B&M Pre-drop

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Postby Mr. Fun » Sat Sep 08, 2007 9:24 pm

I asked this same question to a gentleman that had worked with constructing a a couple B&Ms in the US, and this is what he had to say. There were two main factors in the origin of the B&M pre-drop:

1. Evacuations. Supposing a train crested the hill and balanced prior to going over because the chain stopped, getting people out of those first few cars really is quite inconvenient. If you have a flat area with a catwalk under it, getting those first two rows unloaded becomes much easier, than with the modern straight drops.

2. Perhaps the biggest reason was having to do with chain wear and tear. You'll notice B&Ms all have "feeder" wheels at the base of their lift. This is designed to match the speed of the train with the speed of the lift chain, so that when they engage, it is one fluid motion. The result is less stress on the chain. The pre-drop was implemented at the top of the ride to once again reduce tension and wear on the chain when the train began to disengage. After the first chain dog on the train has disengaged, there still may be one or two more still lodged in place, and the pre-drop made it so that for the most part, all of the chain dogs release from the chain at the same speed. These two features significantly reduce the amount of strain placed on the chain, and as such increases reliability.

(By contrast, I might point out, that most all other coaster companies do not have these design features on their lift hills. Whereas on most coasters, a cracked chain is not unheard of, B&Ms have a stellar reputation- their chains hardly ever break.)

Ok, anyway so why did they get rid of the Pre-Drop?

From what I understand, there is a simple, "duh", solution that solves both of the problems stated above, that was simply looked over in the design of the first B&Ms: Speed up the chain as the train starts to crest over the hill! A proximity sensor detects when the first car has reached just slightly passed the apex of the lift, and signals the lift motor to speed up. The end result is that the dogs release from the chain because the lift hill speeds up to match the speed of the train cresting and falling down the first drop. A keen eye will see on Nitro and Hydra that the chain speeds up as the train starts rolling off the top of the ride.

And because the chain motor is speeding up, it gives that extra little "nudge" to the back cars just in case the power were to suddenly kick out, so that there is enough momentum to push the train all the way over the lift hill, thus preventing any chance of the train balancing before the first drop.

Simply speeding up the lift chain at the right time relieves the necessity of a catwalk for evacuations, and unneeded stress on the chain. So I hope this solves your mystery!

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Postby yankees15 » Sat Sep 08, 2007 9:54 pm

^I never heard that or even thought of that before, but it makes perfect sense. Thanks for that information.

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Postby Tmcdllr » Sun Sep 09, 2007 1:57 am

Excellent explanation! Thank you for the information. Good stuff.
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Postby Garet » Sun Sep 09, 2007 2:29 am

Yea but then why are there still predrops on the B&M coasters with curved drops, its only the straight ones that don't have them anymore, surely you could do the same thing even if it had a curved drop?

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Postby kraxleRIDAH » Sun Sep 09, 2007 3:12 am

^ The company's Flying coasters don't have this "pre-drop" and they're banked.

My personal take on it for the coasters with banking first drops is that the fluidity of transitioning into the banking is a lot more seamless if the coaches of the train are allowed to level out flat before entering the downward bank.

For instance, you'll notice a sizeable difference in the sensation of the opening drop between any B&M inverted coaster and any Vekoma SLC.

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Postby Tmcdllr » Sun Sep 09, 2007 4:09 am

^Tatsu has a pre drop.
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Postby viking86 » Sun Sep 09, 2007 5:05 am

Tmcdllr wrote:^Tatsu has a pre drop.


A quick look at this picture should make you change your mind... ;)

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Postby Garet » Sun Sep 09, 2007 6:01 am

I think AIR is the only flyer with a predrop, as for my above comment, thats what i get for flicking through wikipedia!

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Postby Tmcdllr » Sun Sep 09, 2007 8:02 pm

If there is no pre drop on tatsu then wouldnt you hit the turn at the same angle coming off the lift? I mean you come off the lift at a steep angle then level out a bit as you start to bank into the turn. Isnt that considered a pre drop?
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Postby Vekoma Fan Boy » Sun Sep 09, 2007 8:18 pm

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How does that level out? :|

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