Rake and trail (longer post.. there is a question towards the end)

[LazloH]

Ive already run this by a few other people... I have a better understanding of what is happening to the geometry changes of a bike with different forks... but I am having a hard time putting it to paper to figure out the trail and ensure that whatever I build is safe.

That being said... here are the details.
Picked up an RD400 frame. No front end... just a swing arm. I happen to have a front end from a GS1100 right here (or an 850.. not sure). So I mocked it up and started to do some measurements and calculate the trail.

I was tired when I did this.. so measurements are +- one beer.

Internet says rake is 27.5 from vertical. I have no tires or rear shocks so I did it from the horizontal backbone and was a hair closer to 29 degrees.... roughly.
From the center of the axle to the top of the steering stem is ~ 30.25 inches.
Top tree offset from the center of the steering nut to the center of the forks (halfway between) is 1.5 inches

So I went ahead over to this site.
https://www.rbracing-rsr.com/rakeandtrail.html
punched in the numbers.. and they have a generic number for wheel diameter with tires based on generic info (and rim size). The rim I have laying around is 19 inches (and its for sale!.. and Orange!) so they say to put in 26.10
(there is another equation above for figuring out the real number based on the tire size)

"Trail in Inches - 5.1" I read that between 4 and 6 are desired to avoid tank slapping death traps (as alphadog calls them) or just crap-tastic steering ability....

Details done.. now for the issue.

I was feeling proud of myself.. then I turned around to look at it once more and I realized that these are not straight axle forks..... these are leading axle forks!.

AgentX did a great job of explaining how to visualize the actual movements of the system.. but without experience I have no idea what this will really do to the steering geometry of the machine. I don't even know how to put this new information into the calculator. at first I thought it might just be additional offset (axle comes out an extra inch and a half from center of fork to center of nut)... but I am just not sure.


Any help is appreciated. I was thinking about ordering Tony Foales Chassis design book as well. Has anyone read it and can tell me if its worth while?

 

[8ball]

Basically, your trail will be reduced by the horizontal distance from the center line of the axle to the center line of the fork. Does that make sense?

Or to put it another way, you would need to find the distance from the axle to the center of the fork, (perpendicular to the center line of the fork), and add that to the triple tree offset value that you plug in.

The first way, you would take the trail number that the program spits out, and adjust the result. The second way, you would adjust the number before plugging in the values.

 

[jpmobius]

Well, reading a few books on single track vehicle dynamics would not go awry. However, maybe some generalizations about what you are doing may have value. Keep in mind, these are suggestions and generalizations. Seems there are two likely goals: You have parts and you want to combine them to get the best result without building a chassis from scratch, or you still want to use an existing frame and want to improve the performance. Either way, you have some fixed variables (??) that will help simplify the reasoning behind decision making. First, you have a fixed rake in the frame. This is not actually fixed, as you can raise the back to decrease the rake ("quicker" steering), or lower it to increase the rake ("slower" steering)(or do the same with the front for similar but opposite effect) Let's assume you want to do neither and keep the stock rake. Changing the rake of course also changes the trail, which is your primary weapon against the dreaded speed wobble. The element left out of most cursory discussions about rake and trail is how rigid the rest of the chassis is. Make the chassis rigid enough, and you can substantially reduce the rake for better turn in without the subsequent loads introduced causing other problems. That also lets you move the forks back (less moment), while bringing the trail back into a drivable length. The problem with modifying front suspensions is that it is challenging to bring the chassis rigidity up enough to make use of less rake. SO, if you keep decide to keep the stock rake, then you can reduce your concerns to the trail dimension. The simple way to consider it is to look at the stock figure for the RD400, and base the changes you make (if any) on how the stock bike handles, and what the results of changing that figure might be. Make a decision, and then select parts to be within what you think is an acceptable range. You have to decide on your riding style and threshold for scary behavior. I know some guys that think 400's are twitchy enough stock, and others that fit longer shocks to pick up the back, decrease the rake, and enjoy the noticeably quicker turn-in. High speed stability is sacrificed. Here is the point: If you simply decide on rake and trail numbers , say from a modern good handling sport bike, and build that into your otherwise stock (antique) chassis, you likely will be very unhappy with the result. So I suggest shooting for the stock number. If you think the result is too slow steering for you, lower (raise? whatever, lower the front of the bike) the stanchions in the triples and try it out. If that's for you, raise the back to get the same effect with the higher front end position and get your ground clearance back. As far as the offset axle is concerned, calculate a single number for the tree offset by adding the offset of the axle from the centerline of the fork tubes (your plan of adding the two figures is correct). The steering assembly does not know where the fork tubes are, only how far the axle is ahead of the steering pivot. Generally, having the fork offset be a low number keeps them closer to the pivot reducing steering mass, but by itself also increases trail. Likely there were production reasons for combining the "close" trees with an offset axle. In any event, figure out what it will take to mimic the original configuration number wise, and adjust to suit your personal preferences. Remember, fairly small adjustment will be pretty noticeable if you ride fast, but a lot depends on your riding style as well.

