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Comment by mrfusion | Hacker News Reader

Isn’t it risky for the timing to rely on a rubber belt? Does it never slip? Even a mm of slippage seems like it would make the valve timing stop matching the pistons?

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In addition to what frosted-flakes said, many engines are “non interference” design so that if the belt snaps or jumps a tooth the engine won’t be destroyed, and the belt will just need to be replaced. However, non interference engines are not as compact as interference engines.

Many engines also use a timing chain instead of a timing belt, but this carries extra weight and requires lubrication.

It’s all about tradeoffs!

The engine in one of the vehicles I own has timing gears rather than a chain or belt. Another option, different trade-offs.

Much more common with pushrod engines, where the camshaft is situated close to the crankshaft; less so with overhead camshafts. Though I think some very high-revving sports and competition engines have gears driving overhead cams; IIRC the McLaren TAG F1 turbo V6 (by Porsche) back in the 1980s was one example.

The engine used in the Ferrari Enzo and Maserati MC12 is a notable example:

https://36.media.tumblr.com/8a928fcccf1b41e82be0273ccbe2005a...

Yup, beautiful. The TAG/Porsche looked quite similar IIRC.

And, given your nick, the OHC Volvo B18/B20 can't go unmentioned: Dunno if this was standard back in the day or if it was unique to this engine, but it had an interesting combination of materials. The smaller gear on the crankshaft was steel, but the larger one on the camshaft was made out of some weird bakelite-like stuff; as I recall, with some sort of fibrous reinforcement in it.

(OK, if you're not a Swedish motor enthusiast, you may be wondering "WTF does my nick have to do with that?" A: The B18 was used in the Volvo Amazon.)

My first car would diesel after turning it off. Basically the engine would keep running on fuel in the carburetor and residual heat in the engine. Sometimes it would do this backwards. This was very problematic because the timing belt tensioner only worked in one direction. When it ran backwards the timing belt would jump a tooth then the car would not start. I got in the habit of just killing the motor with my foot on the brake and the car in gear then letting the clutch out.

I also had the machine shop at my high school weld up a custom tool to help me reset the timing belt in a parking lot with a couple of hand tools. It could be done in a few minutes.

This is a long way of saying yes, if they slip it is bad.

Yeah it’s one of those things that you just have to replace every X miles just to be safe. I got one replaced a couple of days ago because the service history was missing and I couldn’t tell whether it had ever been done. If your timing belt goes you’re in all sorts of trouble.

When I started driving German cars (may be coincidental) I noticed my mechanics would write the date and miles of the service directly on the parts.

My Toyota actually had a small decal on the timing belt cover itself specifically for the purpose of tracking that service.

It is risky and they do slip or even break from time to time. They are generally a 100k maintenance item. The alternative is a chain, which can also fail (or other related components), but is generally not considered a maintenance item.

The belts are finely machined to very small tolerances, and things are tightened to specific foot-pound tolerances to make sure they sit exactly where they need to. So no, nothing slips.

I just went through a belt change with my brother not long ago. The tolerances you're dealing with are measured in thousandths of inches. Something that wasn't installed quite straight can cause almost imperceptible wobble that can destroy things under load.

Still, every day shop tools will help you get the precision you need.

Rubber with embedded steel wire is less stretchy than the rubber bands we're used to. Tires even more so than belts; it adds integrity like rebar in concrete.

You're correct that once it jumps even 0.5cm (~1 tooth) it could touch piston to valve; it depends on the engine, but I'd wager most nowadays are interference.

Timing belts are toothed, and if installed correctly they will never slip. Modern kevlar timing belts are near-indestructible under normal use. The real problem is the pulleys—once those bearings wear out, they can seize, causing the belt to slip or even break.

Timing belts have teeth.

Occasionally chains are used instead; more expensive and heavier, but more durable. You do need to maintain the timing either way it it will destroy the valves.

If the engine is a non-interference design, losing the belt won’t be catastrophic for the valves

>Timing belts have teeth.

To expand: It is the teeth that make belts timing belts, they keep the 'timing' (relative rotational orientation) of 2 or more toothed pulleys. In an ICE the camshafts are locked to a 2:1 ratio to the crank shaft, in a 3d printer it keeps an axis fixed relative to the stepper motor shaft.

I think another reason belts are common is that chains tend to be a bit noisier.

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