Topic: longer shocks, smaller front wheel .. what is rake and trail result ?

[Ewan 500K1]

Hi Racing Folk

I'm building a 500/4 cafe racer and have just got longer rear shocks at 14 and a half inches between mounting holes , standard is 12 and 3/5ths inches. As a result folk on the project forum are expressing concerns about the effects on the bikes handling an for my health and welfare, expecting to see rear end twitching and tank slappers. (front end is unmodified, but running with 550K3 forks). I hear that you guys that are into racing 500's generally have longer rear shocks and often run with an 18 inch front tyre . A few Q's for you
a) What length rear shocks do you generally run with ?
b) What  is effect of longer rear shocks and smaller front wheel on rake and trail ... any figures at all ?
c) What is effect on straight line handling ... rear end twitches and tank slapping tendencies common ?

[turboguzzi]

overall it's a quite common mod for racing 500/4s and works well. cant remember from top of my head what's the eye to eye length in my racer, can measure, but it is certainly quite a bit over stock, so 14" shouldn't be far off.

the 500/4 has a pretty step steering angle to begin with and this mod brings it to almost modern figures of around 22-23 degs, making the bike quicker steering. having similar concerns about stability, i race with a steering damper (on softest setting).

main hurdle is that such a reduction in rake requires new triple clamps with less offset to bring back the trail to around 90-100mm. Bike will feel twitchy without and you'll have less feedback at the bars about what the front wheels is doing. I use modified NSR125 clamps, others here have CNC carved their own.

do some homework by reading about rake and trail in general, like Tony foale, John robinson, Kevin cameron books, etc.

[bwaller]

My shocks are 14.25", 18" front with a narrow 90/90 tire to help it turn. To maintain adequate bellypan ground clearance the front of the frame is in about the same position as stock.

I plugged in my values (using stock offsets) into an online calculator and it tells me I have 25.7 degrees of rake and 99.06mm trail.

[Ewan 500K1]

My shocks are 14.25", 18" front with a narrow 90/90 tire to help it turn. To maintain adequate bellypan ground clearance the front of the frame is in about the same position as stock.

I plugged in my values (using stock offsets) into an online calculator and it tells me I have 25.7 degrees of rake and 99.06mm trail.

what calculator are you using , could you post a link so that I can have wee go myslef ?

[Ewan 500K1]

Hi , I've posted the below in  the project shop... but also posting hear as you guys seem to understand the issues in more detail.


So today I played about with taking trail and rake measurements . I tried various combinations of rear shocks and steering stems:

a) standard length rear shocks (12.6 inches  between mounting hole centres) + bent steering stem
b) standard length rear shocks + good steering stem
c) new longer shocks (14.4 inches between mounting hole centres) + good steering stem

Diagram of what I was measuring is attached, along with raw results and some photos of the method I used:

Method:


Heres how I went about measuring the trail with ref to attached photos

1. Got a broom handle and cut slots in it to accodate various bits of the bike...so that it would sit parallel to the headstock / forks . Third photo shows the broom handle with cut outs

2. Fourth photo shows broom handle sitting on bike .. orientated to extend the line of the headstock down to the ground. The broom handle is lined up with the centre line of the headstock and parallel with the stanchions

3. Fifth photo shows the plumb line hanging from the centre of the axle and the bottom of the broom handle when orientated as described at point 2. Trail is then the distance between the bottom of the plumb line and the bottom of the broom handle.

The large washer hanging on a string in the 5th photo is used to measure the rake angle as shown in the diagram in the previous post.


Some key points:

1. All measured rakes were substantially below the stock rake of 26 degrees that is quoted in the shop manual. I calculated mine with someone sitting on the bike and hanging a plumb from the top of the fork (line Y in the diagram) and then measuring the perpindicular distance from the plumb line to the centre of the axle (line X).  Length X divided by length Y then gives the tangent of the rake angle and the rake angle can be derived from that.  In looking closer at the frame today it has some slight curvature in the front rails .. bowing in by about 0.08 inches half way down on both sides. This must be another artefact of its front ender, as with the bent steering stem. This bowing of the rails is presumably reducing the rake angle ?.

2. The longest trail was found using the bent steering stem despite this set up having the least rake. Presumably this bent stem has the effect of reducing the triple T offset , such that the trail is lengthened rather than shortened that would be expected from  the lower rake figure?

3. All measured trails appear to be within an accepatable range and exceed that of the CB350.(3.3 inches according to Clymer) The lowest figure (with the new shocks on) is comparable to some of the CB750 models.( 3.74 inches ?).. all subject to margins of error in my methodolgy of course.

