Pitch, not length
A conveyor presents one slot every time it travels one pitch, and pitch is the vehicle plus the gap behind it. At the defaults — 90 ft per minute, a 16 ft car and a 12 ft gap — the pitch is 28 ft, so 5,400 ft of belt an hour presents 192.9 slots. Fill 85 percent of them and the tunnel does 164 cars an hour.
Run the same numbers dividing by the 16 ft car alone and you get 337.5 slots and 287 cars an hour. That is 75 percent high, and it is high for a reason that never shows up in the arithmetic: it assumes bumper-to-bumper contact for the whole length of the tunnel. The gap is 43 percent of the pitch at these settings, which means 43 percent of the belt is carrying nothing.
Dwell and rate are separate levers
A 130 ft tunnel at 90 ft per minute holds each car for 1 minute 27 seconds, and 4.64 cars are inside at any moment. Lengthen the tunnel and every car gets more time under the equipment while the hourly rate does not move at all. Speed the belt up and the rate rises while every car gets less time. There is no setting that gives both, and the trade is the whole design argument for express tunnels.
The cheapest lever is usually the loader
To reach 180 cars an hour from the defaults you can run the belt at 99 ft per minute, or tighten the gap to 9.5 ft, or fill 93.3 percent of the slots instead of 85. The last one costs nothing mechanical. It is a second pair of hands at the entrance on Saturday, a clearer approach so people are not reading a menu at the pay station, and not having to re-aim cars at the track. Sites chase belt speed first because it is a setting, and slot fill last because it is a staffing argument.
What beats all of it
Downtime. A stall empties every slot behind it and each one takes a full pitch of belt travel to refill, so a two-minute stop costs about five cars directly and more than that in the recovery, because the loader restarts against an empty belt rather than a moving queue. A tunnel with good gap discipline and three stops on a Saturday does worse than a sloppy one that never stops.
Questions people ask
How do I work out cars per hour for a car wash tunnel?
Belt speed divided by pitch, then multiplied by the share of slots you actually fill. Pitch is the vehicle length plus the gap left behind it, not the vehicle length on its own. At 90 ft per minute with a 16 ft car and a 12 ft gap the pitch is 28 ft, which is 192.9 slots an hour, and at 85 percent slot fill that is 164 cars.
Why is my tunnel slower than the number I was quoted?
Usually because the quoted number was worked from vehicle length rather than pitch, or from a slot fill nobody reaches. At the defaults on this page the two methods differ by 75 percent. Count the cars that go in against the slots that go past over one real hour and the gap between the brochure and the site is normally in those two figures.
Does a longer tunnel wash more cars an hour?
No. A longer tunnel holds more cars at once and gives each one more time under the equipment, but the rate at which cars leave is belt speed divided by pitch and the tunnel length does not appear in it. Length buys dwell time, speed buys rate, and the two trade against each other rather than adding.
What gap should I leave between cars?
This page will not tell you, because it depends on your correlator, your equipment, your vehicle mix and how quickly a car can be aimed at the track — all of which are questions for the equipment maker and for whoever loads. What it will tell you is what any gap is worth: the table walks from zero to twenty feet so you can see that the first few feet are worth several times what the last few are.
Is it better to speed the belt up or tighten the gap?
They are the same lever mathematically, since the rate is speed divided by pitch. Which one is available differs by site. Belt speed is limited by what the equipment can do in the time each car spends under it, and the gap is limited by loading. Slot fill is the third lever and it costs no equipment at all, which is why it is worth checking before either of the other two.