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PostPosted: Mon Aug 12, 2013 3:02 pm 
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Joined: Sat Jan 22, 2011 11:22 pm
Posts: 358
Location: London
I know there are a few Lawwill fans here, so I guess this is aimed squarely at them...

Recently, ive decided against buying a new bike and instead Im modernising my trusty four banger. No other full suspension bike Ive tried has a rear end that comes close to the one on my four banger (although I thought the PRST-4 with its quad link and plus fours was very clever, if a little fragile).

Of course there's only so far modernisation can go, and only so many spare parts kicking around on eBay when things break. But then I noticed this...

There's a British Framebuilder, "BTR" who it seems builds jump and free ride bikes in his shed. When requested by a customer who had an old Schwinn Straight 6 kicking about, he came up with this...

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http://singletrackworld.com/forum/topic ... ess/page/4

So, Straight / Banger owners now have an interesting option to keep the bike alive should the frame ever be so borked that weld repairs are no longer an option (ironically, these are slightly easier on the later 7005 alloy frames than on the more desirable Homegrown 6061 frames) and confirmation of something I always knew to be true, the Lawwill rear end on these old bikes is still one of the best around.

This has inspired me. Not so long ago I rode a 29'er, a cannondale I think it was, an I was amazed at just how easily the big wheels roll over rutted, crappy terrain. I've always thought that 26" wheels look a bit small on my XL sized bike, even more so when Im sat on it (6'3" tall), so I started to look at what it would take to fit them to my 4 banger.

The front end is easy enough, its a swap out of forks, but there are some geometry issues to consider there.
The main issue is the lack of clearance in the rear end. theres not really any room to put in a bigger wheel and still have a meaningful tyre mounted on it. The easy way to accommodate the change, if it can be considered "easy", is to fabricate new dropout plates to move the rear axle up and back to accommodate the larger wheel, hopefully without making huge negative alterations to frame geometry;

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A friend of mine owns a machine shop with some serious CNC toys, so I don't think fabricating the parts will be an issue, my only real remaining problem would be where to put the new rear axle line in order to gain the most benefit from the 29" wheels, without compromising the rear suspension operate to much, altering the bottom bracket height to much, making the wheelbase unmanageable, and without ruining the steering (bearing in mind that longer forks for the 29" front wheel may well alter the headtube angle a little to much for comfort)

So, any frame geometry gurus or Lawwill framed schwinn owners here got any informed opinions on dimensions? And would anyone else be interested in a pair of CNC'd 29er dropout plates? (They'd also be IS compatible, this removing the problematic Hayes direct mount 22mm caliper only issue)


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PostPosted: Mon Aug 12, 2013 5:04 pm 
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Joined: Sat Apr 11, 2009 4:27 pm
Posts: 988
Location: Devon
that BTR is mine :-)

I also did the write up on it on here:

http://www.retrobike.co.uk/forum/viewto ... p;t=246390

Quote:
"The main issue is the lack of clearance in the rear end. theres not really any room to put in a bigger wheel and still have a meaningful tyre mounted on it"


This is the biggest problem as you have identified, under full compression big tyres will hit the big carbon bridge on the swingarm, I've mitigated this slightly on the BTR because we moved the main pivot locations a tiny bit from stock but not a huge amount.

Longer dropouts will have other effects on the suspension as it will change the axle path a bit, the 4B and S6s already have a much bigger rearward placement of the axle compared to the original DH4 and DH S6's. The IS mount isn't too much of a problem as the vast majority of the 4B/S6s sold in the UK had the IS dropouts anyway, it was only the very early ones over here that had the 22mm mount.

There's still a fairly active lawwill group in the US who hang around on the MTBR forums who might be interested a 29er conversion but you're going to have to have a proper think about it because keeping the BB height in check without getting silly at the front end is going to be very very difficult unless you're going to get a new front end built as well. just moving the rear axle back and up isn't going to be enough.

If you're really interested in this I suggest you get yourself a program called 'Linkage' and get measuring. Its a suspension modelling program and great for looking at things liek this to work out changes in axle path, spring rates, travel, clearances etc.


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PostPosted: Mon Aug 12, 2013 10:05 pm 
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Joined: Sat Jan 22, 2011 11:22 pm
Posts: 358
Location: London
The BTR is yours? Very cool. I'm quite envious.

Thanks for the input, that's exactly the sort of lead I'm after. I have played with a few force and dynamics apps before, but never with a view to modifying my bike to take bigger wheels. That is a very cool piece of software... I'll give it a try.

As per your thoughts, I have some doubts as to whether this could actually work or not, but I am going to investigate anyway, there are MTB designers out there who've put out some horrifically engineered and terribly compromised frames, so there's at least some chance I'll do better than them!


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