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Discussion Starter · #1 ·
This is not quite "fork/shock" related but I did not know where to post it.

I've always wondered how to predict the new head angle when you change the fork on your bike. I've read a "general rule of thumb" that says 1" more axle to crown length = 1 less degree in the head angle and seat angle but is there a way to have an accurate measurement? I've changed the fork on my bike and using the original manufacturer numbers, it looks like going from a 496mm AtoC fork to a 538mm AtoC fork changed the head angle by 2.5 degrees. This would mean a 1.7" difference resulted in a 2.5 degree difference which would not be the same as this general rule of thumb 1"=1deg.

Any ways also to predict the wheel base change?

I've never been the greatest in math classes, maybe this is basic geometry and I'm just too dumb to know about it! :D :D

Thanks much.
 

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BanzaiRider said:
This is not quite "fork/shock" related but I did not know where to post it.

I've always wondered how to predict the new head angle when you change the fork on your bike. I've read a "general rule of thumb" that says 1" more axle to crown length = 1 less degree in the head angle and seat angle but is there a way to have an accurate measurement? I've changed the fork on my bike and using the original manufacturer numbers, it looks like going from a 496mm AtoC fork to a 538mm AtoC fork changed the head angle by 2.5 degrees. This would mean a 1.7" difference resulted in a 2.5 degree difference which would not be the same as this general rule of thumb 1"=1deg.

Any ways also to predict the wheel base change?

I've never been the greatest in math classes, maybe this is basic geometry and I'm just too dumb to know about it! :D :D

Thanks much.
The rule of thumb will only be relatively accurate for a certain headangle. For example, the slacker the headangle, the wheelbase will change more, and the headangle will change less, for the change in A2C height. In other words, the steeper your headangle is, A2C height change will have more effect on headangle, and less effect on wheelbase.

Sketch it out. Measure your current wheelbase and scale it so your drawing will fit on a piece of paper, then draw the fork with the proper headangle and A2C height. Extend the fork to the new length below the axle, get a protractor and measure the angle that results between the old wheelbase and the new wheelbase. That will be your change in headangle. It's much easier to do this if you've got a CAD program. I can do it for you if you give me the pertinent measurements.

Keep in mind that the data the manufacturer gives you, especially for headangle, is sometimes off, so I wouldn't rely on it too heavily.
 

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The best rule of thumb is to ride it and see how you like it. ;)

It's simple trig to work out how much slacker, draw a right angle triangle, the length is your wheelbase, the short side is the increase in fork length. I think I previously worked it out at roughly 2/3" per degree for a 42" wheelbase but it's easy to check.

Remember that when you're riding a longer fork usually has more sag and seldom rides at full height.
 

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Discussion Starter · #4 ·
WheelieMan said:
Sketch it out. Measure your current wheelbase and scale it so your drawing will fit on a piece of paper, then draw the fork with the proper headangle and A2C height. Extend the fork to the new length below the axle, get a protractor and measure the angle that results between the old wheelbase and the new wheelbase. That will be your change in headangle. It's much easier to do this if you've got a CAD program. I can do it for you if you give me the pertinent measurements.
Good idea, didn't think about it!!!

Basically HA is 68.5 and WB is 42.3" with a 518mm AtoC fork. New fork would be 550mm so a bit more than an inch difference. If you have a minute to try it in your CAD program I'd be grateful but don't waste too much time on this, I can deal with it on a piece of paper and I do have a protractor. Thanks.
 

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BanzaiRider said:
Good idea, didn't think about it!!!

Basically HA is 68.5 and WB is 42.3" with a 518mm AtoC fork. New fork would be 550mm so a bit more than an inch difference. If you have a minute to try it in your CAD program I'd be grateful but don't waste too much time on this, I can deal with it on a piece of paper and I do have a protractor. Thanks.
Yeah, you can also use the Law of Cosines to figure it out, however I find that constructing a diagram or using a CAD program is much faster.

Your headangle will become 1.57 degrees more slack to approx 67 degrees.

Your wheelbase will lengthen .47 inches to 42.77 inches.
 

