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Discussion Starter #1
Hi all! I'm trying to get my head around the general principles of front end swapping, I wondered if someone can help me figure something out here:

After you figure your steering head bearing size (all balls charts...etc...) and found a list of compatible models, after determining whether or not a given model's forks meet the necessary performance expectations, and will handle the weight of the bike, the next question is fork length, as that affects the bike's trail, when attached to the bike. With that said, the problem I'm having is finding the length of the fork. If you look in the service manual, it simply doesn't tell you it's fork length (or at least I couldn't find it). I wondered if you can find whether or not the forks will be compatible based off trail measurements and height of the bike? Are there any other methods for assessing fork length?

Please bear in mind that I'm not referring to any specific model of bike, I'm simply trying to understand the guiding principles of fork changing for any bike, any model to any fork set, with matching bearing size. I do realize this is a can of worms regarding fit (via triple trees) and rear wheel trail, however I'm just not seeing a workable system for assessing how compatible a potential fork set would be to your current bike. Any suggestions or tips? Cheers everyone!
 

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Not really. A forks length can vary bike to bike and rider to rider, as the fork's sag has to be taken into account. Then it'll change as it goes about its work, 3-4 inches or more.

Too many variables to be 100% accurate. And remember, the fork crown offset has to be taken into consideration, as that measurement also affects your rake/trail figures.

So, you need to find a fork the same extended length, the same compressed length, with the same spring/damping range AND the same crown offset to successfully second guess what the factory deemed correct.

If you can do that, you should be in the business.

Oh, the only thing the rear wheel trails, is the front.
 

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you buy a tape measurer, a note pad, and a pencil and then you go around measuring the fork height of the bikes you want to know about. What did you think? you could do this all off the internet or something?
 

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Discussion Starter #4 (Edited)
Hi John, thanks so much for your reply. So, if I'm understanding you correctly, you 're saying that before you can change the front fork set of your bike, after you found a list of potential compatible forks, there are a lot of excluding measurements that need to be done. Is that what the majority of cafe racer mechanics do? Are there any corners that are ok to cut? I'm really trying to understand a workable system for knowing which forks can be used on a given bike. So far, my list looks like this:

1) search for steering head bearing size and find a list of bike models with same upper and lower bearing sizes.
2) exclude any that come from a much lighter bike
3) exclude any that don't meet expected performance requirements (although that requires yet more study to know what i'm looking for)
4) exclude any that are not the right 'fit'. To me, the only 'fit' that there is to find at that point is if the rake and trail measurements are drastically different to the bike you want to fit it to.
5) ensure the forks have enough clearance above the gas tank (= length?)

Am i approaching this from the wrong angle?

Cheers!
 

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Discussion Starter #5
Hi Geeto, I kinda did, yeah. I would like to know what i'm looking for before i go out and look for it. Is that what the majority of mechanics do to know which fork set is suitable for the bike?
 

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Hi John, thanks so much for your reply. So, if I'm understanding you correctly, you 're saying that before you can change the front fork set of your bike.... there are a lot of excluding measurements that need to me done.
yes. even before you have started to look at potential forks you should know the basic suspension geometry and where you want to be.

Is that what the majority of cafe racer mechanics do?
There is no such thing as a "cafe racer mechanic". There are mechanics, period. Some Mechanics have expirence setting up race bikes. Those people are who you want to aspire to be.


Are there any corners that are ok to cut?
No. There is a right way to do things and a way to just guess at it and hope you don't get hurt. You would be shocked at how many people take the second approach.


I'm really trying to understand a workable system for knowing which forks can be used on a given bike. So far, my list looks like this:

1) search for steering head bearing size and find a list of bike models with same upper and lower bearing sizes.
2) exclude any that come from a much lighter bike
3) exclude any that don't meet expected performance requirements (although that requires yet more study to know what i'm looking for)
4) exclude any that are not the right 'fit'. To me, the only 'fit' that there is to find at that point is if the rake and trail measurements are drastically different to the bike you want to fit it to.
5) ensure the forks have enough clearance above the gas tank (= length?)

Am i approaching this from the wrong angle?

Cheers!
you are approaching this from the wrong angle. First question should be: What are the people who are racing my type of motorcycle using? if the answer is nobody is racing my type of motorcycle then the next question should be "Do I have the right motorcycle". If people are racing your motorcycle - ask them what their current setup is and if the rules didn't restrict it what they would do further. It might surprise you as to what answers you get. From there you need to figure out your geometry and get an idea of where you want to be. This includes the rear suspension because frankly bike components don't work independently of each other and changes to the chassis affect the whole chassis. There are also ways to cheat into a number - for example: shorter forks will reduce rake and trail but so will longer shocks in the rear. There is also sag you have to account for, as well as tire size and height as well width and braking force.

after you have done all the math - then maybe to start to look for the hard component solutions out there. Don't approach it from "what fits" but rather were do I want to be and what parts and what work will help me get to my goal. There are plenty of places that can CNC you a set of trees and a stem to fit any front end - all it takes is money.
 

