Tim's guide for putting a ridgid fork on a bike

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TimT
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Tim's guide for putting a ridgid fork on a bike

Post by TimT » Sat Jul 03, 2010 5:58 am

OK I've been asked this a lot lately. It goes something like this. "Tim will brand X fork fit my bike with a _____ size wheel" My answer goes something like this "welllllll I don't know". You see to answer that question I need some more info. First piece is what is your "axle to crown" measurement on your currant bike with you sitting on it. Why sitting on it? If you have a suspension fork it will compress and change the measurement this situation is called "sag". But wait what is the fork "crown" and "axle"
Image
The crown is were the bottom race for the head set sits. the axle is were the hub rotates. So in the above picture that measurement is 425mm. I know its blurry but trust me. From now on axle to crown will be know as AC its part of the secret frame builders lingo so don't tell any one cause they will kick me out of the club. So the second measurement is Axle to Ground.
Image
That's the measurement from the axle of the bike to a flat surface. That from now on will be know as AG. For the record that AG is on my daughters 26" rim with a 2" tire and its AG is 330mm. Oh yea by the way you should do all these measurement with a metric ruler. All forks that I know of have these measurements in metric. So its do my kids bike. Its a Redline PL26 with a Manitou Mars Elite MRD 80mm travel fork with a 26"x2" tire. For the record she's at grandmas for the summer and sense I'm by my self we will do this dry so its just an example you of course wlil be on the bike with a buddy doing the measuring. Oh you have no friends ....try beer.
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So I get a AC of 460mm Her AG is 330mm (above picture) so now I add the two and get a new measurement called Crown to Ground which will now be know in the secret lingo as CG. This CG is 790mm. So now you subtract a know AG of the tire/wheel combo you wish to put on the bike. I went and measured some so don't panic just yet.
Image
The AG for a 650Bx2.1 is 345mm
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The AG for a 29x2.1 is 370mm
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the AG for a 29x 2.55 is 375mm This is the biggest AG that I know of.
So lets say you want to try out a 650B. 790mm-345mm=445mm This is your new AC measurement. You need to look for a fork with at least a 445 AC to get about the same handling characteristics as your currant set up. You may not find anything dead on but close is good enough as your suspension fork travels your AC changes. So don't panic. Rule 1 an taller AC will slow the steering down. Rule 2 a shorter AC will speed steering up. So if I find Brand X fork has a 450AC that will slow the steering . with Brand Y's 423 that will speed it up.

Now people also ask me "How will this brand X fork feel" Well its like this. How is it made and what is it made of. I can make a steel fork using a solid bar of 4130 chromoloy about 2" thick. or I can make a steel fork out of some paper thin Columbus tubing. That 4130 fork will push your fillings out. It also has to do with the bend or rake of the fork. A strait leg BMX style fork rides rougher then a nice big radius bend fork. Now its a little different with carbon fiber. Its all about the weave and glue. Also with a carbon fork they need to wrap it around something to hold its shape before they can cook it. They can use an air bladder,balsa wood,honey comb latish or Styrofoam but I've also seen manufactures wrap up an aluminum fork. I'm pretty sure that's what Winwood does on the Purist fork. Here's something else Rebranding. These guys make every one else's forks. http://www.mekkem-parts.com/products/?n ... &lang_id=0
The White Bros,Fetish, Lynsky, Orgin8, Exotic Sasso and Shimano Pro forks all come from Mekkem. Same fork different stickers. Mekkem just switched to building a bunch of post mount forks so look for those to hit the market soon. Another thing you have to look at is how wide the crown area is. This will tell you how fat a tire you can stuff in there. Pretty much a 2.1 tire is standard. anything bigger, check ,especially if you run a fat tire. http://mtbtires.com/site2/news this site has all that info.

So in short get a metric ruler, a calculator a buddy and some beer and have at it.
If your really superly interested in how a bike fork contributes to handling Google "bicycle fork trail measurement" Read all 10,000 pages and get back to me.

Tim
some one please sticky this.

stu
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Re: Tim's guide for putting a ridgid fork on a bike

Post by stu » Sat Jul 03, 2010 6:43 am

As you wish, sticky as Ricky...


