LONGER SUSPENSION FORK: Geometry recalc using EXCEL?

[srands]

SAG? RIDER WEIGHT?

OK, well since Claud Butler (Falcon) at the time didn't state the GEOMETRY in their 2004 Catalogue, I had to measure the Geometry myself, WITHOUT me sat on it, so no sag to speak.

But since I don't weight alot there isn't much sag, the ROCK SHOX JUDY's are quoted as 10cm travel fork, last week I got 8cm of travel, somewhere between St. Andrews Quay and Fort Paull.

I guess a heavier rider, would have more sag, hence maybe less effective travel, and more likely to bottom out (Maximum travel), something I haven't done yet (Thankfully), but round here that isn't surprising, but I do like drop off's so any bigger I could do with full suspension (But I guess the bike mags will still be saying they'd use a small hard tail for the such).

CLAUD BUTLER CAPE WRATH 2004 16":
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

Claud Butler Falcon cycles, NEW RANGE:
http://www.falconcycles.co.uk/Corporate ... hD27.shtml
http://www.falconcycles.co.uk/CORP/cb/capewrathD27.html

Glancing at the new range the GEOMETRY for their latest 16" is a far bigger bike, not enough crotch clearance, and too long for me:
EG: Cape Wrath 03 (2011)
Sizes: 16"
Seat Tube: 406mm (5.6cm LONGER!)
Wheelbase: 1178mm (11.3cm LONGER!)
Head Angle: 69.5 (1.5 DEGREE SHALLOWER!)
Seat Angle: 73 (SAME)

Cheers Stephan

MTB routes in/near Hull, Humberside

www.srands.co.uk

 

[FluffyChicken]

There should be dials somewhere to set the pre-load, that sets the sag height. Think they are at the top on the JUDY's of that era (well earlier on they where, doubt they changed it)
If a much heavier person used it they would need to buy some stiffer springs

You should be able to find the setup and service manual here
http://www.sram.com/service/include-arc ... shox/7,413

I think the JUDY 100mm should be at around 20mm sag.

 

[srands]

SAG -> 1 TOP CAP PRELOAD ADJUSTER

Thanks for all the archives Ben, from Rock Shox, ah yes Judy, that's what I want.

But on my Rock Shox Judt TT 2004, there is only ONE, TOP CAP PRELOAD ADJUSTER, on the L/H crown:
Clockwise ~ Increases spring load
Anti-Clockwise ~ Decreases spring load

My fork is described as a DUAL STAGE, COIL and ELASTOMER SPRING SYSTEM. And they're the best suspension forks I've ever ridden, far better then my Marzocchi Bombers, they were too soft.

Cheers Stephan

MTB routes in/near Hull, Humberside

www.srands.co.uk

 

[andrewl]

Re: CALCS? Could you write them in a message?

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

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

Sorry I went to bed before i saw your reply. I'm not going to do the calculation for you but the basics are as follows:

1) The distance from the centre of the rear hub to the bottom centre of the head tube will remain a fixed constant whatever fork you run.
2) The angle of the head tube relative to this theoretical length is also fixed.
3) Using the fork offset and length you have in short the one angle and the length of two side so you can solve the other angles and determine the new wheelbase using simple trig.

I've done this calc many times and its where the general rule of thumb that a 20mm longer fork will slacken the angles by 1 degree comes from - it is however a rule of thumb as smaller frames are affected more than larger ones when it comes to geometry changes.

 

[andrewl]

Re: SAG -> 1 TOP CAP PRELOAD ADJUSTER

My fork is described as a DUAL STAGE, COIL and ELASTOMER SPRING SYSTEM. And they're the best suspension forks I've ever ridden, far better then my Marzocchi Bombers, they were too soft.

I'd say your Bombers were poorly set up if an elastomer sprung fork is the best you've ridden! :shock:

based on your posts I'd say you should throw your leg over a modern long travel dual suspension bike or hardtail and then compare it you your old bike.

There is also I'm sorry to say little you can do to make you bike ride like a modern longer travel bike so save your money and leave your current bike as is.

 

[srands]

MTB GEOMETRY CAD

OK after all that I think the best MTB GEOMETRY CAD programs/websites are those I mentioned earlier:

http://bikegeo.muha.cc/
http://www.bikeforest.com/CAD/bcad.html

A number of other IMPORTANT FACTORS we haven't mentioned here much 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."

 

[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: [email protected]

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

 

[andrewl]

Reading through both of your threads on this subject, I'm still wondering what you are actually trying to achieve.

I know you want to put a long travel fork on your existing frame, but you haven't really said what you are trying to achieve or trying to ride. Once you define that then the best frame and fork for the purpose will be a much simpler thing to define.

Playing with the geometry and ride is good fun, but you have to have the right basic frame/fork combination to start with otherwise you are just wasting time.

For example my most adjustable bike (2002 Turner RFX) is rated for a fork in the range of 125mm - 180mm (and dual crown forks), has 5", 6" (x2) and 7" rocker plates for varying the travel (and bottom bracket height and angles in the case of the two 6" rockers. With and adjustable fork up front (140-170mm single crown) and being able to swap between rear shocks which have different stroke for the same eye to eye, there is a ridiculous amount of variation to cover different riding styles and terain.

 

[srands]

Much longer suspension forks is too much of a trade off

Anyway normally fitting suspensions forks is a good thing.

But having much "longer suspension forks" (10.5cm longer, excluding steerer), it ain't ALL GOOD!

THE CONS OF FITTING MUCH LONGER SUSPENSION FORKS:
For myself it's too much of a trade off (IE. Despite changing to increase travel with longer forks some subtleties are lost (Not so obvious apparently minor changes that make drastic changes):
~ Slackening head/seat tube
~ Reducing my crotch clearance by 2": From 10cm (4") to 5cm (2")
~ Reducing my seat post height by 2": From 8" (20cm) to only 6" (15cm)

THE PROS OF FITTING MUCH LONGER SUSPENSION FORKS:
~ More travel
~ Higher Bottom Bracket (raised by 2" = 5cm, from: 30cm, to: 35cm approx)

The above I didn't conclude from a GEOMETRIC calc, but by improvising (Raising the fork height ~ See start of this email), then measuring the height difference of the BTM BRACKET from the ground.

So to summarise I leave my the bike the way it is, as the CONS out number the PROS: Especially considering my height, my frame geometry, my leg length (Seat Post height would have to be lowered) and especially MY CROTCH CLEARANCE would be less, which means alot to me, since I'm short.

Q. How do you feel about your CROTCH CLEARANCE?

Cheers Stephan

MTB routes in/near Hull, Humberside

www.srands.co.uk

 

[mikee]

the thing you are missing here ,or passing over is that you and me will
ride a bike differently just because head angles and seat tube angles ,rake
and trail travel etc are what ever they are you cannot say for sure how
a bike will ride
the only sure way to judge a bikes charecteristics is to get on it and ride
sure try 7" travel tripple clamp forks on your bike try 390mma/c rigids
i bet neither are what you imagine
factor in the fact that you may not even be able to analise the changes
to this and you prove nowt