Topic: Longer Rear Shocks = Handling Issues ?

[Ewan 500K1]

Hi Folks ,

As mentioned on my cafe build thread I've got some new Hagon shocks . These are 361mm between centres of mounting points . Standard ones are 315mm between centres, so my rear end is being raised by 45mm.  I'm being told that installing longer rear shocks will:

A) give me a twitchy rear end and that I should modify the frame to raise the front end too
B) give me quicker steering that may result in tank slappers at high speeds and that I should fit a steering damper

Anyone got longer rear shocks with or without other modifications as suggested above ?  Is prediction of twitchy rear end and tank slap tendencies apparent and real ?

Keen to sort out whether or not I need more mods before getting the frame powder coated

Bear in mind that I'll have near on 17 stone positioned just above the rear shock mountings. See photo, when still with standard length shocks on.

[bwaller]

It'll steer quicker especially if you use an 18" front. The rear won't dance around. If you ride up & down the boulevard you won't notice anything. If you carve around in the canyons, you'll love it. Tank slappers don't just happen out of the blue. Add a damper if you're concerned.

[Ewan 500K1]

cheers Bwaller . sticking with 19 inch front.  Spent all night on this , reading about rake and trail ... which was all new to me. So what I need to do is rebuild the bike and then calculate my trail and see if it looks reasonable (more than  3.5 inch ?) and also compare that with the previous set up.

Changes are :

a) increase of 45mm ( 1 and 4/5ths of an inch) on rear shocks
b) change of lower yolk from old bent one to new straight one (see attachment)

.......a)  will compensate for b) ... maybe ha ha

At least now I undertand some more abbot the bike and bike geometery. .. and having read about tank slappers I think I'll go with an insurance damper.

I also see that the racing boys generally raise the back end and run with an 18 inch front wheel .. wonder what kind of trail that results in ?

Theres not a lot of canyons or boulevards round here ... but plenty twisties on the scottish borders country roads

[seanbarney41]

Ewan, that lower triple is totally shot.  Do not use it.  Others have already told you this.  I have ran longer rear shocks on my 750, so the exact numbers are not gonna be relevant to your build.  In my situation, the longer shocks created a situation where I would get moderate to "kinda heart stopping" head shake if I hit a big bump in the road at speed, so I put stock length back on.  You have limited your options here with your electrics tray limiting rear wheel travel.  If you have not yet powdercoated, I recommend re-designing your rear frame so that it does not limit rear wheel travel...this is what CrazyPJ was getting at in your project thread.  Don't feel bad as many have made this exact mistake, and were unable to admit their error and make changes for the better.  I think you have a good looking project here and it would be a shame to sacrifice some performance when all you need is some slight re-work.

[Ewan 500K1]

Ewan, that lower triple is totally shot.  Do not use it.  Others have already told you this.  I have ran longer rear shocks on my 750, so the exact numbers are not gonna be relevant to your build.  In my situation, the longer shocks created a situation where I would get moderate to "kinda heart stopping" head shake if I hit a big bump in the road at speed, so I put stock length back on.  You have limited your options here with your electrics tray limiting rear wheel travel.  If you have not yet powdercoated, I recommend re-designing your rear frame so that it does not limit rear wheel travel...this is what CrazyPJ was getting at in your project thread.  Don't feel bad as many have made this exact mistake, and were unable to admit their error and make changes for the better.  I think you have a good looking project here and it would be a shame to sacrifice some performance when all you need is some slight re-work.

I'm hearing you sean ... bent triple T is binned... got a new one complete with steering lock the other day. I've got my heart set on having the open centre, so the battery need to go under the seat hump... so I need to keep the battery tray... so I need the longer rearshocks ... so I need to asses the effect on rake and trail at rebuild .. and take it from there. If it all goes tits up, I'll put it down to learning ... I've another 500 to do after this one.  I'm really interested in the geometry stuff now that I've had a look into it and keen to see what the effect is .  With the older bent triple T it must have  had reduced rake and shortened trail and yes it did have "quick" steering. I've been driving it like that for 17 years and never realised it had a bent triple T !  I want to see what the rake and trail difference is between the old and new set up ... think I'll do that before it goes for powder coating.   .. would also still like to know what rake and trail the 500 racer boys are running with using their longer rear forkd and 18 inch from wheel 

[seanbarney41]

You are doing it right...just keep an open mind about possibly re-arranging those electrics for more tire clearance. 

