LONGER SUSPENSION FORK: Geometry recalc using EXCEL?

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!).

For example:

~ 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
http://bikegeo.muha.cc/

(B) ~ BIKECAD
http://www.bikeforest.com/CAD/bikeCAD.php

(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.

NOTE:
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

Cheers

Stephan Rands

MTB routes in/near Hull, Humberside

www.srands.co.uk


CROSS POST REF's:
http://www.bikeradar.com/mtb/forums/vie ... 0#17024860
http://www.mrexcel.com/forum/showthread.php?t=558219

so why do you need this infomation ,and what are you gonna do with it?
geometery is not the main fasctor in how a bike rides ,as a rider will
compensate for the bikes defficencies ,not saying its a be all and end all
but you cant excell that factor

It would be easy to :

A) Get access to a smart phone.
B) Get a level app.
C) Sit on the bike in a normal position.
D) Get a mate to check the angles.
E) Put required thickness of books underfront wheel.
F) Repeat C & D.

The change in height / angle will pretty much be constant for the bike you checked it on (ignoring sticktion differences as the fork angle moves and the fact that the fork isn't vertical to start - but you could you trig to approximate that) so you could then have a simple straight line graph for the bike you tested.

How accurate do you need to be ... Angles won't mean sh1t if you don't like the ride

WD

srands":17ovb9de said:

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!).

Click to expand...

You don't need Excel to see that your bike is going to ride badly (seat tube angle is the worst issue if you want to pedal it on anything other than downhill). You are also likely to shear the headtube off the frame as it won't be rated for the forces of a long travel for dual crown or otherwise.

If you are after a longer travel bike, buy the frame which is rated for the fork you wish to use rather than chucking it on any old frame...

and the maths is actually pretty simple (the hand calcs only take a minute or so) so you could write it in excel yourself if the other applications don't do what you need

CALCS? Could you write them in a message?

andrewl":2eikj8hh said:

and the maths is actually pretty simple (the hand calcs only take a minute or so) so you could write it in excel yourself if the other applications don't do what you need

Click to expand...

OK really, so what would be the hand calcs?

To calc what the new dimensions will be:
~ HEAD TUBE angle
~ SEAT TUBE angle
~ BTM BRACKET HEIGHT

Cheers Stephan

A bit like this

For 1" rise

HA = -1.25deg
SA = -1.25deg
BB = +.36"

give or take a bit depending on frame size. but tyre/weight on tyres/pressures probably vary that difference anyway.

That is for Front Sus only, Full sus is a different ball game as I assume the rear should counter the front then

You also don't seem to be factoring in the fact that on a downhill when you most need a suspension fork, the fork is compressed so the static a/c lengths are largely irrelevant.

take into account sag of 25-30% and that 105mm difference will drop considerably.

Factor in the lengths of the forks when they're bouncing about on a trail and the numbers will be even closer.

I have to say though that I think your getting too Hung up on the numbers. many riders run forks with adjustable travel from 80-120mm and dial the forks in for whatever terrain they are riding with no adverse effects on geometry at all.

WHAT HEAD/SEAT TUBE IS TOO SLACK?

FluffyChicken":7q72ylx9 said:

A bit like this

For 1" rise

HA = -1.25deg
SA = -1.25deg
BB = +.36"

give or take a bit depending on frame size. but tyre/weight on tyres/pressures probably vary that difference anyway.

That is for Front Sus only, Full sus is a different ball game as I assume the rear should counter the front then

Click to expand...

Hi Ben, I like that CALCULATION, is it that something you've derived? Or seen it somewhere? And your conversion calcs seems accurate with my following calc's/dimensions:

So I have a question, I don't ride many bikes often these days, as it's many years since I was mechanic at Halfords, getting my hands Carrera BugaBoo, and other peoples bikes. So WHAT HEAD/SEAT TUBE IS TOO SLACK?

My point being fitting SUSPENSION FORKS is meant to be a good thing, reducing impacts and making riding less fatiguing, but WHAT IF the new LONGER LENGTH suspension forks, changed the GEOMETRY enough to mess up the weight distribution of the rider. PERHAPS Suntour, Roch Shox, Marzocchi and Fox, realise this and that's why there is no GEOMETRY onlinc calc on their websites, or an approximate GEOMETRY calc for owners to do their own manual calc's on how fitting a suspension fork will affect the GEOMETRY of their bike.

My only real question is: WHAT HEAD/SEAT TUBE ANGLE, is TOO SLACK?
(In theory of course, my 10cm travel ROCK SHOX JUDY's are brilliant, as they are.)

FIGURES and PHOTOs explain it better then WORDS:

RIGID FORK SETUP

RIGID FORK: 44cm (Crown to Axle):
Head Tube Angle: 72
Seat Tube Angle: 74
B/B Height (Axle to Grnd): 29cm



SUSPENSION FORK 10CM SETUP

SUSPENSION FORK 10cm TRAVEL: 46cm (Crown to Axle):
Head Tube Angle: 71
Seat Tube Angle: 73
B/B Height (Axle to Grnd): 30cm

SUSPENSION FORK 18CM SETUP

LONG TRAVEL SUSPENSION FORK 18cm TRAVEL: 56.5cm (Crown to Axle):
HEAD TUBE ANGLE = 65.8
SEAT TUBE ANGLE = 67.8
BTM B/B HEIGHT (Axle to Grnd): 34.5cm

Cheers

MTB routes in/near Hull, Humberside

www.srands.co.uk

On my Altitude (as you know that bike) it is designed around a ~390mm fork. (It is 1991 after all). But I've always ran MAG's on it, mainly as I got it that way, which at standard travel are 405mm in length, sag on these is negligible in use
But this drop the angle from 71 to ~68.5 (never actually measured it)
But on your other post I added a screen grab. Measurements are not quite true, but somewhere near there


For about a year I've been using early JUDY forks, 60mm and these slackened everything off. It was ok for general riding, but it just lacked something and it wasn't quite right. So the MAGs are back on and it now feels 'correct'
So to me that is to much for my type of riding.#I could have tried a shorter stem to liven up the front but to be honest that wasn't the biggest problem. for me it was the seat position, I had to run it far forward to get to pedal position right for me.

But other consequences are the front is higher, the effective reach is reduced.
To compensate for the next few years the moved angles to 71.5, and same for the seat tube iirc. This worked with short travel forks until 60mm became norm in 95-ish.
top tubes also got longer a little (to counter the reach I guess).

I wouldn't go any more than the 60mm on these early frames, not least because it looks daft.

Now as you go longer you get other problems. How do you define the angle and manufacturers don't seem to state that.
Some might state it as off the bike (as the did when travel was down at 40/50mm). It could be after sag, or normal usage level.
You then get the problem if it's '71' off the bike, you're then step in use and get steeper the further down the fork travel you travel getting a twitchier and twitchier front end and sliding of your seat

So when your at 100/150mm it get complicated and way beyond anything I know.

If you go to most websites now you'll find they specify a range of fork travel (or the better one's A2C length) it'll fit and work well. The longer for r the All Mountain crowd and the shorter for the race/XC crowd.

You can compare two bieks at Orange
the R8 an P7
http://www.orangebikes.co.uk/bikes/r8_pro/
http://www.orangebikes.co.uk/bikes/r8_s/

Ragley have a different take
http://www.ragleybikes.com/geometry/
with very slack angles after sag.
Ignore their seattube angle we are different blurb as the frame gets bigger, companies have been doing that for ages.

I also remember another uk company that had a nice bit but cannot remember the name (not On-One or PACE)

EDIT remembered
http://www.sanderson-cycles.co.uk/