Saturday, 8 October 2011

Numbers and the Frame Jig

I have now done enough research on the frame to be happy with my chosen figures, the dimensions have chosen are ones that I either know to work, or are features fom other bikes I have ridden that I liked and wanted to incorporate into my own design, or are more feasible for me to construct with fairly basic engineering tools as most of the build will be in my shed. Welding it will most likely be done at my dads workshop though as there is a TIG welding set there and that opportunity shouldn't be missed.

The angles I have chosen are in my first blog, but here they are again, all inches and millimetres
Head Tube Angle - 65 degrees
Seat Tube Angle - 74 degrees
Wheelsbase - 45 inches/ 1143mm
Chainstays - 17 inches / 432mm
Front Centre - 28 inches / 711mm
Seat tube - 16 inches / 406mm
Rear Wheel Travel - 6 inches / 152mm
Shock Stroke - 2.25 inches / 57mm
Pivot to Rear Axle - 20 inches / 508mm
Pivot to Shock mount - 7.6 inches / 193mm
Pivot Location - 3.9 inches / 99mm above line of wheelbase, 5 inches / 12.7mm outside the BB centre
BB Height - 0.25 inches / 6.35mm below line of wheelbase

Did a cleaner Drawing of the main points on the design

Frame Drawing, done with Pencil, Ruler and Protractor, No CAD here

Once i knew what size and shape it was going to be i could then set about making up a jig. Jigs are needed to ensure that things go together in line and to the shape they were designed. i proably could do it freehand, the stuff i make at work isn't jigged and my angles have to be +/- 0.3 of a degree which i ahchieve regularly, when you consider a top end frame like the trek session 88 is only done to 0.5 degree tolerance it gives you some perspective.

My jig needs to hold the following parts tightly in position, in a way that still allows access for a welding torch:
Head tube, Seat tube, Bottom Bracket and dropouts once the swingarm is welded up seperately. this will need its own jig. WORK!
So here are the pictures of the finished article:
Full jig

Full Jig on kitchen floor.

Fork length Adjustment, currently set at 546mm for a Marzocchi 55 set at 160mm travel

Fork height adjustment via an old boxxer crown

BB clamp piece. think I'm going to revise this as it isn't how I really want it

Rear Dropout jig holder. this is part i'm pretty happy with. is simple, square and goes about its job quietly

 I put my hardtail frame in the jig just to alignment check it. Everything seems good.

And again led down, just to show off the angles and how it goes in.

Friday, 7 October 2011

Ste Tomlinson and the quest for a home made trail bike to razz

Hi Everyone and thanks for taking the time to read about my Home Made Trail Bike.
Just a little bit about myself, my name is Stephen Tomlinson and I'm 27 years old, married to my lovely wife Nicola, and we live with our dog Jasper in a 2 bedroomed terrace in happy, sunny, Clitheroe, Lancashire. I work making Conservatory components and spend a chunk of my free time messing about with mountain bikes.
I have been into MTB's for over 15 years now and see an awful lot of changes and fashions in mountainbiking, some, such as front derailleurs and ISIS cranks, have fallen by the wayside for me and most of the people I ride with, but otherss, like hydraulic disc brakes and decent suspension have stuck around bringing new found speeds to riding bicycles off road. However, I do feel a lot of the bikes you see at the trail centres are not as good as they could be for me and feel they've become very expensive for what they are, essentially still in development until the development of bikes reaches a sort of plateau like road bikes almost have. It is this, along with my love of things home made or modified that I am starting out my process of building a trail bike that will a) be relevant for years to come, and b) bring me a new sense of enjoyment from mountain biking.

Over the last six months I have pondered over making my own bike, knowing it was possible and i had the skills to do it. The only thing that was stopping me was my own motivation. Then along came a man called Adrian Bedford on the Dirt Mountainbike Magazine website with his own version of a home made bike. It looked cool, carbon rear end, mated to a steel front trangle. 150mm travel, slack head angle and a low centre of gravity. It was likewhat I wanted to build. Except for the carbon bit. I'd rather use steel all round.

I knew you couldn't build a decent bike frame without some sort of jig to hold all the tubes in place as you construct it so I knew I'd need to make one of those but I couldn't make one of those without knowing my numbers....

Coming from a mostly Downhill based background and this being the way our lot at Chatburn Downhill ride at trail centres, the climbs being a necessary evil to get to the descents on the other side. I ride a Kona Howler 4X for XC rides, and although it's relatively well behaved, it could do with a bit more than its 4 and a half inches of travel via a longer stroke shock and the top tube could do with being a little lonnger to help my back on the longer climbs. Bottom bracket is nice and low and you feel properly 'in' the bike through the turns which is ace, really like that part of it.
I know I want it to feel closer to my downhill bike than most trail bikes do, stability wise so i'll bring the headtube close to that at 65 degrees and put the rear end a bit shorter so the front wheel will be a bit more ready to come up. Now all my measurements are becoming vague, and I sympathise with those in the industry who have to develop bikes. You don't know what works until you try it- to try it means you need to physically make it, and to make it gets expensive. All pushing up the price of your off the peg All-mountain bike. Oh Dear.

Decided to bite the bullet and plump for :

- 65 degree head angle with my Marzocchi 55 RC3 (546mm axle to crown)
- 16.75 inch chainstays
- 45 inch wheelbase - slightly shorter than a Nukeproof Mega
- 74 degree seat tube angle, slacker doesn't help climbing and the rear wheel curve can bump into it with shorter chainstays
- Bottom Bracket in line with the wheelbase, works out about 13.5 inches, depending on the tyres fitted. using a set BB height can be altererd with bigger or smaller tyres so its best to use a constant measure like the wheel line.
- 135mm dropouts and 68mm bb shell. more component options. highly likely i'll use DMR swopouts too just for options on wheel usage.
200mmx57mm  fox air shock

I put these altogether on paper because i don't know how to use CAD and it came out like this:

The next step will be building the jig. hope to have that finished soon.