 

[LazloH]

Alright.. glad to know that I should be adding the offset of the axle to the total offset of the trees. JP, to give you some idea.. I have a frame with a title and a motor. No wheels.. and all of the parts are not fixed in stone. I just wanted to mock up a front end so that I could feel like I did something with it and I was more just trying to get a handle on some important aspects.

Without wheels.. or really wheel selections and the modified swing arm (that is not actually in my possession or modified yet).. its all just an exercise in math.. and to ensure I have an ok grasp on what the results will be of the changes I make. Basically.. I want to make sure that I am doing things in a way that keeps my ass in the seat instead of sliding.

This is also just kind of a question for a build that is too early to even be remotely classified as a "build thread". Also.. I picked up some shocks for this project today... guy said they were Hagon shocks (even came in a Hagon box).. but they look more like the shocks off my bonnie... same size and all. Still worth it. 340 mm. I do realize that longer shocks decrease the rake and lessen the trail, but when the mounts are added to the swing, it can be adjusted.

Appreciate all of the information... a fantastic passerby loaned me a copy of that book as well.

 

[LazloH]

here is maybe a better example.

Stock RD400 data (as best gleaned from the interwebs)
Fork length 29.5 inch
Rims both. 18 inches.
Rake 27.5
Offset (I think I recall it at 1.5)
Rear shocks are 12 3/4 (325mm)
-Assumptions include stock swing arm and shock mounting location.

According to google.. stock RD400 has a trail of 4.2 inches. My math doesn't quite come out to that... but I don't know the rim size with the tires to get total diameter.


So at this point.. changes to the forks are easy enough to sort out. Longer rear shocks though.. I would have to find a way to calculate the degree change in the rake. I guess its just an isosceles triangle.. solve for the acute angle, reduce from stock trail.... or just put it on, man up and measure it.

How do you guys sort this stuff out?

 

[Big R]

As far as tire sizes go - can you put a 1/2" steel bar thru the axle plates and support the frame on jackstands? Figure out what tire diameters you want to go with, and raise or lower the jackstands to the corresponding tire radius.

Last time I was at Harbor Freight they had two angle finders for sale. A digital one which is more accurate for about $40, and the analog one that I bought was about $10 (supposedly accurate within half a degree).

 

[teazer]

Two simple ideas that involve zero math.

1: Get a decent set of forks the right length without all that leading axle nonsense

2: See 1. Have someone measure a set of stock forks if necessary and have them measure stock offset because your new forks will probably have less offset and more trail and that will allow you to push the nose down and tail up a little and still be stable.


Random data form one we prepared earlier.....
http://pinkpossum.com/GT750/phattrakka/part4a.htm

 

[Big R]

Just throwing this out there: the leading axle GS forks from an 850 / 1100 use a different triple tree then normal forks from similar bikes. As in, the GS850GL has a different front end as a GS850G.

So if you get forks that are non-leading axle and try to use the triple tree you have, something may not line up properly.

 

[LazloH]

Teazer. These forks are a good 4 inches longer than stock so... As much as I love Ichibans bad ass cafe racer.... I don't think they are going to get used. I am just looking at the numbers for fun. That being said, there is a guy with lowered drag forks for a sport bike around my office...


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[LazloH]

Rich. I picked up a Bosch angle finder for 10 bucks from a pawn shop. It works great with a built in level.


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[LazloH]

Just want to reiterate. I am not doing the "can't I just...." Thing here. These questions are just on bike geometry as a whole with a simple mocked up front end as a visual guide. Not a build or anything. That will come later when I have researched the proper parts and lengths I plan to use.


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[xb33bsa]

the reason the japs went to leading axle forks was a few
lots of times the style was longer forks and they went leading axle to increase the internal overlap of the forks to add rigidity less flexy,and the increased overlap reduces stiction making for a more compliant ride
an added benefit is rotational moment of the heavier longer forks is reduced by having the leading axle as well because the triple offset is always less with leading axle forks
there were a few very good handling bikes with leading axle forks the gpz550 is a great example
but of course the RD uses much shorter forks than any of those old 4 strokes

 

[teazer]

Rich. I picked up a Bosch angle finder for 10 bucks from a pawn shop. It works great with a built in level.

Do we want to know what you picked up at the Porn shop?

Tony Foale has software on his website that I think will allow you to work out exactly what you want or draw it all up. You did allude to the quintessential metaphysical question though "What would Ichiban do?"

 

[LazloH]

Teazer, I don't have a bike lift so I just use a couple of those leopard print swings when I work on my triumphs front end.

zing!

 

[teazer]

Leopard print slings. Is that why bikes are breeding out there in your garage?