LONGER SUSPENSION FORK: Geometry recalc using EXCEL?

srands

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
http://www.retrobike.co.uk/forum/postin ... &p=1158754

an_original_name

i'll be honest, i didn't read all of that. but am i right in saying you want to fit a set of rockshox totems onto a bike designed for a 100mm fork? if you are thats a very bad idea. you'll likely rip the frame in half pretty quickly.

jj1048

General rule that I apply is +10mm on the a2c = -0.5° off the head angle; I wouldn't want to change the head angle by more than 1°.

I find this site quite useful: http://bikegeo.muha.cc/

The Northern Monkey

an_original_name wrote:

i'll be honest, i didn't read all of that. but am i right in saying you want to fit a set of rockshox totems onto a bike designed for a 100mm fork? if you are thats a very bad idea. you'll likely rip the frame in half pretty quickly.

Brilliant... I didn't read any of it, but totally agree with your recommendation.

Putting Totems on a bike made for 100mm suspension is a stupid idea, regardless of the geometry.

nicklouse

Bad Idea. but if you do please have a video camera filming.

srands

OK you've sensed WHAT I'm working upto.

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

supersonic

Download Linkage suspension simulator, does all this for you.

Steepnesss/slackness is purely personal preference, though the bike is designed to work best around 100mm of travel or a crown to axle measurement of 470-490mm static. You can go a little higher, or a little lower and most people find it handles fine.

Fitting triple clamps or going over 120mm will invalidate your warranty, probably make it handle horribly, and you'll damage the bike and you. Don't do it.

YOUR FRAME IS NOT DESIGNED FOR IT!

The Northern Monkey

srands
What you're forgetting to incorporate into your interesting pictures, is that if you fit a 180mm for to a frame designed for 100mm, then the frame is likely to snap or it will just be shite and unridable.

A longer length fork will put higher stresses on the frame.

Oh, and it looks ridiculous.

warpcow

srands wrote:

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.

I think you're missing the point (unless I've missed your's somewhere). Different suspension forks, with different amounts of travel, are designed for different purposes/types of riding. A 100mm fork on an XC-ish bike will fulfill all those good things you mention if used for 'XC-ish' type riding. A 180-200mm fork on a DH/FR type bike will do the good things for DH/FR type riding.

Then you get the slightly more complicated stuff of different 100mm forks, for example, being designed for different purposes: say a lightweight XC-race fork versus a stronger dirt-jump fork. They're the 'same' in your way of looking at things, but not really.

There's still a lot of generalisation above, but basically I think it would help you to get around thinking of "suspension forks" as one "thing/grouping" that serves only one purpose.

supersonic

approximate GEOMETRY calc

Easy. 20mm of static length increase of fork gives approx 1 degree of angle change.

But as above, you are thinking of this the wrong way.

captainfly

I don't quite get the point of this thread, if it is all about the maths then fait enough it's anteresting thought. Frame geometry is all about handling which is a black art in just the numbers but quite intuitive when comparing how bikes feel. If you want o run 180mm single crowns buy a frame that can take them like .243 or DMR exalt. it will be a pure garity sled but why else would you want 180mm of travel?

ricardo_smooth

I think you need to stop shouting in random places and listen to the simple advice that you'll end up with a face full of dirt if you fit the forks to the propose frame


Like this!!!!

http://www.youtube.com/watch?v=FDZgOvoen5E&feature=related

getonyourbike

I think fitting 180mm forks to your bike is a great idea.

What is the point of this thread anyway?

Johnny Napalm

I have an old Cape Wrath '03, which was a great bike when built up (in bits at moment, but will one day see the light again), and I would never dream of sticking Totems on that frame.

I originally had Judy forks on, but upgraded to Tora forks after a while, which I generally ran at 100mm of travel...even though they were 80mm-135mm. It's a great little frame but not designed for those forks,

leaflite

As above, frames and suspension forks have a purpose. If you put 180mm travel forks on a frame only designed for 100mm, at the very least it will ruin the handling. Likewise, if you put 100mm travel xc forks on a downhill bike designed for 200mm travel forks, it will ruin the handling too.

leaflite

As above, frames and suspension forks have a purpose. If you put 180mm travel forks on a frame only designed for 100mm, at the very least it will ruin the handling. Likewise, if you put 100mm travel xc forks on a downhill bike designed for 200mm travel forks, it will ruin the handling too.

cooldad

Just buy this
http://www.bikeradar.com/forums/viewtop ... t=12784816

The Rookie

It wouldn't be that difficult to do the excel spreadsheet you want, the reason you probably can't find one is that its pretty pointless, also a lot of people won't have all the numbers accurately enough that they need to start with such as wheelbase, head angle (at a known axle to crown race dimension), current axle to crown race dimension (less sag) and trail from axle centreline to steerer centreline and the new forks same dimensions, without all that data, the spreadsheet is pointless!