So Tim, does that mean any 26" rigid fork will fit my wheel...? :twisted:

TimT
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Re: Tim's guide for putting a ridgid fork on a bike

Post by TimT » Sat Jul 03, 2010 8:36 am

Thanks Stu

Almost forgot something. Here's one I got not to long ago "Tim I have an old frame and some parts and I want to go single speed I don't have a fork yet which one should I get?"
Well the answer to that is you have to have at least the published frame specifications from the manufacture. With a little math you can figure out the original AC. In fact some frame makers give the AC its very own number so you don't have to do any math. Lucky you sleeping in Algebra payed off after all. If you can't find the frame specs. there is another method but it will cost you. You have to lay the frame out on the floor with a long strait edge a framing square and a protractor. You will also need at least two blocks of wood and a couple of rulers . Two framing squares actually work better but I can't find the other one now but any ways.
Image
It sets up something like this. Sigh I know its a road frame in the picture but its all I had. You will have to pick out a head tube angle . Anything from 70-72 degrees and you'll need to know how thick your head set is. Some string helps also as you need to make some imaginary lines to simulate your new fork. You set up the AG and the head tube angle. Then find the manufacturers specs on the forks your looking at and fill in some blanks a note pad and calculator helps at this point and maybe some Trig. But wait you don't have any fork specs!!!?? well I guess your SOL buddy. You really need to know the rake. You'll just have to guess. Or barrow a buddies fork and ride it and see how you like it. But seriously if you get in a bind you will have to bring it over.

Tim

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Re: Tim's guide for putting a ridgid fork on a bike

Post by Pivot 5.7 » Thu Sep 29, 2011 1:54 pm

Followup comment to my post below:
In case it was not obvious, this post was intended as a redneck contrast to Tim's professional methodology and not to recommend an alternative philosophy. In other words, I follow Tim's advice when I seriously need a professional result. :)

Tim,
I guess my SS project turned out ok by my criteria i.e. Throw something together and see what happens. Your attention to detail requires attributes that I don't possess.
I didn't have the old suspension fork so I took a chance on buying a short, suspension adjusted, ridgid fork. I didn't even remember the travel of the old fork. So I ended up with a pretty low BB and very steep head angle. I wasn't going to "bring it over to your house" and admit I was too lazy to do the geometry.

Rightly so, you pride yourself on using the proper tools. I had everything needed except when it came to headset installation. I am forced to pride myself on ingenuity. the finished headset install actually was a quality result. I wish I had the procedure on film.

The last issue was the chain tensioner. I filed and bent a Sette chain tensioner to push up
but I couldn't get much travel range out of it. I ran without a tensioner yesterday. I am going to look at building my own design next. Fortunately I have not dropped the chain yet.

The bottom line at this time is:
-23.2 Lbs
- Steers very quick
- Pedal strikes not to bad
- No dropped chain yet
- 18 year old rim brakes stop ok
- Need to wear teeth chatter protection over roots
- Front stays down on climbs
- Actually pretty fast
- I have absolutely no use for a SS, but at least I have built one. Hey, I
did use it yesterday on a trash pickup run at the Comite trail.
- I will enjoy talking someone into making a lap on it
Image
Last edited by Pivot 5.7 on Wed Oct 05, 2011 11:00 am, edited 1 time in total.

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Lets_Ride
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Re: Tim's guide for putting a ridgid fork on a bike

Post by Lets_Ride » Thu Sep 29, 2011 3:13 pm

Perfect Race of the Dead machine!!!!
SS/HT/FS/RD (Anything on two wheels)

Ryan

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Re: Tim's guide for putting a ridgid fork on a bike

Post by vudu » Thu Sep 29, 2011 7:43 pm

Very nice set up there Wayne.. Will be looking forward to seeing you soon...
What we have here, little yellow sister, is a magnificent specimen of pure Alabama Blacksnake. But it ain't too goddamned beau coup.

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http://www.laxcseries.com

TimT
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Re: Tim's guide for putting a ridgid fork on a bike

Post by TimT » Fri Sep 30, 2011 5:23 am

Wayne
Do you know what kind of frame you have and the year?

