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Old 04-15-2011, 05:43 PM   #1
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Default HPI Experts: Need help w/custom parts fitment

Hi, I'm hoping a resident HPI expert or dealer can spare some time and do some measurements to see if any purple control arms will work for a custom build I'm working on. It's based on a Kyosho Turbo Optima, so there's no way to know if parts will fit without breaking out the ruler or caliper. I hope someone can help me out!

I need some front & rear control arms that will substitute the stock Optima arm sizes. I've been told that HPI RS4 and possibly Cup Racer arms & hubs/C-carriers might be suitable. I just want to focus on the arms for now. I made the attached image showing the allowable measurements for A, B & C.

A = The width of the arm "base", typically the thicker end of the arm;

B = The maximum space where a hub/C-carrier can fit, on the outer, thinner end of the arm;

C = The length of the arm, from hinge pin-to-hinge pin.

The pin holes need to be 3mm diameter, and mounting holes for ball studs must be 3mm thread. Please refer to the attached image for clarification.

I know it's asking a lot but I don't have access to a hobby store's selection of parts and I don't want to buy stuff without having a pretty good idea if it'll fit or not. Hopefully someone here has this stuff either lying around in their parts bin, on their shelf, or on their cars, and can spend 15 minutes taking some measurements.

If you have something in alloy that will work for me, I might even buy it from you if you want.

Thanks for your help, and I hope you all enjoy your weekend!
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Old 04-16-2011, 02:38 AM   #2
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Default RS4 PRO 1 control arms

I have the RS4 PRO 1 and measured the front and rear arms. It is my understanding that the RS4 and the Pro2 also share the same arms but different part numbers. Not positive on that though. I'm restoring my PRO1 and what I've found is these parts are extremely hard to come by. I've ordered RS4 Nitro3 aluminum arms hoping they will fit. I can measure those when they come in if needed.

Here are the measurements I have:
Front
A, 39.2mm
B, 11.9mm
C, 45mm

Rear
A, 38.75mm
B, 14.5mm
C, 49.5mm

**C measurements taken from the outer most hole on the arm, there are two options, wide and narrow setups.
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Old 04-16-2011, 11:47 AM   #3
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Originally Posted by IH8VTEC View Post
I have the RS4 PRO 1 and measured the front and rear arms. It is my understanding that the RS4 and the Pro2 also share the same arms but different part numbers. Not positive on that though. I'm restoring my PRO1 and what I've found is these parts are extremely hard to come by. I've ordered RS4 Nitro3 aluminum arms hoping they will fit. I can measure those when they come in if needed.
Thank you very much for taking the time to do those measurements! Those measurements would actually be workable if the arms were longer (C). I think those specs might work for a different build I'm working on. It would be great if you could measure the Nitro3 arms when you receive them. I need longer arms for my off-road build, something in the 60 - 68 mm range for (C).
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Old 04-16-2011, 12:19 PM   #4
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Originally Posted by Coelacanth View Post
Thank you very much for taking the time to do those measurements! Those measurements would actually be workable if the arms were longer (C). I think those specs might work for a different build I'm working on. It would be great if you could measure the Nitro3 arms when you receive them. I need longer arms for my off-road build, something in the 60 - 68 mm range for (C).
I can try and check out a lhs today and see what other HPI arms might be longer in C, they also carry a ton of Traxxas parts so I'll give those a look.

Do you just need them to be in range of you measurements in the picture? How many different arms are you looking for? Is there a perfect measurement you are looking for?
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Old 04-16-2011, 01:36 PM   #5
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Originally Posted by IH8VTEC View Post
I can try and check out a lhs today and see what other HPI arms might be longer in C, they also carry a ton of Traxxas parts so I'll give those a look.

Do you just need them to be in range of you measurements in the picture? How many different arms are you looking for? Is there a perfect measurement you are looking for?
Anything within the ranges in that image would be "perfect", in an alloy arm. Anything longer or shorter would mean I'll also have to source different CVDs, something I'd rather not do.

I'm basically looking for aluminum arms to upgrade the Optima arms that aren't made by or for Kyosho. I just figure there has to be something out there that'll work, but nobody's done the trial 'n' error yet.
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Old 04-16-2011, 05:41 PM   #6
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Originally Posted by Coelacanth View Post
Anything within the ranges in that image would be "perfect", in an alloy arm. Anything longer or shorter would mean I'll also have to source different CVDs, something I'd rather not do.

I'm basically looking for aluminum arms to upgrade the Optima arms that aren't made by or for Kyosho. I just figure there has to be something out there that'll work, but nobody's done the trial 'n' error yet.
I didn't find anything better at the lhs today. The slash arms were close but 80mm where you need 68. I'll be able to check another lhs sometime this week if you don't find what you are looking for. I posted a link to this thread in my PRO thread, maybe some other HPI heads have an idea.

Have you emailed a manufacturer of the aluminum arms? They might be able to help with specs. I'm taking measurements of stock parts hanging on the wall. You would then have to see if aluminum replacement arms are available.
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Old 04-18-2011, 11:26 AM   #7
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Originally Posted by IH8VTEC View Post
I didn't find anything better at the lhs today. The slash arms were close but 80mm where you need 68. I'll be able to check another lhs sometime this week if you don't find what you are looking for. I posted a link to this thread in my PRO thread, maybe some other HPI heads have an idea.

Have you emailed a manufacturer of the aluminum arms? They might be able to help with specs. I'm taking measurements of stock parts hanging on the wall. You would then have to see if aluminum replacement arms are available.
Thank you again for your help. Finding parts specifically meant for a particular car is easy, but much more difficult when you're trying to see if something off-brand will fit. Most people can't be bothered to take a few minutes to do a few measurements, and I can understand this because 9 times out of 10, they won't match closely enough.

I'm going to try emailing HPI but I don't find manufacturers particularly helpful unless you're buying a specific part number or for a specific product. They're just as reluctant to break out the tape measure/calipers as the rest of us, I guess. I need to discuss with an actual RC technician or enthusiast, not the salesperson who just wants to know a SKU number and take my credit card info.

Your idea to measure plastic part specs is a good one, I'd be doing the same thing if I was able to. Once you know a plastic part that fits, and know the car it came from, it's easy to search for alloy upgrades.

I'm trying to avoid going with arms longer than my "acceptable range" because then that means I'll need CVDs and maybe shocks and other stuff to make this mod work, and the costs will go up too much.
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