Rake and trail for dummies

[barnett468]

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TRAIL

Jason and others have posted a lot of good info and I just wanted to mention that the images Jason posted depict the measurement for "false" trail . . This is a simplified way of measuring trail and is one that some custom bike builders use, however, measuring it this way can lead to slightly different results in other measurements . . Motorcycle and bicycle mfg's etc measure what is referred to as "true trail", and the difference between the two is depicted in the image below.

The long angled solid lines represent the angle of the fork tubes . . The broken lines that run somewhat parallel to those solid lines represent a line drawn thru the center of the steering stem.

Although the tripple trees have a slightly different stagger in both examples, the image still shows a reasonable example of how the elevation of the steering head will change when the steering head angle is changed if the length of the forks and the amount of axle offset remains the same.

The greater the steering head angle is, the more trail it will cause.

The greater the tripple tree stagger is in the positive direction [the lower tree being positioned further forward of the steering stem than the upper one], the less trail it will cause.

Changing the amount of trail the front wheel has also changes the amount of trail the rear wheel has.

[Billy Golightly]

Great info gentlemen...thank you. From my continued research it appears that 3-6 inches of trail is "normal" on 2 wheeler applications, and how you arrive at those numbers (fork length, rake angle, triple tree offset, front tires size, etc) is not particularly important although they all have effects on it.

[Yamada]

What about the old dirtbike where the axle was at the end of the fork tube, they are not trailing or leading?

[oldskool83]

I'd think neither.

[barnett468]

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Below are specs from the owners manuals . . One of your dilemmas is that the steering head angle will be fixed, therefore, you need to pick one that will work decent in both tc and cafe mode . . once this is decided, adjusting the trail becomes much easier and is manly a matter of fork and tripple tree selection to achieve the amount of trail you like . . Since falling off on the street will hurt a bit more than falling off on the dirt, you might consider setting the steering head so it will be more stable at high speed if you plan to corner it hard in street mode or go 80ph plus on it.

I agree with Jason in the regard that I would not use a trailing axle, especially on the street . . Honda has proven that it "works" in the dirt, but to make it handle like the 250R, you need the same geometry which is a 21 degree steering head and 31 degree forks and 48 mm of trail, so therein lies the main problem of trying to use 250R steering geometry for the dirt, and street type steering geometry for the street.

I really suggest you ride an '86 Tecate for a few hours and see how you like it because it's steering geometry is a little closer to a dual purpose one than the Honda's, plus it does not use a trailing axle . . Using a 26 degree rake will slow the Tecate steering down a little but you could speed it back up a little by lowering your forks or raising your back end or by doing a little of both which will help maintain overall ride height and center of gravity, however, the center of gravity would actually change very little even if you only lowered one end anyway.

This being said, I have not seen anything other than a center axle on a street bike for maybe 40 years . . Every one I am aware of all use center axles . . Kawasaki used center axles on their Championship winning 165 mph road race bikes, so I don't see any reason to vary from a well established norm for street mode.

Dual purpose bikes have leading axles but they are not getting ridden hard in turns on the street.

Another of the problems is that you like a "twitchy" bike in the dirt, however that is the last thing you want in a turn or even a straight line at 80 mph, especially on the asphalt.

Below, is one option to look at for street mode . . The rake is the same as the NINJA 250R which turns like a race bike which is NOT exactly what I think is best for your app because it turns pretty quick, however, the trail is slightly more than the NINJA which will slow the steering down a hair...kinda sorta.


WEIGHT BIAS VS WEIGHT ON FRONT WHEEL

Don't forget to consider the weight bias.


TIRE PROFILE

This has a noticeable affect on how a street bike corners . . The more crown it has, the easier t will be to lean over and with the steering specs below, I would try a tire with a uniform crown meaning one that is not higher in the center than the rest of the tire.