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Bike Comparator Spreadsheet

Clary (who used to post here occasionally) created an Excel spreadsheet that allows you to compare two bikes (or the same bike with two different setups). You enter the frame geometry, axle to crown height, etc. for each bike and the spreadsheet calculates the resulting head angle, seat tube angle, bottom bracket height, wheelbase, effective top tube length, etc. for each bike. It also creates an overlay plot of both bikes for comparision. If you have Excel, it's pretty handy and it's the quickest and easiest way I know of to make these types of calculations. You can download the spreadsheet here (download 3-Bike Comparator v.1.7):

http://www.geocities.com/clary_mtb_info/

A few notes on using the spreadsheet:

You must enter the fork length and axle to crown height (both expected and actual). If you just enter the travel length as 100, the spreadsheet looks up the axle to crown height for a 100mm fork in a table (on a separate sheet). However, you can specify the axle to crown height directly by entering the fork length as 100L471 (i.e. for a fork with 100mm of travel and a 471mm AC height).

Tire sizes are stored in a table as well. I find it easiest just to make sure both bikes are using the same tires. That eliminates differences due to tire height and makes the plots line up nicely.

You can enter the bb height directly using a number such as 325, or as bottom bracket drop such as D5.

You can set the reference point for the comparison plot to several different places including bottom bracket, rear axle, front axle, handle bar, etc. The best reference point depends on what you want to compare (do you want to know how your saddle position changes relative to the bb, or the handlebar to the front axle, etc). You set the reference point in a field on the left down near the bottom of the spreadsheet.

The spreadsheet takes the wheelbase as an input, and then computes effective top tube length. This may seem backwards, because most frame specs include effective top tube length and don't always include wheelbase. However, if you have the bike, wheelbase is easy to measure. If you don't, you can just enter a guesstimate for the wheelbase, and then adjust the wheelbase until the computed effective top tube length matches the specs.
 

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Discussion Starter · #7 ·
WheelieMan said:
Yeah, you can also use the Law of Cosines to figure it out, however I find that constructing a diagram or using a CAD program is much faster.

Your headangle will become 1.57 degrees more slack to approx 67 degrees.

Your wheelbase will lengthen .47 inches to 42.77 inches.
Thanks much, really nice of you. So my "general rule of thumb" of 1"=1deg. definitely does not apply very well.
 

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Discussion Starter · #8 ·
Backmarker said:
Clary (who used to post here occasionally) created an Excel spreadsheet that allows you to compare two bikes (or the same bike with two different setups). You enter the frame geometry, axle to crown height, etc. for each bike and the spreadsheet calculates the resulting head angle, seat tube angle, bottom bracket height, wheelbase, effective top tube length, etc. for each bike. It also creates an overlay plot of both bikes for comparision. If you have Excel, it's pretty handy and it's the quickest and easiest way I know of to make these types of calculations. You can download the spreadsheet here (download 3-Bike Comparator v.1.7):

http://www.geocities.com/clary_mtb_info/
I guess there is only one word to describe this tool, WOW. I have to spend some time to understand it, man this thing is FULL of stuff! Thanks much, I'll definitely keep it for future use, every freak mountain biker like me should have this tool handy!
 

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BanzaiRider said:
I guess there is only one word to describe this tool, WOW. I have to spend some time to understand it, man this thing is FULL of stuff! Thanks much, I'll definitely keep it for future use, every freak mountain biker like me should have this tool handy!
As you might notice, the seat angle gets relaxed too... which is no good. For a M frame and a inseam around 30"-33" changing from a normal 25mm offset seatpost to a zero-offset one, will correct your seat angle around 2 degrees (give or take a half one). The higher the setapost, the less the correction will make effect.

I'm switching from a crappy fork to a Magura Phaon and I will change my seatpost from a 25mm offset one to a zero offset. This will set my angles at around 68° HT / 71° ST.
 

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I don't know the screenname to the resultant emails as I've exchanged PM's with a couple of people, but I've got 1.7 now thank you. Really needs to be hosted somewhere...
 
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