Big Bird's Bikes

Big Bird
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Big Bird
Edited Date/Time 3/20/2015 4:48pm
These are the photos from a DirtMag site article that I can't seem to find anymore...

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The DH bike geometry is... wb 1190mm, bb 345mm, down tube 700mm, ha 62.5 deg, cs 412.5mm
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Big Bird
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Big Bird 3/19/2015 12:06pm
3/19/2015 12:06pm Edited Date/Time 3/19/2015 12:30pm
Damnit. Didn't work. At least you get the geometry. It's a single pivot based on a gen 2 Santa Cruz Bullit swing arm, but with the main pivot moved up the Cro-mo down tube to shorten the cahinstays.
Primoz
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Primoz 3/19/2015 12:36pm
3/19/2015 12:36pm
Ah, i thought you had a different rear end on a demo.

How does a 412 CS feel like?
Big Bird
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Big Bird 3/19/2015 4:25pm
3/19/2015 4:25pm
Well done Sir! Sometimes the obvious eludes me. I went searching on the DirtMag site. And in terms of the short rear end, it helps it turn very quickly. I was going to say "Turns on a dime." but you probably don't have dimes in SVN. As I said in the article, I'm 2 meters tall. This geometry allows me to have a long front end to fit my height, a short rear end for quick turns and a reasonable wheelbase.
Primoz
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Primoz 3/20/2015 10:05am
3/20/2015 10:05am
We don't have dimes, but i do understand the phrase, i'm lucky to be good with english.

How does the long front centre deal with putting your weight over the front axle, is it a problem? Can you use flats or do you have to hang over the front so much that you need to be clipped in to prevent from blowing off the pedals?

I'm thinking about a 420 mm chainstay, 650ish mm top tube, 75° seat-tube angle and a 65° head angle for an enduro class bike (it would give about a 1230 mm wheelbase), which would be a large L compared to other brands and in most cases about 30 mm longer on the wheelbase (i'm 190 cm, so in most cases right between the L and XL frame).

I'm affraid how a bike like that would handle and go uphill, with such a long front end and a short rear end. Granted, it would have a really upright seat tube, which moves the centre of gravity (of the rider) quite a lot further forward, which would help a lot.
Big Bird
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Big Bird 3/20/2015 10:30am
3/20/2015 10:30am
I do use flats with no problems. On a typical XL frame with standard geometry, I end up with the seat so high and as a result back, that I end up doing wheelies up climbs. I have a drawing for a trail frame that I haven't been able to build yet, that should test my geometry for climbing. Like you, I use a steeper seat tube angle to both make the bike pedal better and make more room for the rear wheel to travel while achieving a short rear end. The front is longer, but the seat angle keeps my weight more centered over the long front end. Does that help? I'll check out my drawing for my trail frame to give you all the numbers...
Primoz
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Primoz 3/20/2015 12:23pm
3/20/2015 12:23pm
Yea, i know the steeper seat angle helps, i've done some quick calculations and it shouldn't be a problem, it's quite similar to more old school, rear-oriented bikes, a percentage point or two of difference. But numbers tell you something, while real world feelings may tell you something else. So it's better to ask if you have the chance.

And i feel the pain of tall frames, i currently still have a Commencal Meta 5.5 of a 2008 vintage, that has a 68.5° seat angle, where the seatpost is fully extended. My ass is right about at the halfway point on the chainstay since the seat gets so far back. I don't like that much and i even more strongly dislike the fact that when lowered, the seat is so weirdly forward (the seat tube is offset to the front about 5 cm compared to the BB, the angle is actual, not virtual as far as i know).
Big Bird
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Big Bird 3/20/2015 4:33pm
3/20/2015 4:33pm
OK. So here are all of the numbers for my next bike to build. A lot of it is my own theory, but I really got the go ahead to realize it after the DirtMag Dan Atherton interview where they give some of the numbers on his personal bike. This one is based on an original Bullit swing arm that I have in the shed. I'm an American and generally work in inches. I had translated those others in a notebook for that article. So here the new one is in inches...

Down tube 30.5", seat tube 21" at 76 deg, actual top tube 25.5", head tube 5" at 66 deg, BB 13", wheelbase 48" plus fork offset, chain stays 16 5/8, and the main pivot at 5" from the BB.

I hate the feeling of pushing forward to go forward that the tall are generally subjected to. I want to push more down like everyone else. The steeper seat angle also makes room for the shorter rear end to keep the longer front end feeling lively and puts weight forward to weight the front tire. Win win win, I think, but I haven't built it yet so who knows. It may be totally whack. I think golden.
Primoz
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Primoz 3/20/2015 4:40pm
3/20/2015 4:40pm
M, i know what you mean with the pushing forward.

A friend of mine who's finishing up his kinesiology PhD actually said that a steeper seat tube angle is more efficient, so there's that.

The chainstay is basically the same as i chose, a smidgen over 420 mm, same goes for the top tube, 650 mm. I've been throwing head angle numbers around, going from 65 to 66,5 in half degree increments, and the wheelbase differs by 6 mm for each half degree, going from 1238 mm at 65° to 1220 at 66,5°. With your one degree steeper seat tube you should gain a few mm on my numbers (66° at 1226 mm), so i suppose 1230+ mm or 48,5 inches should be easily achievable.

Have you thought about going with a virtual pivot point suspension design? Well, horst link is more or less the only way to go with a chainstay that short...

Also, 26" wheels then? I'm basing my numbers on 650B wheels and an appropriate fork.
Big Bird
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Big Bird 3/20/2015 4:48pm
3/20/2015 4:48pm
I'm a big fan of the simple single pivot. I'm proud that at least one major company still embraces it. Thank you Orange. And at some point I decided that I could fit 650s in a 26" swing arm, so the design is based on that assumption.

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