VEHICLE SPECS

PROPOSED FOR STREET
Rake ............................ 26 . . 27 would be better imo, but it would hurt the dirt mode.
True trail approx .............. 4.0 in
Axle location ................. center
Tripple tree .................... non staggered


NINJA 250R
rake ............................. 26 deg
trail .............................. 82 mm . 3.23 in
w/b .......................... 1400 mm . 55.12 in
dry weight ................ 335 lbs
wet weight ................ 373 lbs
wet bias .................... ft 181 . rr 192 lbs
bias percentage ........ ft 48.5 . rr 51.5
axle position ............. center
tripple trees .............. non staggered


1984 ATC250R
rake ............................ 69 mm . This is measured from horizontal instead of vertical so using current methods, the rake is actually 31 degrees but it is unknown if this is the forks or the steering head.
rake forks ....................... ?
trail ............................. 45 mm . . 1.8 in
w.b .......................... 1197 mm . 47.1 in
dry weight ......... ..... 302 lbs
dry bias ..................... ft 117 . rr 185 lbs
bias percentage ........ ft 38.7 . rr 61.3
axle position ............. trailing
tripple trees .............. unknown but look staggered which would make sense


1985 ATC250R
rake .............................. 21 deg
rake forks ........................ ?
trail ............................... 37 mm . . 1.45 in
w/b ........................... 1295 mm . 51.00 in
dry weight .................. 291 lbs
dry bias ...................... ft 117 . rr 174 lbs
bias percentage ......... ft 51.3 . rr 48.7
axle position .............. trailing
triple trees ................. unknown but im guessing they are staggered the same as the 86'


1986 ATC250R
rake .............................. 21 deg
rake forks ..................... 30 deg ? . . if this is the fork rake then these tripple trees are staggered 9 mm
trail ............................... 38 mm . 1.5 in
w/b ........................... 1905 mm . 75 in
dry weight .................. 289 lbs
dry bias .................... ft 116 . rr 173
bias percentage ......... ft 50.9 . rr 49.1
axle position ............... leading
tripple trees ................ staggered 9 mm?


1984 and 1985 TECATE
rake ............................. 24 deg
trail .............................. 40 mm . 1.57 in
w/b .......................... 1280 . 50.4 in
dry weight .................. 286.6 lbs
wet weight ................. 312
wet bias ..................... ft130 . rr 181
bias percentage ....... . ft 42 . rr 58
axle position ............... leading
tripple trees ................ non staggered


1986 TECATE
rake ............................. 24 deg
trail .............................. 48 mm . 1.57 in
w/b .......................... 1280 mm . 50.4 in
dry weight ................. 280 lbs
wet weight ................ 299 lbs
wet bias .................... 121 . 178 lbs
bias percentage ........ ft 40.5 . rr 59.5
axle position .............. leading
tripple trees ............... non staggered


1974 CB 250
caster .......................... 63 deg . This is measured from horizontal instead of vertical so using current methods, the rake is actually 37 degrees.
trail ............................. 85 mm . 3.35 in
w/b .......................... 1320 mm 52.00 in
curb weight ............... 353 lbs
wet weight ................ 489 lbs
wet bias ................... ft 212 rr 273 lbs
bias percentage ........ ft 43.4 . rr 56.6
axle position ............. center
tripple trees .............. non staggered


This is a high crown type tire which I would not use.




This crown is more uniform and doesn't have much of a peak in the center . . I would us one more like this.

[barnett468]

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POST CORRECTION because I have no edit button.

For 1986 ATC250R

w/b ........................... 1905 mm . 75 in

should be

w/b ........................... 1305 mm . 51.4 in

[barnett468]

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To test different amounts of trail on the cafe style, you may be able to use a set of old Kawi F5 350 big horn tubes . . The drum brake would work for the test and is designed to be switched from one position to the other . . With this setup, the lower tube can also be rotated 180 degrees to make it a leading axle....and for those that are hoping, unfortunately no, there are no old Tecate pipes laying around on the shelves.

.................................... Photo shamelessly plagiarized from Harry Klemm.

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[barnett468]

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Billy, hopefully some of this will be useful.

Rake and trail calculator

https://www.rbracing-rsr.com/rakeandtrail.html


Expanded rake and trail calculator

https://www.rbracing-rsr.com/advchoppercalc.html


If you like to use the long method, here's some free online math calculators including ones for cosines and tangents etc.

http://www.math.com/students/calcula...lculators.html


Here's a very good percentage calculator.

http://www.percentagecalculator.net/


You need to change trail by at least 20% to notice a difference.