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Hi Geeto, I kinda did, yeah. I would like to know what i'm looking for before i go out and look for it. Is that what the majority of mechanics do to know which fork set is suitable for the bike?
Today the majority of cafe builders don't have a fucking clue to what they are doing and chase down "a look". That's why there are so many fucked up cafe's out there right now that are just down right dangerous. You at least sound like it's correct to actually do some research before blindly diving in to "make it my own".
 

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Hi Geeto, I kinda did, yeah. I would like to know what i'm looking for before i go out and look for it. Is that what the majority of mechanics do to know which fork set is suitable for the bike?

I put an FZR600 front end on my H1 back in 2002 when there wasn't a heck of a lot out there on the internet. I chose that front end because:

1) I already got a tip on the rear swingarm from an fzr400 fitting my H1 frame perfectly, and I had already bought the setup. I wanted something that matched the rear wheel setup

2) I wanted conventional forks, not USD units but wanted cartridge damping. The stock forks were 36mm and the FZR units were 38mm so I wasn't going to tax the already mediocre frame too much with a bigger lever.

3) I wanted a wheel set that was closer to stock sizes so I didn't fool with the stock geometry too much.

At the time I did the swap I was living in new orleans and had a motorcycle salvage yard called Clancy's at my disposal. For about a month straight I was in that yard measuring everything. There wasn't a great knowledge base for this stuff so that's what I did. What I ended finding out was that the fzr600 bottom tree used the same bearings as the H1, the top was taipered and not even close but the stem was too long anyway so in the end I ended up paying a machinist to weld the h1 stem into the fzr bottom tree and then bore the FZR top clamp to take the h1 stem.

Here is what I learned:

- the important measurements to me are neck rake, trail, tree rake, fork offset, overall height, wheel height, and lock to lock turn circle. There may be others I am forgetting but I was taking pot shots in the dark when I was figuring this all out. I later learned about sag heights and how I could use big changes in the rear to make little changes in the front.

- offset and steering lock are important. In my first test I used the stock h1 stops and the first tight turn I smashed my knuckles into the stock tank hard. Also making a sharp turn with the front end at low speed I found that the front wheel went so far over it no longer steered but skipped along the ground - i.e it was almost like a shopping cart the wheel angle was so far over.

- this isn't just a matter of finding out what bike is close in weight an hope that the spring weight it right. What I found was that most people who raced FZRs hated the stock forks and wanted to sap to TZ 250 or 350 forks and later gsxr forks. when I started testing it I found out why - the fzr front ends were too mushy and while they were a small improvement over the stock h1 forks they were not as good as they should have been. I also found out about this time from a couple of Kawasaki triple racers that first generation ex500 forks were 36mm, better internals, better spring rates and damper rates, better brakes and could be used with a bolt on first year kz650 wheel and rotors (stock 5.5mm thickness - one year only). what made it sting especially is that the entire ex500 front end would slide right into my stock h1 trees as the wheel spacing was the same. I felt like a heel, because here was a solution I didn't find on my own but those in the racing community knew about, was cheaper, and was probably better than what I already had going on.

does this help?
-
 

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Discussion Starter #10 (Edited)
Hi Geeto, I guess I'm a long way from acquiring all the necessary skills and knowledge to doing this properly, and I really appreciate you taking the time to answer my noob questions!

Let me repeat back to you what I'm hearing, as your advice is really useful, although it does put me further away from the "first check this and then check that and then calculate this and you then know what can be fitted to your bike" set or rules or guidelines that I was looking for.

So it sounds like what you're recommending is:


1) First check how the bike will be ridden and special requirements of the rider to assess necessary standard of performance upgrades, eg. is this bike going to be ridden entirely on the track? Will it be mostly a road bike and sometimes used on track days or driven aggressively on twisty roads? Will be used off road...etc...?
2) forget the steering head bearing size, as you can get a new stem made for your steering head, and you can still use bearings for your head size. Likewise for the triple trees.
3) Calculate the ideal rake and trail, based off riding style (i assume that's what you mean when you say geometry?), taking into account rear suspension, calculate my ideal wheel height, and any "cheats" that I may want to do (wouldn't you cheat if were limited in parts or is there another reason?), and take into account sag (is this a problem if i take adjustable whole front ends from bikes that are similar weight?). This should give me an ideal fork size (right?)
4) using those fork size calculations, go to a parts yard with a tape measure and find front ends that come from bikes that meet the performance requirements, come from a bike that has an approximately equal weight and are the right length that was previously calculated.
5) Check that neck rake, trail, tree rake, fork offset, overall height, wheel height, and lock to lock turn circle is similar to stock and/or will fit the bike.
6) get fittings (trees and stem) machined if needed to fit your chassis.


