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Hi my name is John ,
I am building a fat 1981 mongoose based on the motomag frame and forks, called a fatgoose 170, It has taken a year to get to this stage but have hit a bit of a problem and would welcome some advise please.
It is fitted with the rear wheels taken from a schwinn stingray chopper and I have widened the crank by 60mm total, the frame geometry is the same but the bike is wider.
I am trying to make it look as much as a 1981 mongoose as possible by using era parts and decals of 1981 accept the wheels and hope disc brake of course.
I built the bike complete in an unpainted state and luckily I did because when applying pressure on a power stroke as if you were in a starting gate the bb seems to have a little more flex than i'd like even though my supergoose also has flex when you do this.
I know a lot of frames have flex in a steel frame but with the added leverage on the crank it feels a bit much but I have not widened the rectangle section behind the bb.
I you could take a look please, I am open to suggestions, I have spoken to a frame builder and the standard size of tubing used is a big factor in the problem but I can't change it now, maybe if I brace it but not too much to move the problem to another area ?
I was thinking of widening the rectangle by 20mm either side of the it and then also welding a mongoose gusset between the down tube and the seat mast tube that was used on the 1975/76 mongoose frame.
from 77 onwards mongoose stopped using this brace and I don't know why, it would be interesting to know the answer.
Thank you for reading

John.
 
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Sorry, part of this thread needs to be moved to Builds, new to the site and don't know what i'm doing:blush:
 
When you apply pressure stationary it feels weak at the rectangle where it joins the bottom bracket
 
I can see how that xxxtra wide BB shell/ crank spindle could be flexy, especially with the stock-sized cigarette box behind it. I don't have much advice to offer, aside from using some plate steel, cut into 4 pieces, to extend the box laterally, so more of the BB shell is braced. Basically, imagine building a bigger box around the original box.... weld it to the loop and the shell, and keep the original box as a platform upon which to do this. Does that make sense? I've got no experience with a project like this one, but plate steel is cheap enough, and you could potentially add that to the project as it sits, without having to remove anything and (most likely) without mucking up the clean set-up you've got so far.

Mostly, though, I wanna say "welcome" to RRB, and I need to mention that this is the most monumental Intro post I have ever seen....
 
I could add a 76 goose gusset and widen the cigarette box by adding 20mm x 20mm box either side of the rectangle, problem is if I go over the existing box with say 1.5mm wall box cut in half , time it gets to the rear stay tube it will look wrong because the rear stay tube is also 20mm so maybe better adding rather than going over the top of it ?

 
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Pics above of what i may be attempting the weekend, the crank shaft was made from grade 5 titanium copying the original Team mongoose Shimano crank, using the original 1981 mongoose bearing set with 12 race bearings
 
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Very cool work so far! If you are having problems with flex there I would be more inclined to run small bracing plates from the widened BB up to the seat tube...

Great bike, awesome work! :thumbsup:

Luke.
 
Thanks for your comment's, The welder is booked for next week, the same guy has welded it from the start and takes great pride in his work.
I have also enlarged the rear triangle to match and cut out the gusset that's fitted on a 76, pics below.
and got my parts back from the anodisers.
any ideas for the main frame colour ? want to stay with old school ideally
I have the original decals to fit, including the 2 rare dura ace decals found on a 78 team model





 
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Thanks for your reply Luke, I think either white or 2004 seat Onvi yellow S1H
The anno stuff and the chrome will go with white for sure but not sure the cranks will go with yellow and the bars would look better black rather than chrome.
I have just finished building the wheels with polished stainless DT swiss spokes
 
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just added the final welding pictures above, I'm sending it off tomorrow for nickel, the forks are back from nickel and are fantastic !
I was going to do the frame in white but the forks came out so nice I couldn't resist
 

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