[Ewan 500K1]

You are doing it right...just keep an open mind about possibly re-arranging those electrics for more tire clearance.

its open ... though might be difficult to get fully ajar . This forum is the bees knees though. Would alternative solutions to bringing rake & trail into acceptable limits , ie by increasing front fork length with longer stanchions or getting a triple T and top yolk with a larger offset not do the job .. or would there be tangentiial issues in that ?. Obviously I'm not keen to undo all the frame mods and electrics work that has been done.. but if needs absolutely must....  Do you know what length shocks you put on your 750 and also do you know what model the 750 is , as I understand it trail length varies considerably across the 750's with some being well under 4 inch as standard... so that the impact of longer rear shocks would be more significant on some models than others

[pangloss]

Wow.! Did that stem get bent like that whilst in your current frame.? If so I' d be checking the frame for trueness

[Ewan 500K1]

Wow.! Did that stem get bent like that whilst in your current frame.? If so I' d be checking the frame for trueness

I guess so .. and explains why its a 500 with 550 front forks. Frame has been checked and reads true. No kinks  or signs of stress fractures and all nice and symmetrical ... unknown to me it had that triple T in it when I got the bike 17 years ago.. it wasnt really noticeable until I took the bearing race off

[seanbarney41]

You are doing it right...just keep an open mind about possibly re-arranging those electrics for more tire clearance.

its open ... though might be difficult to get fully ajar . This forum is the bees knees though. Would alternative solutions to bringing rake & trail into acceptable limits , ie by increasing front fork length with longer stanchions or getting a triple T and top yolk with a larger offset not do the job .. or would there be tangentiial issues in that ?. Obviously I'm not keen to undo all the frame mods and electrics work that has been done.. but if needs absolutely must....  Do you know what length shocks you put on your 750 and also do you know what model the 750 is , as I understand it trail length varies considerably across the 750's with some being well under 4 inch as standard... so that the impact of longer rear shocks would be more significant on some models than others

My 750 is a huge mish mash of parts.  It is a k5 frame.  The longer shocks were, I believe, 3/4" longer than stock.  They were off a cb900f.  ...the kicker is that my 750 has 550 front forks installed by a previous owner, so the front end is lower than stock(this is not ideal either)  I have never figured my rake and trail numbers, so I have no knowledgeable advice for you on that level.  I have just always kept my eyes open for available tire clearance, which gives me the freedom to try different parts until I find a combo that works.  I think raising the front of your bike to get the good trail numbers should work ok.

[Ewan 500K1]

checck out the longer rear shocks on the runner up in August bike of the month.

http://forums.sohc4.net/index.php?topic=124227.0

Stunning bike to my eyes. Be good to get stuart's view on handling effects

[crazypj]

If your talking about the 550, check crank center-line, it's way above axle line but bike is sitting pretty much level.
Shocks are not too much longer than stock
You should probably cut out an oval section in the plate and weld a bulge in for clearance

[Ewan 500K1]

If your talking about the 550, check crank center-line, it's way above axle line but bike is sitting pretty much level.
Shocks are not too much longer than stock
You should probably cut out an oval section in the plate and weld a bulge in for clearance

StuartNI's  silver 550 is running 14 and 1/4 inch rear shocks, same as my new ones give or take
Cutting out a bit of my rear electrics tray and putting in a bulge would defeat the purpse .. ie wouldn't leave enough clearnce between the seat base and the plate for the electrics. Racing boys appear to get round the reduced trail issue arising from using 14 inch shocks and 18 inch front wheel by using a damper and a triple T and top yolk with shorter offset . Two pics  of my bike with new longer Hagons attached. Still to calculate impact on rake & trail ... tomorrows job. 3rd pic shows the StuartNI's silver 550's rear end in close up

[crazypj]

If your talking about the 550, check crank center-line, it's way above axle line but bike is sitting pretty much level.
Shocks are not too much longer than stock
You should probably cut out an oval section in the plate and weld a bulge in for clearance

