Hello, I want a simple EXCEL SPREADSHEET to recalc bike geometry, when a LONGER FRONT SUSPENSION FORK, is fitted.
Before you suggest a program/website, read below first to the suggestions.
Sounds so simple doesn't it! But it isn't that simple, because a bicycle frame isn't just trigonometry and Soh Cah Toa, because a bike frame isn't made up of RIGHT ANGLED TRIANGLES!
These days fitting longer travel forks and stiffer triple clamps is a fairly common upgrade, but obviously normally we are well happy with the greater amount of travel, but will this mess up the geometry too much? A worthwhile thought (I DON'T WANT A NAFF HANDLING BIKE WITH A DODGY RIDING POSITION!).
~ CURRENT SUSPENSION FORK: ROCH SHOX JUDY: Length = 460mm = 46cm
~ NEW SUSPENSION FORK: ROCK SHOX TOTEM: Length = 565mm = 56.5cm
DIFFERENCE IN LENGTH = 105mm = 10.5cm
I have improvised to measure what the actual head tube/seat tube angles would be approximately, with the following tools:
~ LONG LENGTH, PREFERABLY A LEVEL
~ PROTRACTOR, WITH VISIBLE DEGREE INCREMENTS!
~ AN OBJECT TO CHOCK UP THE FRONT WHEEL TO THE CORRECT HEIGHT! (IN THIS CASE 4 OLD VIDEO CASSETTES = 10.5CM!)
MY CURRENT GEOMETRY approx:
HEAD TUBE ANGLE = 71
SEAT TUBE ANGLE = 73
BTM B/B HEIGHT = 30cm
MY NEW GEOMETRY approx:
HEAD TUBE ANGLE = 65 (Difference 6 degrees)
SEAT TUBE ANGLE =67 (Difference 6 degrees)
BTM B/B HEIGHT = 34.5cm (Difference 4.5cm)
But I want an EXCEL spreadsheet to do the above for me, so I have searched the internet, and found three partially suitable programs/websites:
(A) ~ GeoCalc
(B) ~ BIKECAD
(C) ~ Martin Manning's Bicycle Geometry 101:
http://www.os2.dhs.org/~john/Bicycle_Ge ... 1_v801.xls
The above don't entirely meet my basic purpose, hence comments below:
(A) GeoCalc: SIMPLE and RELEVANT, compare current setup (L/H) with new LONGER SUSPENSION FORK setup on (R/H). However this does NOT auto calc that a longer fork will raise the bottom bracket higher from the ground, hence without this "correct b/b height length", then the quoted angles will be incorrect. With it, looks good.
(B) BIKECAD: PRETTY, WITH LOTS OF OPTIONS, BUT THE FORK OPTION EVEN IN CUSTOM MODE FOR A LONGER SUSPENSION FORK, does NOT auto calc the new HEAD/SEAT TUBE ANGLES, and neither does it auto calc what the new b/b height will be either. With it, looks out of proportion of the example I did.
(C) TOO MUCH INFORMATION, AND DATA TO INTERPERATE. I need something much simpler. Not criticising, frame designers may need this amount of detail, because obviously a frame is more then just lengths, it's curved tube sets, chain/seat stays, and a fork that clamps around wheels. Obviously there are many normal unsymmetrical (Not the same of each side) inclusions such as, a DRIVECHAIN SIDE (R/H) and a NON DRIVECHAIN SIDE (L/H), hence side specific rear dropouts with a gear hanger on Rear R/H dropout, gear and brake cable/hydraulic brake cable routing stops over the frame & fork, also there will be disk brake caliper mounts on the L/H (NON DRIVECHAIN SIDE) of the frame, and also the fork as well, etc. Also there should be bottle cage/crud catcher mounts, etc.
Q1. So does anybody have a simpler EXCEL SPREADSHEET CALCULATION/FORMULA to work out the new geometry after fitting a longer fork? (NOTE: To include new BB HEIGHT, HEAD/SEAT TUBE ANGLES!)
Q2. Also, beyond what head/seat tube degree is thought to be too slack? (The term slack, seems, ironic or misplaced as the frame would look steeper as the head/seat tube would be less vertical.
The bike in question is a 2003/2004 16" CLAUD BUTLER CAPE WRATH, the approximate current geometry is as follows:
HEAD TUBE ANGLE: 71
SEAT TUBE ANGLE: 73
HEAD TUBE LENGTH: 115mm = 11.5cm. H/TUBE TOP EXT L: 20mm = 2cm. H/T BTM EXT L: 50mm = 6cm
TOP TUBE LENGTH: 580mm = 58cm
DOWN TUBE LENGTH: 670mm = 67cm
SEAT TUBE LENGTH: 350mm = 35cm (= 14" + ext = 16.4")
SEAT TUBE EXT LENGTH: 60mm = 6
CHAIN STAY LENGTH: 435mm = 43.5cm
SEAT STAY LENGTH: 470mm = 47cm
B/B axle to GRND: 300mm = 30cm
WHEELBASE LENGTH: 1065mm = 106.5cm
SUS FORK LENGTH: 460mm = 46cm
MTB routes in/near Hull, Humberside
CROSS POST REF's:
http://www.bikeradar.com/mtb/forums/vie ... 0#17024860