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Hi all, I'm looking for a little help with a custom chassis project. Currently, all my stuff is in storage but I am hoping to get my hands on it for over the winter months. I want to make a custom Ultima Pro XL-length chassis, and graft either a B4 front end and rear arm/hub/driveshaft assembly onto it, or some sort of longer front arm concoction with the stock rear suspension. I'm going to buy some flat sheet to cut the chassis, bit I need to know how long the stock Pro XL chassis is from the very back to where the front kick-up starts. If someone could measure that for me, I would be very grateful.

Also, if anyone has any modifications that involve adapting a B4 transmission to a Pro XL chassis/suspension (or another brushless-capable transmission), I'd like to see a few pictures of what you came up with and get an idea of how it works. I've been advised that the Lethal Weapon, even with the Power Clutch might not fare well with the high levels of instant torque that brushless motors produce. I'd rather not lunch the LW, but I also don't want to deal with brushed motors anymore either.

Thanks in advance!!

Hey Adam,
I'm guessing that you've already seen this topic discussed in the other post and are just looking for someone to measure their XL chassis. FWIW, I don't have an XL chassis to measure but other members here have confirmed that the XL chassis is 13mm longer than the Ultima. When measured from the back edge to the kickup break, the original Ultima chassis is 288.5mm so the XL would be 301.5mm.

Rather than fitting a B4 trans, why not use the re-re Ultima trans? The re-re unit has a slipper clutch and metal gears so it should hold up to the most powerful brushless systems.

That's perfect on the measurement, much appreciated!

As for the transmission, I already have most, if not all the parts for a B4 transmission, and I know it'll handle brushless easily enough. I'm trying to avoid buying more than I have to at this point to keep cost down. It's gonna be run so I'm not worried about it being period correct, but I want it to be durable. I think the Lethal Weapon would work if I went with a 21.5 motor, turned the throttle exponential way down, selected the least powerful start up program from the ESC, and wrapped the belt idler with some Scotch tape to give the belt some more tension. But then I'd have to buy a motor... I have a 10.5 that I'd like to use but I'm not sure if it'll be too torquey, even with all the adjustments and modifications...

Having recently completed a B4 3d model and inspired by your original post, I couldn't resist the challenge of designing a cradle to mount a B4 gearbox into an Ultima chassis.
I was surprised at how well the B4 box actually fits the space and how the motor will continue set very close to its original location. Notice however that the Ultima's dog bones will be too short by 5.5 to 6mm.
The cradle bolts to the chassis plate using the existing forward gearbox screw holes and the two outdrive grub screw access holes. Unfortunately the Ultima's rear gearbox holes are slightly misaligned with those of the B4 box so they can't be used to bolt the cradle to the chassis. I believe the four M4 chassis plate screws plus the top bulkhead brace should be sufficient to hold everything in place.

Also, the bulkhead will require a slot to accommodate the B4's forward mounting screws.
If you think this might be useful for your project, I can make available the stl files for printing.

Wow Doug, that's great!! Do you think I could get away with using the inner hinge pin mounts and still have a structurally sound transmission mount? I could run the B4 screws straight through the chassis and cradle to tie all that together so maybe those lateral mounting ears wouldn't be necessary. Also, I would likely be starting with a clean sheet of 4mm carbon fiber so I would only be transferring over the holes I'll need. Before I stepped away from RC, I had mocked up B4 rear arms, hubs, and wheels mounted to the inner hinge pin mount and it all lined up pretty decent. I figure with several Associated buggy iterations since then, I should be able to find a combination of parts that will work well. If that doesn't work, I can always still use the set of long Pro XL arms I have and get some newer hubs for it.

I appreciate your time and generosity with mocking that up and offering me the file. At this point, I would probably hack that cradle from a chunk of plastic by hand. Is there a way for you to provide me with the basic dimensions of it...? That would be a huge help!
Thanks!!

Since you're starting from a clean sheet, the gearbox mounting screws can definitely pass directly through the chassis plate and the cradle becomes two simple spacer blocks. At first glance these tall stanchions might look a bit precarious but I believe with the bulkhead brace, the mounting system should hold the gearbox/motor assembly securely.
If this is what you're after, I can supply all dimensions necessary to make the blocks and mount them to the chassis plate.

That looks bang on!! I would be very grateful if you would send me those, PM coming your way!

Thank you Adam. I'll just post the dimensions here for anyone else who might be considering a similar build.

Just click the images to enlarge them and let me know if any of this is unclear.
I hope you're able to retrieve you're stuff soon so you can begin working on this project.

That's phenomenal Doug! I really gotta get my arse in great and figure out a way to get reunited with my gear...!

Thank you again for doing this. Though I'm sure you probably enjoyed it a little, I'm really going to enjoy the build (hopefully relatively soon).

XLR8 wrote: ↑Wed Nov 18, 2020 9:38 am Hey Adam,
I'm guessing that you've already seen this topic discussed in the other post and are just looking for someone to measure their XL chassis. FWIW, I don't have an XL chassis to measure but other members here have confirmed that the XL chassis is 13mm longer than the Ultima. When measured from the back edge to the kickup break, the original Ultima chassis is 288.5mm so the XL would be 301.5mm.

Rather than fitting a B4 trans, why not use the re-re Ultima trans? The re-re unit has a slipper clutch and metal gears so it should hold up to the most powerful brushless systems.

do you know how long trinity ultima chassis is?

rwordenjr wrote: ↑Sat Apr 30, 2022 2:33 pm

XLR8 wrote: ↑Wed Nov 18, 2020 9:38 am Hey Adam,
I'm guessing that you've already seen this topic discussed in the other post and are just looking for someone to measure their XL chassis. FWIW, I don't have an XL chassis to measure but other members here have confirmed that the XL chassis is 13mm longer than the Ultima. When measured from the back edge to the kickup break, the original Ultima chassis is 288.5mm so the XL would be 301.5mm.

Rather than fitting a B4 trans, why not use the re-re Ultima trans? The re-re unit has a slipper clutch and metal gears so it should hold up to the most powerful brushless systems.

do you know how long trinity ultima chassis is?

No, sorry. If I had a Trinity chassis, I would definitely measure it for you. Most of what I have is OEM parts - very few aftermarket parts.

XLR8 wrote: ↑Mon Nov 23, 2020 5:43 pm Thank you Adam. I'll just post the dimensions here for anyone else who might be considering a similar build.

Just click the images to enlarge them and let me know if any of this is unclear.

B4 Mounts.jpg

B4 chassis holes.jpg

bulkhead slot.jpg

I hope you're able to retrieve you're stuff soon so you can begin working on this project.

Thanks ! This helps a lot

XLR8 wrote: ↑Mon Nov 23, 2020 5:43 pm Thank you Adam. I'll just post the dimensions here for anyone else who might be considering a similar build.

Just click the images to enlarge them and let me know if any of this is unclear.

B4 Mounts.jpg

B4 chassis holes.jpg

bulkhead slot.jpg

I hope you're able to retrieve you're stuff soon so you can begin working on this project.

Guess who has been reunited with their gear...? That's right, this guy!! I'm going to see if one of the local machine shops has any small chunks of aluminium or delrin sitting around they'd part with. Then I'll get the Dremel and drill press fired up and hopefully get these made. Finally.

I'll keep you posted.

Edit to add: is there anything reading this that wants to print these for me...? Obviously not for free, send me a PM if so.