Am i getting close here? It's really not easy to figure out even a course of action of what the necessary steps are to arrive at a decision for which front end to change your bike to. Giving me a headache! haha

Cheers!
 

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Discussion Starter #11 (Edited)
Hey Geeto, (sorry you posted your follow up message as i was typing mine so i didn't see it until after i posted it) that story does help (as everything you share does! :) ).

so it sounds like what i need to pay attention to from your message is:

1) unless you want to recalculate (chassis?) geometry, keep the wheel size close to stock size
2) check the steering lock travel of the potential forks and make sure they fit the parts I'm fitting the bike to.
3) if upgrading the forks, ensure the frame isn't under too much pressure from the new parts (are we thinking flex of the frame with increased spring travel, as well as how the frame can handle the new break setup?)
4) make sure the offset is similar to stock, to prevent "shopping cart wheel" turns
5) check common gripes of the potential forks to see if there are any well known problems or issues that might make you change your mind.
6) check to see if a certain community of your riding style knows about a particular set of forks that work for what you want

am I understanding that correctly?
 

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What I am recommending, more than anything, is to know how something works before you start modifying it to make it better. Part of that is knowing yourself as well.

A lot of this stuff comes with experience. When I started my h1 project I had plenty of experience working on old American cars, I don't think I even owned metric tools. I made a metric fuck ton of mistakes and learned a lot by mostly being persistent and a sponge for anybody with any experience. It will probably come as no surprise that I never actually finished that original h1. With the lessons I learned from it and the people I associated with, I came to learn how to do basic improvements, then moved to learning how to do chassis setups on my own bikes, and then eventually learned how to do it in a performance setting from many people with a ton more experience than myself.

You keep trying to project plan this stuff like there are linear steps to it and I just don't think you can do that with this kind of stuff. Or at least you have to be ready for when things go off the rails, because they will. I am normally a big fan of punch lists and steps but I don't know how rigid you are looking at this. Or rather the thing I should say is in between each step is another half step where you stop, consider all the information you have in the context of how you have it, and whether the direction you are heading is still the right one.

if you are new to this and you want to minimize the mistakes you make, then the ideal is to start with stock and see if you can modify the bike while keeping the geometry as close to stock as possible. remember for every change you need to compensate for it somewhere else so you can run smaller wheel and tire height but you have to make up for it somewhere else like in suspension to keep the rake consistent (usually bikes run 19/18 F/R wheel sizes so a 17/17 setup means you need to lengthen the front forks 1/2 inch to re-level the bike).

you want to know how you ride to set up the bike to suit you. be self conscious of how you ride, your quirks, are you smooth? do you turn in too early?, etc...
 

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Discussion Starter #13 (Edited)
Sure Geeto, you're right. I definitely do need to learn more about this. And I'm trying, man! There's just so much to learn! And I get that a lot of that experience comes from stripping bikes, trying stuff and making mistakes. At the same time, a lot of those mistakes can be avoided by having a working guideline before you begin. The guidelines exist, it's just not aggregated into easily accessible resources. So it leaves newbies of this day and age, who are used to having instant access to relevant knowledge in the situation of trying to piece together bit by bit the standard operating procedures with whatever resources they can get, asking anyone who knows what they're doing!

That being said, you need to think you know what you're doing before you can do it. You don't randomly grab a front end. You need to understand (or think you understand) how it fits the bike, and have a basic selection method. That way you can make mistakes. But right now, I got the idea that at times, when modifying bikes, you want to change the front end, for different reasons and for different purposes. That might also mean you need to change the rear and make other major changes to the bike to help either front fit, or enhance the requirement you originally had to change the front end. I'm just trying to figure out how you might go about selecting a new front end.

The majority of the people suggest that the best way for noobs is to replace the entire front end (as opposed to individual components), and to use the same bearing sizes. I think you made a really good point in saying that you can (with relevant ease and money) have a new stem machined and attached to a fabricated triple tree. So that removes that limit. But now the choice went from choosing from forks with the same bearing sizes to choosing any front you want. So you need some limiting factors, ways to exclude, "all the front ends in the world" to get to a handful of great potentials, so you can choose off affordability and availability.

That's all I'm doing right now, man. Just figuring out what those exclusion factors are. I can then research each one, learn more about them, why they're relevant and how they affect the handing of the machine, and how that applies to the purpose and my riding style. It's not everything, but it's a start for learning.