You need to change rake by at least 2 degrees to notice a difference.

You can also buy "raked steering head cups" . . These are just bearings with offset races that allow you to change the effective rake of the steering head, so If you want to modify your steering head, you can run a different set of steering stem races for each setup, unfortunately, the most rake they have is 3 degrees, so if you left your geometry stock, this would leave you 2 degrees of the minimum amount of rake I think you should run in street mode.

If you modified your steering head angle to 24 degrees, you could run these bearings reversed to get the stock 250R steering head angle of 21 degrees, then simply remove them and rotate them 180 degrees to get a steering head angle of 27 degrees for street mode which is the angle I preferred in my post above . . If you use these offset cups and you like how the 250R works in the stock configuration, the only thing to test would be different amounts of trail in the street configuration after you reversed the cups from the ATC position.

If you didn't need to run the raked cups but wanted a little more adjustability with the offset, you could run offset steering cups, however, since the most offset you would get from them is around 6mm, it wouldn't be noticeable unless you're someone like Wayne Rainey and therefore they wouldn't be worth the effort.

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[Billy Golightly]

Holy - those offset bearing race cups are BRILLIANT! I didn't have any idea such a thing even existed.

[barnett468]

Holy - those offset bearing race cups are BRILLIANT! I didn't have any idea such a thing even existed.

LOL, yes, they are quite cool aren't they . . I didn't think of them earlier but I have actually been contemplating your idea since you started the thread and now that I thin I have a reasonably good idea of what you want to achieve, I thought...hey, these might be absolutely perfect.

They were actually invented by the harley chopper guys to help with their steering issues on the bikes with heavily raked forks . . this is an alternative to the raked/staggered tripple clamps or can be used in conjunction with them to increase the trail.

[OZQUAD44]

Re the raked steering head cups, could these item be used in a dirt bike to trike conversion as a simple method to reduce trail without having to reposition the goose neck? Or is it a moot point if most of the YZF and CRF conversions have the reduction in trail designed into the triple clamps? Your always going to need manufactured triple clamps.


Sent from my iPad using Tapatalk

[barnett468]

Re the raked steering head cups, could these item be used in a dirt bike to trike conversion as a simple method to reduce trail without having to reposition the goose neck?

In general yes however they obviously change the rake also.

Or is it a moot point if most of the YZF and CRF conversions have the reduction in trail designed into the triple clamps? Your always going to need manufactured triple clamps.

Rake affects trail but rake is also an independent function . . In other words you cant use rake for the sole purpose of changing trail because changing the rake will also affect handling and the length of the bike which will also affect handling.

[barnett468]

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TIRE AND RIM MADNESS

This is might be a bit early to talk very much about tires and wheel types, but since I stated thinking about them I thought you might be interested to see what is out there anyway . . The two tire models I posted "should" still be available in a couple years.


WHEELS

The easiest type of wheel to find is a spoke type and that will require inner tubes, however, many of the bikes came with aluminum or stamped steel rims and all of those use tubeless tires . . Ebay has many different types . . If you look on Googe at old bikes you can see some of the different types.


TIRES

The easiest type of wheel to find is a spoke type and that will require inner tubes, however, many of the bikes came with aluminum or stamped steel rims and all of those use tubeless tires . . Ebay has many different types . . If you look on Googe at old bikes you can see some of the different types.
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The tire and rim sizes on the old bikes were 18" or 19" front and 18" rear, unfortunately, most of the new super street tires are 17" or are too wide for your app, therefore unless you run 17" rims, you are stuck using older technology tires . . These older tires are bias ply and the newer ones are radials . . This being said, unless you are going to be pushing the bike extremely hard, the old style bias ply performance tires will be more than you need, and many people with vintage road race bikes use them and they run over 100 mph in these races . . I posted some of the better ones that are still reasonably priced.

You don't want really wide tires for your app because they make it more difficult to corner, especially on tight twisty mountain roads, although I don't remember seeing any of those the last time I was in Florida, plus your bike won't weigh 500 lbs or more and the narrower tires I posted will stick extremely well.

The first number in the tire size is width in mm . . The second number is aspect ratio [sidewall height] . . 90 is fairly tall by current standards, but is still far better than the old school tires that came stock on these early bikes . . 80 is the lowest you can get in the sizes you need.


