limiter" as shown in Raisin's pic. Your method is easy to do/undo after the initial build is complete. It's funny how the mind works; I'd SEEN the tube "limiters" on a number of chassis' and naturally ASSUMED that yours would be the same. Talk about "limiting"... monkey see and monkey can ONLY do what he's seen... instead of thinking for himself (me!). LOL

 
Welll... I do have a way to do it but it involves a light saber and is very top secret. I suppose I could tell ya but then you'd know too much and of course then I'd hafta... well... you know.
 
But IAC... you figgered it out anyway. The dual tube limiter is way slick but it's... uhhhhh... 'limiting' in the sense of removing it totally if desired. I've probably done about two dozen iterations of the tuning fork chassis over the past six months and tried limiters on many of them. I'm not sure that the effect was all that notable. I've left it on in some cases but in most cases, I've taken it off. Each chassis is different in its response though and a simple "solder it on and then take it off" limiter has worked pretty well... so far.
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#34 MantaRay

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Posted 29 October 2013 - 05:25 PM

I think Bryan Warmak has been doing this sort of limiter for quite a while... just from race report or two...

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#35 Joe Mig

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Posted 29 October 2013 - 08:27 PM

Not sure if this is quite what your thinking, but I installed a "limiter" on my fork rails simply by soldering two pieces of tubing together and slightly crushing them so they fit tight over the rails. By sliding it for or aft it will change the "flex"


Is this within the rules?

3j. The bottom surface of the whole chassis (including the motor, but excluding the motor seal and guide flag) must be flat and straight in all directions, with no bowing or drooping of any parts below the plane defined by the front and rear clearance specifications. This will be checked by applying a straight edge to the underside of the car both across the frame and along the length of the frame.


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#36 Cap Henry

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Posted 29 October 2013 - 08:38 PM

As long as it doesn't hang below the chassis, yeah.

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#37 Tex

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Posted 29 October 2013 - 09:29 PM

Since the wishbone(s) generally extend into some tubing up front, that effectively raises the wishbones off the deck... at least up front. If the limiter is resting on the jig along with the motor bracket and nosepiece, the limiter shouldn't extend below the plane from the nosepiece to the motor bracket.


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#38 JimF

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Posted 30 October 2013 - 01:31 AM

Yup!
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#39 raisin27

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Posted 30 October 2013 - 05:12 AM

Is this within the rules?

 
I can only speak for my own build of course... When the fork rails are soldered in the limiter is already installed so the rail runs paralell to the jig slightly above the bottom plane of the chassis. Nothing droops below the "plane defined by the front and rear clearance specifications".
 
Raisin
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#40 Noose

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Posted 30 October 2013 - 07:03 AM

Correct. That limiter is part of the frame. Just like the tubes if rails go into them.

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#41 gas

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Posted 30 October 2013 - 07:57 AM

Does it matter what the angle of the tuning fork is? The SoCal guys are using 90 degree bends, most everyone else is 45 degrees.


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#42 JimF

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Posted 30 October 2013 - 09:53 AM

IMO, no it does not. I have built maybe two dozen different ones and there does not seem to be any differences that are traceable to that.


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Posted 30 October 2013 - 12:24 PM

In my opinion the sliding "limiter" will react much differently than the solid solder in one... When Jim gets the chance to do his on track testing, that will be interesting. He R&Ds better than anyone I have seen. With his realtime job being so busy right now, we'll just have to wait until time opens up for that segment of the build...
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#44 Greg VanPeenen

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Posted 02 November 2013 - 04:01 PM

Greg:
 
Thanks so very much for those clarifications. Since I didn't know those details at the outset, I couldn't include them in this build. It will be very easy for anyone following this thread to insert your formula into these guidelines. I would certainly encourage anyone to do that if they chose. Keeping in mind that metric tubing is not all that readily available here, I'd note that simply stepping down each wire size by one increment in the fork rails, pan hinge rods, and the pan stop rods would accomplish similar results.
 
As for the 60/40 weight distribution idea... I think we may be using different criteria to compile our results. Mine were based upon the simple balance point between the axles. If I were to try to achieve 60/40 (R-F) based upon that criteria, then even an additional 15 grams at the rear of the pans would not get there. OTH, if your figure is based upon distribution from R axle to guide center, then your balance point is indeed .010" to the rear of mine which calculates at 58/42 using that criteria. That would make some sense given the small amount of weight in the windows and the small change in distribution caused by reversing the position of those windows.
 
Thanks a ton for your contribution to this thread.

Jim, 

 

My builds carry no extra weight in the front like yours. I have built a few of these that are under 100 grams RTR. I use wheel scales to check my weight distribution just like racing the real thing. A car weighted at 100 grams, lead added to the very back of the pans to make 100, will go 40 grams plus or minus tenths of a gram front and 60 grams plus or minus tenths at the rear. These cars even have an extra hole drilled through the guide reinforcement  and through the guide tongue behind the guide hole and holes drilled in the front axle uprights plus the tops of the front axle uprights are removed above the front axle. Every bit of weight I can remove is. Everything I learned in engineering, as an race engine builder, and racing real cars tells me that is 60 rear / 40 front. I also build lead into some of the chassis depending on what kind of track configuration we are running. Just like racing the real thing. :)   

 

Looking forward to the results of your testing. 