Really sound advice for bike project planning! I appreciate your time and effort in sharing your experience with me, it's not easy to find people willing to help for nothing other than appreciation in return.

Cheers,
Chris
 

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the majority of what people? if you ask the majority of people around here most will tell you to work with the junk you already have if you have a bike that has any kind of aftermarket.
 

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You basically have 2 schools of thought :| Those who ride motorcycles, and those who make em look cool.

People who ride their motorcycles service their machines frequently and avoid replacing things like the entire front end of the motorcycle wherever possible.
People who make their motorcycles look real cool, replace everything you can see with something that looks new, but is probably off a newer model trashed motorcycle, or made very recently in China.
 

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You don't need to consider the bearings being the same size. Many stems may work by merely going to AllBallsRacing.com and getting bearings with the correct inside and outside dimensions.
 

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Discussion Starter #17
You don't need to consider the bearings being the same size. Many stems may work by merely going to AllBallsRacing.com and getting bearings with the correct inside and outside dimensions.
Hey Marc, you just cleared up a little misunderstanding I had. I was under the impression that the charts stated on the site were listing compatible models that share the same bearing size. But I can see now that what you're saying is that they sell bearings that can house those models in your steering head. That's pretty useful to know.
 

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Chris,
Maybe it is time to get past all the hypothetical front end swap and just tell us what you want to do. What bike do you have or are you thinking about getting? Why do you want to change the front end? What is wrong with the front end that is on the bike? Are you planning on racing the hypothetical bike? Is this just an exercise in mental masturbation or are you really going to do something to a bike?

My suggestion generally is to look at front ends from the same manufacturer as the frame. Many manufacturers use the same steering stem bearings for a wide variety of bikes, so front end swaps are easier. It is also very easy to lengthen or shorten a steering stem. It is also easy to internally limit the travel of most standard forks. It is also easy, but more expensive to get longer fork tubes.


Ken
 

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Discussion Starter #19
Chris,
Maybe it is time to get past all the hypothetical front end swap and just tell us what you want to do. What bike do you have or are you thinking about getting? Why do you want to change the front end? What is wrong with the front end that is on the bike? Are you planning on racing the hypothetical bike? Is this just an exercise in mental masturbation or are you really going to do something to a bike?
Hi Ken, thanks for the message. Actually I'm about to work on a couple of bikes with a semi-pro mechanic, here in the Netherlands. I volunteered my time to try to learn more about cafe building, so when I get the money and space to get my bike, I also have the necessary skills. I just don't want to enter the situation being a burden. So I'm trying to get as wide a range of skills and basic knowledge as I can get before arriving and saying "uhhh how do you do that?" when he asks me to do something. And since I don't know what bikes I'll be working on, I have to think generally, and since I can't apply the skills to any one particular bike or situation, I need to keep things hypothetical.

I've really noticed that getting basic mechanics knowledge isn't really that difficult. There's plenty of sources. I've read a few tech books, and played around servicing my own bike. But getting custom knowledge isn't easy at all. Finding books with the relevant knowledge isn't easy, neither is getting advice, as people tend to have a "leave it as it is" attitude or "stick anything you want on it" attitude. Alternatively you have people that give you lists of differential equations to describe your rake angle, and again, you're left wondering how you can apply any of it to a set of working skills.

My suggestion generally is to look at front ends from the same manufacturer as the frame. Many manufacturers use the same steering stem bearings for a wide variety of bikes, so front end swaps are easier. It is also very easy to lengthen or shorten a steering stem. It is also easy to internally limit the travel of most standard forks. It is also easy, but more expensive to get longer fork tubes.
Very good advice, thank you. I will look into methods of lengthening and shortening steering stems. Although it sounds like what you're saying is you can basically modify any set of forks to work on any bike?

Cheers!
 

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;) There is a big difference between a set of forks fitting on another bike, and a set of forks working on another bike.


Go down to your local bearing supply shop and ask for their catalogue of standard taper roller bearings. All the specs, dimensions and details are in there.
They do not make taper roller bearings specifically for motorcycles, they make taper roller bearings in a finite range of designs and sizes and you the customer select the appropriate ones suited to your job.

If you are buying your standard schedule roller bearings from a motorcycle specialty shop and not a bearing supply shop, you are probably spending more money then you need to for the exact same item. But if you can't figure out what bearing to use and buy your bearings from a specialty shop based on what motorcycle they fit, then you are paying the supplier for their knowledge of bearing fitment when you buy their product at the inflated price.



lol I always considered the fork travel to be the important number, but when people start talking about reducing their suspension travel. How can that be productive?
 
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