Didumos69

@Filthy Fox, @allanp, the stock Lamborghini gearbox has 4 idlers transmitting torque, of which one is very suspicious. I really don't understand why they made it so complex, because the same ratios can be obtained with much less gear meshes.

Very nice!

Thanks @allanp for this elaborate explanation. It makes total sense to me and confirms my own line of reasoning. This is why I always avoid idler gears transmitting torque. In my designs (except for some mods for Bugatti and Lamborghini) clutch gears are either engaged with a driving ring or they are just making dummy rotations and not transmitting torque. For instance in this 8-speed transmission tunnel. The gearboxes of the Bugatti and the Lamborghini have the high-low gearbox and the 4-speed gearbox layed out next to each other, with several idler gears transmitting torque from one gearbox to the other. In this 8-speed tunnel I layed out the same gearboxes longitudinally which allows them to be connected without using torque transmitting idler gears.

Yes, reasoning from engine to wheels. For that I would reason from wheels to engine, because this is a manual build, so the wheels effectively drive the engine. I would opt for the gearbox with the least amount of gearing up, so the least increase of rpm as the one that should be closest to the wheels. In the above case the high-low gearbox either has a 1:1 ratio or it gears down with 12:20 x 12:20 = 9:25 (reasoning from wheels to engine). The 4-speed gearbox actually gears up by 20:16 in 1st gear (again reasoning from wheels to engine), so I would connect that one to the engine and the high-low gearbox to the wheels, which is the case in the above example.

That's a valid question. Under high torque, like in RC builds with some gearing down, having a 8t-8t mesh is indeed not a good idea. Building gearboxes for RC puts completely different demands on the drivetrain. However, in a manual build, with a supposedly smooth running drivetrain, but even with some friction, this should not be an issue. This kind of setups have also proven itself in many manual builds, for instance the pimped up Bugatti. All of-the-shelf LEGO 8-speed gearboxes (42083, 42115) are in fact a combination of a high-low gearbox (bottom of the above image) and a 4-speed gearbox (top of above image). To make this work, without too much friction, the overall ratio variation should not be too extensive. In the stock Bugatti and Stock Lamborghini, the high-low gearbox has 2 ratios, 1:1 and 20:12 x 20:12 = 25:9, so the variation is 1:1 -> 25:9. The 4-speed gearbox ratios should be within this range; it should have a smaller variation to make sure the combination of low gear and 4th gear does not exceed nor level with the combination high gear and 1st gear. Another aspect to this is that the 4-speed gearbox itself (as do all sequential gearboxes of this type, see here), is required to have the ratio between it's inputs (in the above case (1:1):(16:20) = 20:16) smaller than the ratio between the front and rear ends of the driving rings (in the above case (20:12):(1:1)=20:12), otherwise you would never get the sequence right. And together they should not exceed the 25:9 of the high-low gearbox: 20:16 x 20:12 = 25:12 > 25:9. When we would replace the 20:16 with your 24:8 x 12:20 = 9:5, the 4-speed gearbox ratios (9:5 x 20:12 = 3:1) would exceed the high-low ratios (25:9).

It relies on my setup with the different reverter, the one with the 2 8t gears, where the DNR has different outputs ratios for Drive and Reverse to compensate for different input ratios for Drive and Reverse of the gearbox. It also restores the stock gear sequence. This one: @jb70, here is the file with this drivetrain.

I think the gearbox with center differential, separate inputs for Reverse and Drive and an 8->1 gear block should fit inside the 11x15 frame...

This observation remains valid. But the good thing is that things can be modded to make it work.

I enjoy all this suffering so intensely .

As a further development of the incorporation of a center differential for both Drive and Reverse, this is an option that does not need a 2nd new CV-joint because of it's longer axle (the model comes with only 1 such new CV-joint). In this case the DNR has a different ratio for Reverse (1:1) compared to Drive (20:16). The gearbox can easily be changed to compensate for this difference, giving 1st gear and Reverse again the exact same overall ratio. The dark azure pieces highlight the changes (the transparent piece could be removed). As a side effect, the gear sequence is back to the stock sequence again.

You are right of course, but if I were to make a C3, it would not have openable doors. The driver would have to hop in via the side window .

A whole range of cars and MOCs can take advantage of the new arched mudguards: I say Chevrolet Corvette C3:

Usually we have a thread for new parts if they feel like a game changer. I think the new curved wheel arches are a reason. They allow for many existing models to be redone. EDIT: I made such a thread.

For instance in clocks...