 

Regards.

Greg VanPeenen


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#45 James Wendel

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Posted 03 November 2013 - 01:18 AM

Greg - you make a strong case.  Let us see an example of your work.  A picture or more  :good:


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#46 Greg VanPeenen

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Posted 03 November 2013 - 10:55 AM

James,

 

Look for pictures of my work in the latest Sano race report. You will see pics of the top 3 in Can-Am, top five were the same chassis. You will also find pictures of the top 3 in the PD/Coupe class, Also check the Retro East race reports. I will not post detailed photos of my chassis. I work to hard to get the results I get and the chassis get copied fast enough as it is. :) 

 

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#47 JimF

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Posted 06 November 2013 - 01:18 AM

OK... got in some track testing today with the new car. Venue was Fast Track Hobbies in Rocklin, CA, and the tracks were the 165' high speed Purple Angel and the 125' flat MTT. Both tracks were in very good shape today. Most testing was on the Angel because I think the GVP is primarily targeted at speedway tracks. I used a very basic but successful single .078" 'loop rail' chassis for my baseline.
 
The Purple Angel is an almost unique track in that I think only two were ever built. Other high speed tracks such as King tracks or hillclimbs are a series of fairly conventional simple 90°, 180°, and 270° corners joined by straightaways of various lengths. The Angel is different in that the track has one very high bank followed by a main straight after that. Then the rest of the track is a more or less continuous chain of interconnected corners. While the speeds are very fast (faster than any King in NorCal), the cars are constantly changing direction through the infield. This track is challenging to build for in that "high bite" cars often don't work as well as some more basic stiffer designs. Target time today for a Can Am was 5.0 sec. I was using a fresh motor in the GVP that was pretty good.
 
GVP: General characteristics.
  • At a starting weight of 101.5 grams (no body) this car was very light for our venues here in NorCal.
  • From the first few laps, it was obvious that the GVP can generate very high cornering speeds.
  • It is exceptionally quick off the corner and thus feels very fast in the short straights.
  • In stock form shown earlier, the best times were 5.15 with JK 8713, 9/27, and O/S Lola 163 body.
  • As a counterpoint, the baseline car was running consistent at 5.05 with same body and tires.
Purple Angel: Initially the car had too much bite. It would be in the middle of a fast turn and just let go.
  • First step was to go to JK 8713T. Immediately the car was less critical and would exit more cleanly.
  • Times dropped to 5.05 and were much more consistent with less tendency to deslot suddenly.
  • Next, I tried an 8713T narrowed. This didn't change the times but the car felt easier.
  • Next, I shortened the spoiler on the body from 1/2" to 3/8". Times went up slightly so I changed back.
  • Added 3 grams to each pan in the spots shown below by the red arrows. Car was more comfortable at 5.01.
  • Next I soldered in a "limiter" (blue arrow) and it all came together at 4.94 with several at 4.97-.98.
  • Took the weight back off and the times stayed almost the same but the car was little touchy again.
  • While the limiter was the bigger factor, I'd probably run the car with the weight as well.
  • Very good race car in this configuration and track conditions.
94b5b1f8-0140-42c2-811a-7ed2c0c5961f_zps
 
Flat MTT: I was running out of time by now so only got one short session. Target was 4.75.
  • With the limiter on but weight off, the GVP was pretty harsh on exit with 8/29. Times = 4.85 range.
  • Weight back on and that helped quite a bit. Times dropped to 4.77 range. Still felt "too fast"
  • Went to 7/29 and times stayed the same but car was a little more tractable on exit (still not real smooth).
Ran out of time with some more things to test for the flat track (next time)

  • Gearing somewhere in between the 8/29 and 7/29
  • More weight maybe 5 grams/side instead of the 3 grams.
  • Longer body to smooth out the turn exit.
  • Eventually, maybe a longer guide lead for the flat tracks.
  • Maybe even try a slower motor... :shok:
There will be more...
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#48 Tim Neja

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Posted 24 November 2013 - 11:27 PM

Jim,

If you're adding lead to the back why not turn the pans around so the holes are towards the front?

T
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#49 Rich Clark

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Posted 25 November 2013 - 11:55 AM

Jim and crew,

This has been a good read. I am just busting out my first "fork" build.

So... Thanks for the info!!!

Rich

#50 JimF

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Posted 25 November 2013 - 04:51 PM

Jim,

If you're adding lead to the back why not turn the pans around so the holes are towards the front?

T

 

Two reasons.

 

  1. One: when done this way, the front pin placement is a bit further aft than I like. Flipping the pans to move the windows makes the front pin placement even further back. 
  2. Two: I have done a fair bit of experimentation recently with two scales. Taking the small amount of weight involved in the windows and moving it to the rear a net C.O.M. distance of 10mm or so, does not materially affect the fore/aft balance.

 

The subsequent versions of this kit have solved #1 and hence have also improved #2 to a point. For many track configurations and surface conditions, that will probably be enough. In a broader sense for the tracks and conditions that we see up here in NorKalli.........the guide lead is still a little short.


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