NOTE - Spoke rims require a tube . . You can use inner tubes that are designed for tube type bias ply tires in tubeless bias ply tires although some mfg's frown on this idea . .


I would consider any of the tires and sizes below,

AVON AM26 - These are the tires and sizes I would prefer to use.

http://www.bikebandit.com/tires-tube...FRKRfgodruUD5g

Front --- 3.25-19 w 2.15 wide rim V rated --- $86.00 . . Use with 4.00 tire below.
Rear ---- 4.00-18 w 2.50 wide rim V rated - $105.00

Front -- 90/90-18 w 2.15 wide rim V rated --- $78.00
Front -- 90/90-19 w 2.15 wide rim V rated --- $89.00
Rear - 110/90-18 w 2.50 wide rim V rated -- $102.00
Rear - 110/80-18 w 2.50 - 2.75 wide rim V rated -.-$101.00


MICHELIN PILOT ACTIV

http://www.bikebandit.com/tires-tube...otorcycle-tire

Front --- 3.25-19 w 2.15 wide rim H rated -- $86.00
Rear ---. 4.00-18 w 2.50 wide rim H rared - $106.00


.................................................. ...... AVON AM26

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................................................. MICHELIN PILOT ACTIV
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[barnett468]

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ADDITIONAL TIRE INFO

Forgot to mention . . You need to find dead center between the swing arm mounts, then mark the back of the engine for reference . . Then measure from that point to the inside of the chain . . Then subtract 6 mm from that number . . Then multiply that number by 2 . . This will be the maximum width the rear tire can be . . Then you need to exact width of the tire mounted on the rim you plan to use so you can see if it will clear the chain or not . . Unfortunately you can not go by the advertised widths of the tires.

If you determine that a 110/80 and 110/90 tire is too big, but one that is a total of 10 mm narrower which will be 5 mm narrower per side will fit, you can use a 100 mm series or 4" wide tire . . Neither will be to small for your app . . The 11o mm tires I posted are the biggest I would use on the rear.


TIRE BALANCING

Not everybody does it but I would . . Static balancing is better than nothing but dynamic balancing on a machine that does not use the clamps on the outside of the tire to spin is better.
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[barnett468]

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WHEELBASE

I figured I would mention this in case you hadn't thought about . . If it's too short, it will turn too quick and possibly be a little unstable at high speeds with the proposed steering geometry . . If its too long, it will turn like a pig . . In your case, I would make a swing arm that would have at least 1 1/2" of adjustment and 2" if possible and so the wheelbase can be adjusted from 54" to 55 1/2" or 54" to 56".

Although you would need to calculate how long the swing arm needs to be to achieve this length, without having more dimensions and doing a bunch of math, I would guess that with the street front end on with around 4 - 4.5 inches of trail and a steering head angle of 27 degrees, the front axle location would be within around an inch of its current location . . If this is the case, you would need a swing arm that is only a few inches longer than the stock 250R one.


SUSPENSION

There is not much point in putting a lot of money into different valving etc until you see how it handles in street mode when ridden casually, but I would at least run it a little on the firm side for initial testing which is how Cafe and road race bikes are set up anyway . . This can easily be done on the front end by using a heavy oil like 20 wt and preloading the fork springs by maybe 1 1/2" with a piece of pcv pipe.


FORK TUBE DIAMETER

33 minimum, 42 maximum . . 33 mm is the same size as a 1980's Honda CB450 and the earlier Honda 350 and 400 twins . . I think it is also the same size as the stock Hatta forks on the F5 350 big horn that Harry Klemm won every AHRMA road race with including the Championship and his bike went over 100 mph.

The 2,000 lb 1500 cc Honda Gold Wing and the 166 mph 1988 1000 cc Kawasaki Ninja ZX 10 had 41's . . The 86 250R tubes are a fairly stout 39 mm and the Tecate's are a stupifying 41.


FORK BRACE

I would definitely use one and any tube smaller than 35 mm . . The first photo is the typical old school on you can find for maybe $20.00 that only fits certain forks . . The one below that is one that I think Harry Klemm's brother made . . They had to machine a perfectly round spot on the fork legs so it would clamp properly.

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