Tim

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Re: Tim's guide for putting a ridgid fork on a bike

Post by Pivot 5.7 » Fri Sep 30, 2011 11:27 am

Tim, The best estimate of a purchase date that my wife and I could come with is something like 16-18 years ago. After riding a few years I became ill and had to quit riding. It is a size small Rocky Mountain brand but I don't recall a model name. It was stickered "Tange Steel". It came with a few XT parts with the remaining components lower end. Naturally I soon had to take it apart and tinker with it so it soon ended up with all XT running gear except for XTR hubs and new Mavic rims. Suspension forks were starting to become popular so replaced the stock ridgid with a Marzocchi Bomber something. After hanging in my shop for 10 or 12 years I decided to get back into riding. After not having much luck finding fork manuals or rebuild kits, I took the fork apart one day to see what shape it was in and what made it tick. Within a week I had forgotten how to put it back together and threw it in the garbage. I then used this excuse to go 100% "old school" since a ridgid fork was cheaper and I really didn't know why I was doing this anyway except that I got a bug of some kind when I saw the "Race of the Dead" flyer. Damn

I did travel with it to the Spillway and timed myself on the second loop. I wanted to confirm that, should I enter, I would at least have a lock on DFL. I was encouraged to confirm that I indeed have a lock on DFL with a 10 minute cushion i.e. 42-43 minutes. The only insecurity remaining on my part is, following your trail markings, I may have actually made 1-1/2 laps. I could swear I passed the same tree (9) times. :mrgreen: I started to stop and carve my initials on it but I had left my pocket knife at home.

Hope I got your answer in there somewhere. If anyone needs help explaining to their wife why a brown truck visits your house almost daily I would be willing to share some of the logic I have used.

Wayne

TimT
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Re: Tim's guide for putting a ridgid fork on a bike

Post by TimT » Mon Oct 03, 2011 8:32 pm

Wayne read this
http://en.wikipedia.org/wiki/Bicycle_an ... e_geometry

Hope you like math. Do the trail calculation. Should tell you which way to go. General rule of thumb on slowing down steering is to lift the front of the bike. So longer fork or thicker head set stack (bottom cup), taller front tire. Or you can just get used to it if its not dangerous fast.
I'm pretty sure last years DFL came in at 45 minutes.

Tim

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Re: Tim's guide for putting a ridgid fork on a bike

Post by SULLY » Tue Oct 04, 2011 9:43 am

Cool bike. I am pondering the thought of switching my rigid bike to a ss for the race and maybe for good. The only problem is the bike is technically my wife's, and she is turned off by the whole "Single Speed or Death" mantra. I am thinking a "temporary" switch to ss is in order, and I may just forget, not have enough time, or be too broke to switch it back to being a geared bike!n :twisted:
And then it came time to descend Mountain Lion trail. Descending mountain lion is like drinking a Margarita through a crazy straw on a trampoline. It's like giving a dog Pop Rocks. It's like a knife fight with a monkey. It's like necking with an octopus. http://forums.mtbr.com/showthread.php?t=301909

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Re: Tim's guide for putting a ridgid fork on a bike

Post by Pivot 5.7 » Tue Oct 04, 2011 10:50 am

SULLY wrote: I am thinking a "temporary" switch to ss is in order, and I may just forget, not have enough time, or be too broke to switch it back to being a geared bike!n :twisted:
Logic plays a large part in the success of many of our hobbies. Thanks for your example, I'll add it to my tool kit.
Careful you don't fall into the "endless loop scenario" i.e. Wife says, You paid how much for a new bike!!! Well I'm going to get me a new (insert whatever). :mrgreen:

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Re: Tim's guide for putting a ridgid fork on a bike

Post by Pivot 5.7 » Thu Oct 06, 2011 4:30 pm

TimT wrote:Wayne read this
http://en.wikipedia.org/wiki/Bicycle_an ... e_geometry

Hope you like math. Do the trail calculation. Should tell you which way to go. General rule of thumb on slowing down steering is to lift the front of the bike. So longer fork or thicker head set stack (bottom cup), taller front tire. Or you can just get used to it if its not dangerous fast.
I'm pretty sure last years DFL came in at 45 minutes.

Tim
--------------------------------------------------------------------------------------------------------
Tim,

I finally found time to following your suggestion to measure my steering geometry following the installation of a rigid fork. This is what I found:

2-3/4” Trail
72 degree head angle
74 degree seat angle
12-1/4” BB height

This would lead you to believe that the original bike was a 71 / 73 design.

I had commented that it felt quick turning, steep, and low on the BB.
Since the numbers look un-remarkable I think the feeling was somewhat the result of riding a 6” suspension, slack head angle, bike for the past few months.

I may not make any further modifications since I don’t plan to make it my primary bike unless I develop masochistic tendencies (not likely).

Thanks for the interest,
Wayne

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