I also had a play with RB racing's rake and trail calculator at:
http://www.rbracing-rsr.com/advchoppercalc.html

I couldn't get the calculator  to reproduce Hondas stock rake figure by entering standard dimensions  for the 500 four,,.. so maybe it doesnt work for Honda SOHCs ? ... but what did seem consistent is that for change of one degree of rake a change of about 6mm in trail resulted. This is roughly in line with results for the 400/four at : http://www.denoonsp.com/user/image/v3-issue1.pdf. In this article 0.4 degree of rake change results in 0.09 inch of change in trail... so 1 degree rake = 0.22 inch trail.  This is consistent with what happened when I changed the shocks, I got 1 degree of change in rake and 0.2 inch of change in trail. The 440/four article also notes that a 0.25 inch increase in rear shocks procuded 0.4 degree reduction in rake. I've increased  my rear fork length by 1.84 inches, so if the 400 four figures can be extrapolated this would result in a change of 3 degrees in rake and 0.62 inch of trail. I didn't get this level of change in rake or trail when changing from standard to longer shocks , mine are more like 1 degree of rake and 0.2 inch in trail.

If 1 degree of rake change = 0.22 inch of trail change , then my measured rake of 23 degrees with the new shocks should have produced a trail reduction of 0.62 inch against the stock figure of 4.1inch.. but I only get about half that.

So what does all that tell me ? .... that the tolerance in my measurements were too wide such that my figures are unrelaible , or just that that I should  get the bloody thing on the road with the longer shocks and see how it feels ...:   

[Ewan 500K1]

I decided I wasnt happy with the broom handle .. too much scope for variation in the figures so made a telesopic version which was then fixed to the tripe T in line with ,and parallel to, the headstock. Much more rigid and stable. Pics 1 & 2 below show this. The 3rd photo shows how I measured the sides of a right angled traingle to calculate the rake.  So .. after much repeated measuring and using three different trail and rake calculators .... I've got some averages of averages:

Old shocks with straight triple T :  Trail = 97.9mm (3.9 inches)    Rake = 23.8 degrees

New longer shocks with straight triple T : Trail = 90.5mm (3.6 inches)  Rake = 22.7 degrees

So still showing a one degree reduction in rake with the new shocks and now a 7.4mm (0.3 inch) reduction in trail. I think I'll get a steering damper to be of the safe side.


All observations most welcome

[turboguzzi]

agree that your way of measuirng just isnt consistent enough. you'll get way more accuracy by shooting a high def photo from a long distance, camera at mid bike height, importing it into a cad or vector graphic program and anlyisng it. even adobe illusrtator could work.

regardless,you could try the shocks & 18" front without much worry, preferebly with triples that have about 10mm less offset than stock. simple

[johno]

I dig computers, Cad, solidworks, all good stuff, but hey dont forget the humble bubble. The bubble is used to set and zero all computer controlled measuring devices like computer controlled wheel alignment machines for example.

Soooo if you need to check and dont have the big budgets,  a lot of the best frames in the world were and are done with  a horizontal and a bubble protractor placed on the fork,   easy peasy nice and cheapy. Its accurate to a degree if small and less if its a big unit. either a plumb bob down to the floor for trail or a right angle square up.  Also its relative , so it doesnt neccesarily matter what it reads, just measure first then  change shocks and remeasure, bit like you have done presto your wiser.

My only advice is to put your bum on the seat while you measure !

The fun that turbo is alluding to is if its a hassle to measure then give the bike a good flogging, swap the shocks and give it another flogging ,    walla you know as much as any technical suspension expert in the world then.   if you get the drift.
Have fun....cheers johno

[Ewan 500K1]

yeah yeah , I know .. just get in the saddle and pay less attention to the theory  . Its still needing a few ££ / $$ things done to it before its ready for the road  though .. so off on a theory diversion .

So really all I need to do is measure the rake angle of the forks using a bubble protractor         ( Bubble protractor !!, never knew you could get such a thing .. I'm not usually much futher forward than an abacus  ) or by taking a photo and doing a bit of CAD type jiggery pockery

so with a bubble proctractor .. you just stick it on the forks and it tells you the rake angle ? Brill !