Simon

TuckerUK

I've skimmed through the thread, but I'll just add that I have read (and believed ) the +/- 20mm guideline, but one also needs to take into account fork length, and sag. As an example a 2004 120mm Marzocchi MX is only 5mm longer than a 2007 100mm model. And of course a longer travel fork run with more sag (plusher ride) might actually be running at the same height.

supersonic

That is why I quoted static (none sagged), which is what many manufacturers use.

While it is true that extra sag can compensate, the fork can still fully extend at times and your sag is greatly reduced when climbing. 65 degree head angle, and 67 degree seat angle when climbing steep hills will have you looping out backwards in a jiffy.

nicklouse

supersonic wrote:

That is why I quoted static (none sagged), which is what many manufacturers use.

While it is true that extra sag can compensate, the fork can still fully extend at times and your sag is greatly reduced when climbing. 65 degree head angle, and 67 degree seat angle when climbing steep hills will have you looping out backwards in a jiffy.

and to add to Sonics comments you still have to compress it from fully extended and that end up putting extra loads on the bushes and the headset and the frame.

It is just a bad idea.

wordnumb

From the perspective of quantum mechanics and the eventual heat death of the universe(s) you should not fit RS Totems to a Claud Butler Cape Wrath. People will laugh at your bike, it will handle like a dog and bring about the apocalypse.

supersonic

Or as Dr Brian Cox woudl say: If you take this rock, a big one, and smash it against your head, that is what it will feel like when the frame snaps.

Anonymous

supersonic wrote:

Or as Dr Brian Cox woudl say: If you take this rock, a big one, and smash it against your head, that is what it will feel like when the frame snaps.

PMSL

wordnumb

supersonic wrote:

Or as Dr Brian Cox woudl say: If you take this rock, a big one, and smash it against your head, that is what it will feel like when the frame snaps.

But then, taking the default position of random internet warrior trying to prove himself smarter than d:reamy physics boy, Dr Cox would be wrong. He would have failed to take into account that although the face/rock particle acceleration would produce an approximation of the pain and facial rearrangement, it would mask the simultaneous ruined frame and forks cost-effect and would circumvent the walking home in shame clutching bits of twisted metal phenomena.

bonezy

but WHAT IF the new LONGER LENGTH suspension forks, changed the GEOMETRY enough to mess up the weight distribution of the rider.

You've just answered your own question...

It's the random shouting that I can't get my head around :?

Either way it's a terrible idea, sure to end in a £250 You've Been Framed Moment and the pleasure of Harry Hill taking the pi**...

srands

OK I initiated this question so this chat discussion is on the internet about a subject that needs more clarification, for those who don't know!

A number of other IMPORTANT FACTORS we haven't mentioned here is:
~ RIDER HEIGHT
~ LEG LENGTH
~ CROTCH CLEARANCE (STANDOVER)

At 5'7", with leg length 28" approx, my 16" Claud Butler Cape Wrath (Really 13.5", but with seat post extension measures 16") I have 4" crotch clearance.
I.E:
~ RIDER HEIGHT = 5'7"
~ LEG LENGTH = 28"
~ CROTCH CLEARANCE (STANDOVER) = 4"

I know if I fitted a LONGER TRAVEL FORK (10.5cm longer then current suspension forks), this would radically slacken the head/seat tube, and reduce crotch clearance to less then half, approx 2". Hence the bike is good/better as it is currently, for my ride quality.

Also KONA replied to an email I sent them "Long forks are to be avoided. The slack angle means the rider’s weight is back and the front tire is un-weighted. This means it looses traction or washes out on descents or wanders and wheelies on climbs. Also the fork gets more energy put into bending it instead of compressing it so the damping control can be compromised. The right angle to keep the head tube is more of a soft science than a hard and fast rule. It depends on the type of bike and the rider."

supersonic

know if I fitted a LONGER TRAVEL FORK (10.5cm longer then current suspension forks), this would radically slacken the head/seat tube, and reduce crotch clearance to less then half, approx 2". Hence the bike is good/better as it is currently, for my ride quality.

