My aim was to build a simple, fast car. I wanted to have NXT brick and the servos inside the bodywork and not to end up with a build that was too heavy.
This car has the usual working front suspension, but is a hardtail for simplicity and space saving. This has made it much easier to fit in the servos and gearing, and left me with enough boot space to fit the Lego trip computer that I bought on eBay (see below)
I've done some speed tests, and the car (still accelerating) reaches 6+ kph (from the trip computer, wheels set to 8 2/3 studs) before I run out of floor space. I'm waiting for the rain to stop so I can get it outside and see how fast it'll actually go. With the back wheels off the ground at full speed the trip computer shows 10.5 kph, so I'm hoping to get close to that.
Car length: 430mm, car weight: 1.5kg.
Boot view: Lego Trip Computer (5206), with the sensor on one of the half-shafts next to the diff
Roof hinges to allow access to the NXT Brick. This chassis is constructed as a series of rigid hoops, with the mechanics inside each hoop. The black beams crossing above the NXT brick take all the stresses from the single thickness beams that run under everything. Makes the chassis nice and low. You can also see my main issue that I'm still working on. There's a servo mounted across the chassis, with a shaft that has two universal joints driving the steering. This is a problem, particularly at speed as the joints have quite a bit of play in them which makes the steering pretty vague. I need to rebuild this, possibly relocate the servo, so that it's gears only, no universal joints.
One good thing about the NXT brick is that I've written the control script so that as the car gets quicker, it slows down the steering response, which makes it less twitchy.
Underside detail: rear gearing, diff and the sensor for the trip computer.
I've seen a number of posts from people who (using good engineering practice) suggest using a diff to multiplex two servos together. In my experience, particularly using the NXT servos which generate a lot of torque, don't. The result is a lot of broken bevel gears, as the torque is so much that the centre gear in the diff shatters. These two are on a common shaft. They're very accurately matched so this doesn't cause any problems that I've noticed.
The gearing steps the output from the servos something like 5.25 times before it goes on to the diff at about 1:1. I had to reinforce the left side of the Chassis (near the buff coloured gear in the image above) to cope with the torque from the motors as this was twisting the chassis under acceleration.
The diff has the spring from a grey shock absorber in between the bevel gears - this creates friction between those gears and the housing = limited slip diff. Not my idea, and unfortunately I've forgotten whos post I read it in, but it works great!
Thanks! I love the NXT because you can do a lot with it, but planning your build round the size of the motors and the NXT brick itself can be a headache. I've updated the build with the steering changes I mentioned above and bodywork updates, so I'll post a WIP in the next day or two. Cheers!