Then ... to get the trail you can just use a combination of rake angle, front axle height and fork offset to calculate the trail using trigonometry as in attached pics, ie only 3 simple measurements.  Sound loads more easier and much more accurate than bits of stick and string 

I've made a calculator , its acessible in the link below the third picture below. It only works for bikes whose triple Ts have zero rake in themselves. Also to note that a one degree change in rake translates into a quarter inch change in trail, ( at least on th 500 four) so the bike needs to be on a perfectly level surface and equipment used should be able to measure to say 0.5 of a degree of rake angle

[bwaller]

Sorry Ewan but I believe the stock offset is 45mm

[Ewan 500K1]

no worries bwalller .. that the first time I've seen it quoted .. I've been searching all over the tinternet for a stock fingre and couldnt get it anywhere ... where did you get 45 from ?

[Ewan 500K1]

no worries bwalller .. that the first time I've seen it quoted .. I've been searching all over the tinternet for a stock fingre and couldnt get it anywhere ... where did you get 45 from ?

PS .. the offset figure can be changed in the calculator thing , meantime I've refreshed the snapshot of the calculater to show an offset of 45mm

[Retro Rocket]

Offset is the easiest thing to measure,  put a steel ruler {or something dead straight} across the center of the fork tops, then measure the distance from the center of the steering head to the line between the fork tops....The offset in this picture is 35mm

[turboguzzi]

confirm 45mm brent.  and its no coincidence that that ABM triple clamp in the picture is 35mm, just like the NSR triples in my bike.

not much in it really, to get 90-ish mm of trail with 22-ish degs, you need 35mm not just in a CB500... just look how much sweat we are saving you ewan

[Ewan 500K1]

confirm 45mm brent.  and its no coincidence that that ABM triple clamp in the picture is 35mm, just like the NSR triples in my bike.

not much in it really, to get 90-ish mm of trail with 22-ish degs, you need 35mm not just in a CB500... just look how much sweat we are saving you ewan

you lot are so much more on the ball than the project shop folk on this one . The calculator says that with 22 rake and ground to front wheel pinion height of 12.9 inches (standard 19 inch wheel) and a 35mm offset , then trail is 92.34mm.     so .. is there much in the way of mods need to the NSR stock top and bottom yokes.. and is there a specific model of NSR125 , looking on ebay there seems to be a few different looking yoke sets for the NCR.

Any other alternative (and affordable) yoke option out there ?

[turboguzzi]

dont know year as i gotem at a swap meet. should have a pair downstairs, will take a foto

there was some light engineering involved like a collar to adapt bearing dimeter and a longer top center bolt to compensate for the slightly shorter stem.

you might try to look into ducati (pantah?) triples, otherwise it's CNC one-offs = mucho $$$

[Ewan 500K1]

Another rake / trail calculation. This time I took some photos of the bike. First placing the bike on a plank of wood. Then printed out the photos and drew a centre line on the fork and using the plank as the horizontal drew a line along its edge. Then I  used a decent protractor to measure the castor angle (angle A in the attached photo). Out of four different photos (2 of each side) the castor angle was consistently between 65 and 66 degrees giving a rake angle (R on attcahed photo) of 24 to 25 degrees and a calculated trail of 93.2 to 99.5mm ...... a much more acceptable result that using the crude pole measuring methods above.  Just waiting for a mate to loan me an inclinometer to measure the angle of the forks to the horizontal now to see how that compares. .... but looks now as if I dont need to be so worried about the longer rear forks resulting in an unnacdeptably low trail at the front end. So I currently think there's now no need to mess about with the front end geometry to compensate for the longer rear shocks. ust need to to put the old rear shocks on and use the photo/protractor method to see how the rake angle compared with the dtandard figure for the bike (26 degrees) and how the standard shock result compares with the longer rear  shock result .. and repeat using the inclinometer method.

[turboguzzi]

take a complete photo fo the bike, draw a line between the two tires contact points and measure angle between the two lonis, no protractor-shmotractor needed

BTW, confirmed that stock is 45 and nsr125 is 35mm trail. there's just one model, make sure that its not an nsr400...

[Ewan 500K1]

see reply #14 at
http://forums.sohc4.net/index.php?topic=126389.msg1440459#msg1440459
for an update

... now pretty confident that the longer rear shocks are giving a rake of 23.5 degrees and a trail of 91mm (3.64 inches)  Using a digital inclinomter was the most reliable method at the end of the day. The photograph method just didnt give consistent results when I revisted it today . Anyway , decided just to go with a steering damper meantime an forget about messing with the Triple T

[Retro Rocket]

Good idea....

[Ewan 500K1]

 

Good idea....

[turboguzzi]

if you are not racing i think you'll be fine.

and that Herald in the background.... had a 13/60 convertible way back, miss it !

[Ewan 500K1]

and that Herald in the background.... had a 13/60 convertible way back, miss it !

  ...... it skakes rattles and rolls... its #$%* to drive