Yes. So don't do it. Plus all the other reasons we have given.

srands

OK this is a very serious discussion as nobody would intentionally want to ruin the handling of their bike and be a total waste of a new fork etc.

As we've said go for a SUSPENSION FORK too long for your frame geometry will ruin the handling, but WHAT ANGLE (+) is THOUGHT TO BE TOO SLACK?

But seriously front suspension with a decent geometry, is a good thing.
The danger is that people upgrading forks or having custom built frame according to their spec, they can get it well wrong!

Anyway remember rigid forks!

It had Cannondale Pepperoni alu forks (1/4" Evolution ~ Pre-Aheadset), they weighed a wafer light 750g!

Would I go back to RIGID FORKS? No way they're too unforgiving!

Never mind that suspension forks are heavier, the comfort factors of reducing impacts and reducing fatigue are far more important.

COMPARISON NOTE:
RockShox Judy TT (2004)
Travel: 100mm
Weight: 3.96 lb. (1.80 kg = 1800 Grams)

So my FRONT SHOX are about 1050g heavier (1kg approx) then my ULTRA LIGHT RIGID FORKS, I think that is very impressive of my front shox.

srands

SUNTOUR SR's email reply:
"as a service technician of SR Suntour forks I can only speak for SR Suntour. I assume that our competitors forks are built similar in some points but differencies may occur in other construction details.

For our higher end forks it´s like you say. The spring (air or coil) is in one fork leg, and the damping system is in the other, so both fork legs have different duties to do. So adjustment for softness / hardness can only be done on one side, and adjustment for rebound, compression or lockout can be done on the other. However, please see not just the fork but the whole system containing the fork, the front wheel which is in both dropouts and the (hopefully stiff) skewer which is connecting both. This system altogether is pretty stiff and making the wheel always push on both fork legs together. So, having the spring in only one side is no problem for the fork. We actually have forks with springs on both sides, but these are more entry level forks without oil damping.

Regarding the fork travel some of our forks have adjustable travel, one example is our Epicon model. The travel can be adjusted from 80 to 140mm. The stanchions and the oil damping cartridge are long enough to cover all possible travel setups, and of course the fork is built strong and stiff enough, so even with the maximum travel (and more leverage to the fork crown) the fork can handle it without problems. To adjust the travel an internal setting can be changed and the built-in-height of the fork is changing by the same amount. So. if you change the travel from 100 to 140mm will change from 481mm to 521mm. When we talk about XC or Marathon forks with about 100mm of travel, then a triple crown is certainly not needed. For DH forks with like 200mm of travel, which have to take some serious beating, a triple crown might make sense to keep the necessary amount of stiffness. However, higher stiffness can also be achieved by bigger outer diameter stanchions, so you can find lots of fork with like 180mm of travel with single crown too.

Every frame designer is making his frame construction based on a certain built-in-height of the fork he wants to use. Small tolerances are no problem. Our XC forks with 100mm of travel are all in between 475 and 485mm of length (on 26 Inch versions of course). I guess that the length of our competitors are in the same area, so typically changing the fork from one brand to another is no problem when you keep the same amount of travel. A basic “rule” is that for every 10mm more in length your head- and seat angle is going down by 0.5°. So, in most cases it´s even possible to use a fork which is up to 20mm longer than the original one. If you change the height of your fork by too much, and so also your frame geometry, you will get negative effects on the handling of the bike, like some chopper-like feel of the steering if your fork is too long. This will not handle better at all, not even in steep terrain. However I don´t think it´s possible to say how much of angle might be OK or how much change of height is OK until you get negative handling effects. Since frames and forks can be different there might be situations where a 30mm longer fork will handle OK, while on other frames a 30mm longer fork will be a NO-GO. It´s true that other kinds of bikes (like DH bikes) might have different head angles too, but you won´t be able to turn a XC machine into a DH bike. DH bikes have different geometry, different rider position, different suspension setup and for sure also different handling needs.

Hope this helps,



Best regards,



Bernhard Scholz

Service & Promotion

SR Suntour Europe GmbH
Am Marschallfeld 6a
83626 Valley
Germany

Tel: +49 (0)8024-47399-0 (Ext.2)
Direct Tel: +49 (0)8024-47399-16
Fax: +49 (0)8024-4730984
e-Mail: bernhard@srsuntour-cycling.com

web: www.srsuntour-cycling.com
SKYPE: b_scholz"