@I_Igor, @astyanax, guys, one small request. I would really like to see mods and improvements here and not all kinds of speculations on B-models with large images. This is not the mods and improvements and b-models speculations topic .

I did some further cleanup on my suggested mods for a gearbox with integrated center differential, which is used in Drive and Reverse. Of course all based on @jb70's work so far. I will PM my file to him. Summary of what I did: I moved the Drive input of the gearbox and the Drive output of the DNR to the driver's side and the axle that engages with the center differential to the co-driver's side. This effectively reverses the entire gearbox, making the paddle-shifter mod unnecessary. To get the gear ratios right again, I replaced the 20t clutch gears in the high-low gearbox with 16t clutch gears and swapped the two 20-12 meshes. This allows the axle that engages with the center differential to be used as input for Reverse, because it now has the same ratio as 1st gear. To utilize this Reverse input, I moved the Reverse output of the DNR to the co-driver's side and added a reverter. Finally I made changes to the structure to brace all well. To further tweak the overall gear ratio (engine wheels), there are still some parameters that can be changed: The gearing close to the front and rear differentials and the gearing close to the engine.

Thanks! I'll share my file with you after some final changes. Shouldn't take long. EDIT: The gear sequence is also not correct yet, I have to look into that too. EDIT2: Can be fixed easily. Btw, this setup already reverse the gear sequence, so it would make the change to the paddle shifters unnecessary. This idea would fit my ideas above, because it would allow to swap both 20-12 meshes at each end (front and rear). However, I think the stock diffs are good enough and it would be a pity if people would have to buy 2 diffs only for this. But is remains to be an option.

Based on the digital file from @jb70 I tried a few things that might work: I swapped the gears close to the front and rear differential. This gives a ratio change of 25:9. This allows the center differential to engage with its 24t side. This gives a ratio change of 9:27. I swapped the input-output sides of the gearbox. So the Drive input is at the driver's side now. To get the gearbox ratios right again I changed the gear ratios in the high-low gearbox. Now I can use the old input as Reverse input, which directly engages with the center differential. It has the same ratio as 1st gear. I moved the Reverse output of the DNR to the right side of the DNR. I added a direction inverter, so Reverse actually is reverse. I'm still working on the bracing inside the gearbox, but around the DNR I think I'm done.

You need to apply some torque, but then it certainly can happen. It depends on the way axles are layed out too of course. I had a motorized build with 2 Buwizz, where 1 servo, geared down twice, was not able to get any movement in the rotary catches under high torque, simply because the driving ring got a hold on the clutch gear it was engaged with. By disengaging the driving ring at the top of this image, under high friction, you will pull the clutch gear and the axle it meshes with along with the driving ring. When you disengage the other end it will push the same axle back again. Doing this many times may cause the displacement. If the displaced axle has never been completely inserted into the red 3L connector, this course of events is even more likely.

You should never do that! ? When there is severe friction, the driving ring, the driving ring extender, the engaged clutch gear and the engaged gear get a firm hold on each other. When you shift gears under such friction, the driving ring, extender and engaged clutch gear will pull the engaged gears forwards and backwards, causing them to gradually slide off their axle, if the surrounding structure doesn't properly lock up these axles. Welcome in the world of flawed stock gearboxes!

Did you shift gears while pushing it around, I mean while it had some speed?

@jb70, one potential issue I see with the gearbox redesign, with and without my additions, is that the 16t gear which engages with the differential, can be placed too tight against the adjacent 20t clutch gear, causing undesired friction. I don't see an easy way to resolve that though.

Instead of using CV joints in the orange reverse train, you could use 2 8t gears to make a 1stud jump. The gearbox uses a pair of 8t gears, but the gearbox is not engaged in reverse. So there will never be two pairs of 8t gears engaged.

If I'm not mistaken Reverse has the same ratio as 2nd gear in the above layout. Which is not bad for an 8-speed gearbox I think. EDIT: I didn't take into account the different ratios in the DNR. The reverse ratio is actually smaller than 1st gear (by 4/5), which is fine too in my opinion.

Cool! I noticed that the pimped Bugatti also has reverse engage with the front output of the differential and was thinking this could be improved for the Sian. I agree! If you can somehow reverse one of the outputs of the DNR, than that would solve it. The 42053 had a DNR which really reversed, if I remember correctly. I'm also not sure how the gear ratio in reverse compared to the other gears. Because in a manual model this does not give problems if you brace them well. It's a general misconception that having 2 8t gears in a manual model lead to undesired friction. It works in the pimped Bugatti too and even helped reducing the overall friction in that drive train. If you want to make a compact 8 speed gearbox with center differential you have to be resourceful.