Advanced Geometry Calculations - Front end swap

M

MattCB750OT

New Member
I gave up trying to find what I need. My head hurts. I am trying to calculate the numbers for the swap but since I am changing too many variables (more than just shock length), there are no calculators that can help. I did see someone post a gif with the similar changes, but I tried to do it in CAD and almost threw the computer through the window.

Current: 82 CB750
Wheelbase: 60.4
Front Diam: 26.5
Rear Diam: 25.5
Offset: 2.3 (ish - 60mm)
Fork: 30.5
Rake: 26.5
Trail: 4.04 (??)

Swapping front end to 05 CBR1000rr:
Wheelbase: ?
Front Diam: 23.61
Rear Diam: 24.48
Offset: .75 (25mm)
Fork : 27.5
Rake:?
Trail:?

Can anyone point me in the correct direction? Am I missing the easiest way to do it?

Maybe I am overthinking it. Since I am "lowering" the front, the wheelbase changes. I know I get some "positive" help from the smaller offset. How far off from "safe" are my final numbers?
 
12.75 tan(26.5*) = 6.35"
6.35-2.08(horizontal offset)= 4.27" trail

Assuming the frame angle to be unmoved and the rear tire contact as zero

9ccac3aa28226231deee93821dafd0ef.jpg


Which added up gives
Delta x =2.73"
Delta y = 2.99"
For your front contact point vs the rear
 
This has shortened your wheelbase to 57.67"
Arcsin (2.99/57.67)=2.97*
Your new rake angle is about 23.5*
So with a new wheel radius of 11.8"
And an offset of .75"
Your trail is now 4.44"
Your wheelbase is now~58.2"

Note:
If anyone wants it, i could write up a small windows .exe to calculate the effects of front end swaps on the bike geometry
 
Wow. Farmer92 - really appreciate the time you spent to do it. Is there a page or link that describes what you did (so others can save from asking you to do their homework? I spent quite a while and couldn't find anything.

clem said:
Did you check out Tony Foales software for suspension and geometry?
Click to expand...

Yeah that is the issue - I only have mac products. (well I do but the PC's are buried in the basement). I was trying to find something like that program for mac but nothing turned up that I could find.

My biggest worry was that since the CBR1000RR fork is an inch or two shorter than the GSXR shocks, that it would be out of wack. Based on what farmer92 did, I think I could ride it fine, but would feel much better with a custom triple with a 30mm offset. .... Unless I am reading my numbers wrong?
 
Actually I just found out that the offset on the 04-05 CBR1000RR is 25mm which is 0.98 not 0.75 which brings me back to a more normal setup. Not sure if spending $800 on a new triple will be worth it now (if I did it would drop trail down to 3.85). Looks like my new trail is 4.06 which is almost stock. Interesting.
 
Not really a page that describes what I did, or none that I consulted.
I just assumed the frame was held static and calculated the height and length changes at each end, after that it's simple geometry
 
If you get a copy of Windows it's free to install on OSX via bootcamp. That's how I do most of my CAD/CAM work at home, though I do have a PC dedicated to my shop for these purposes as well. Given that Windows 10 can be had for free legally, it seems silly that you wouldn't have a copy installed somewhere.
 
:'( please
farmer92 said:
This has shortened your wheelbase to 57.67"
Arcsin (2.99/57.67)=2.97*
Your new rake angle is about 23.5*
So with a new wheel radius of 11.8"
And an offset of .75"
Your trail is now 4.44"
Your wheelbase is now~58.2"

Note:
If anyone wants it, i could write up a small windows .exe to calculate the effects of front end swaps on the bike geometry
Click to expand...
 
Curious and how you got everything to work? Did you use stock stem, new offset triples or did you end up sticking with the cbr triples? I have an 81 cb750c same geometry and I'm looking to do something similar.

Sent from my SM-G930V using DO THE TON mobile app
 
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