StuartNI's  silver 550 is running 14 and 1/4 inch rear shocks, same as my new ones give or take
Cutting out a bit of my rear electrics tray and putting in a bulge would defeat the purpse .. ie wouldn't leave enough clearnce between the seat base and the plate for the electrics. Racing boys appear to get round the reduced trail issue arising from using 14 inch shocks and 18 inch front wheel by using a damper and a triple T and top yolk with shorter offset . Two pics  of my bike with new longer Hagons attached. Still to calculate impact on rake & trail ... tomorrows job. 3rd pic shows the StuartNI's silver 550's rear end in close up

It's a little nose down and probably twitchy but doesn't look dangerously out of proportion.
I can't find a good side on pic of my 550 except for the one after coming back from south of France in 1980

[Ewan 500K1]

If your talking about the 550, check crank center-line, it's way above axle line but bike is sitting pretty much level.
Shocks are not too much longer than stock
You should probably cut out an oval section in the plate and weld a bulge in for clearance

StuartNI's  silver 550 is running 14 and 1/4 inch rear shocks, same as my new ones give or take
Cutting out a bit of my rear electrics tray and putting in a bulge would defeat the purpse .. ie wouldn't leave enough clearnce between the seat base and the plate for the electrics. Racing boys appear to get round the reduced trail issue arising from using 14 inch shocks and 18 inch front wheel by using a damper and a triple T and top yolk with shorter offset . Two pics  of my bike with new longer Hagons attached. Still to calculate impact on rake & trail ... tomorrows job. 3rd pic shows the StuartNI's silver 550's rear end in close up

It's a little nose down and probably twitchy but doesn't look dangerously out of proportion.
I can't find a good side on pic of my 550 except for the one after coming back from south of France in 1980

1980 was a great year !

[KRONUS0100]

running the showa rear shocks off an 81 750F on my 76 f model. roughly 1 1/2 longer than stock, but i am also running the 81 swingarm as well,  maybe 3/4 rise in rear end

[Ewan 500K1]

running the showa rear shocks off an 81 750F on my 76 f model. roughly 1 1/2 longer than stock, but i am also running the 81 swingarm as well,  maybe 3/4 rise in rear end

hows it feelin matt ?

[Ewan 500K1]

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

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 to 3 below show this. The 4th 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 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

[SOHC4 Cafe Racer Fan]

Experience substantial head shake on one of our older bikes with a comparatively flimsy front end (as opposed to a modern front suspension) without a steering damper and you will want one.  If all you want to do is put around about town, you might not need one with the set up you have, but if you are planning to ride the bike at speed, why not improve the bike?

[Ewan 500K1]

Experience substantial head shake on one of our older bikes with a comparatively flimsy front end (as opposed to a modern front suspension) without a steering damper and you will want one.  If all you want to do is put around about town, you might not need one with the set up you have, but if you are planning to ride the bike at speed, why not improve the bike?

I want one already having read someof the freaky stories 

[KRONUS0100]

rides like the gl1100 i used to have.  very stable and very comfortable.  i can lean her into the corners almost as well as the gs500e i sold to buy her 9 years ago

[Doctor_D]

My 2 cents here may not be entirely relevant, but on my CB750 I added an inch to my shocks and lowered the front end an inch.  Rake and trail with a rider on-board is very reasonable by modern standards, at roughly 25 deg and 100mm.  Because I essentially "rotated" the frame up in the back and down in the front - the crankshaft centerline to axle centerline relationship was largely much unchanged.

The caveat to all of this is that I also gusseted my frame, used (and properly tuned) RaceTech  internals in the forks, Works shocks, bronze swingarm bushings, tapered steering bearings, lighter rims, stiffer engine mounts, and a steering damper.

The result is a bike that is taught and responsive without being twitchy.  For a pretty big bike, it quickly disappears underneath you at speed.  In short, no drama.

[Ewan 500K1]

another (vastly improved, simpler an more accurate) method of finding out what the trail is ...thanks to turbo guzzi and johno on the high performance & racing forum

Step 1. find out what the rake angle is by sitting a bubble protractor against the fork tubes , OR  by taking  a high definition photo from a long distance, camera at mid bike height, importing it into a cad or vector graphic program and anlalyisng it. .

Step 2. Use my new calculator ... link attached below , under the the picture of the screenshot of it

Note .. this only works if the triple T has zero rake itself and neither ir nor the forks are bent.

Also for a 500 four a  degree change in rake translates to a quarter inch change in trail ... so you need get an accurate rake measurement ( say within 0.5 degrees) and the bike will need to be on a perfectly level surface

[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.