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Rear Shock, Travel Geometry & how to set for dirt #748647
09/10/18 7:04 pm
09/10/18 7:04 pm
Joined: Nov 2012
Posts: 306
Sacramento, CA
Denis J Offline OP

BritBike Forum member
Denis J  Offline OP

BritBike Forum member

Joined: Nov 2012
Posts: 306
Sacramento, CA
Rear Shock Options...who else makes them and who is off-roading their machines?

Hi All

I am working on a few machines all set up for offroad
Having listened to a few suspension companies about shoosing the rear shocks I am now fully confused. The information is all over the board

Machines in question
06 Royal Enfield Bullet 500 (pre unit) with stock front forks
66 BSA A65 with Betor Front forks and B50 21" wheel
69 B44 With Ceriani forks

I am looking for a rear shock set that is longer (of course) and will be a plush as allowable for the total travel. The items currently tried/recommended are all too stiff.

I am looking for answers on how to choose the rear spring rate and then maybe pick a damper that will work.
Going the easy route a buying the techno fancy stuff is all good but spending 500-1000 on rear shocks for a weekend banger is not reasonable. I am hoping these can come out less than 200 for a kit

Any experience would be excellent! Thank you
Denis J

Last edited by Denis J; 10/10/18 8:49 pm.

The formula for how many motorcycles to own is:
n+1=n
n = the number of motorcycles you currently own.

“You can wheelie any bike if you just try”

Broken Motorcycle Mechanic at The Vintage Monkey in Sacramento, CA ...Old Motorcycle Specialists
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Re: Choosing Rear Shocks for Offroad [Re: Denis J] #748677
09/10/18 9:48 pm
09/10/18 9:48 pm
Joined: Dec 2004
Posts: 9,667
Scotland
kommando Online content
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kommando  Online Content
BritBike Forum member

Joined: Dec 2004
Posts: 9,667
Scotland
For my B44 I specified a shock 15 lbs lower spring rate than BSA specified, was told it was too much lower but I find it a much less harsh ride.

Re: Choosing Rear Shocks for Offroad [Re: Denis J] #748762
09/11/18 12:26 pm
09/11/18 12:26 pm
Joined: Aug 2015
Posts: 1,301
Bolton Lancs UK
A
Andy Higham Online content
BritBike Forum member
Andy Higham  Online Content
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A

Joined: Aug 2015
Posts: 1,301
Bolton Lancs UK
Manufacturers always err on the side of caution when specifying spring rates. Probably ok for 18 stone rider with GF on the back.
A good suspension supplier will take into account the bikes axle weight, the riders weight and riding style


BSA B31 500cc "Stargazer"
Greeves 200cc "Blue Meanie"
Greeves 350
Greeves 360 Challenger
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
'35 OK Supreme
'36 OK Supreme
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #748845
09/12/18 3:44 am
09/12/18 3:44 am
Joined: Nov 2012
Posts: 306
Sacramento, CA
Denis J Offline OP

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Denis J  Offline OP

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Joined: Nov 2012
Posts: 306
Sacramento, CA
Ok
Race Tech literally blew me off. “Well it’s $875 to start so if you’re not serious then we can’t help”
My question was “I’m trying to find an affordable way to get close to 4” of travel in a 14” shock no damping adjustment needed”

Progressive suspension was super helpful but options came to a dead end. They have 1 compatible damper and it requires their 80/120 spring...stiff for a leightweight bike...good for the sportster hooligan class

Hagon has a myriad of options but I,m waiting for email confirmation on travel. According to their charts they have a very long shock 370mm with 100mm of travel and a spring as light at 10kg per cm...all that translated is 14.5 long, 4” travel and a 56lb per inch spring....cushy or bottom out happy?

Is there another company I should look into?


The formula for how many motorcycles to own is:
n+1=n
n = the number of motorcycles you currently own.

“You can wheelie any bike if you just try”

Broken Motorcycle Mechanic at The Vintage Monkey in Sacramento, CA ...Old Motorcycle Specialists
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #750385
09/25/18 6:15 am
09/25/18 6:15 am
Joined: Jan 2012
Posts: 457
Western NY
P
Pete Suchawreck Offline
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Pete Suchawreck  Offline
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P

Joined: Jan 2012
Posts: 457
Western NY
Falcon shocks, NJB shocks from Speed and Sports

Re: Choosing Rear Shocks for Offroad [Re: Denis J] #750649
09/27/18 12:28 am
09/27/18 12:28 am
Joined: Nov 2012
Posts: 306
Sacramento, CA
Denis J Offline OP

BritBike Forum member
Denis J  Offline OP

BritBike Forum member

Joined: Nov 2012
Posts: 306
Sacramento, CA
Man
There is very little information on this
2 pages in Race techs bible...helpful but likely not applicable

Supposedly there is a book by Tony Foal twin shock setup. Looking for this



The formula for how many motorcycles to own is:
n+1=n
n = the number of motorcycles you currently own.

“You can wheelie any bike if you just try”

Broken Motorcycle Mechanic at The Vintage Monkey in Sacramento, CA ...Old Motorcycle Specialists
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #751095
10/01/18 12:34 pm
10/01/18 12:34 pm
Joined: May 2013
Posts: 5,135
Rotherham - S. Yorkshire
Allan Gill Online happy

BritBike Forum member
Allan Gill  Online Happy

BritBike Forum member

Joined: May 2013
Posts: 5,135
Rotherham - S. Yorkshire
I heard some bad reports about NJB, many folk stating they were only good for off road/ trials use.... However I fitted a pair of fully enclosed "ultimate shocks" to my A65, not yet had to adjust the pre-load on them, just use and go.... I always thought the hagons were a reasonable shock, but these are a far superior alternative and I imagine to be as good as the Koni/Ikon's but without the price tag... between that and the referbed front end with progressive springs, the bike is a dream to ride, almost too smooth on the road.


beerchug
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #751139
10/01/18 9:01 pm
10/01/18 9:01 pm
Joined: Aug 2015
Posts: 1,301
Bolton Lancs UK
A
Andy Higham Online content
BritBike Forum member
Andy Higham  Online Content
BritBike Forum member
A

Joined: Aug 2015
Posts: 1,301
Bolton Lancs UK
Falcon will build a pair of shocks to suit your requirements
https://www.falconshockabsorbers.co.uk/


BSA B31 500cc "Stargazer"
Greeves 200cc "Blue Meanie"
Greeves 350
Greeves 360 Challenger
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
'35 OK Supreme
'36 OK Supreme
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #752223
10/10/18 8:48 pm
10/10/18 8:48 pm
Joined: Nov 2012
Posts: 306
Sacramento, CA
Denis J Offline OP

BritBike Forum member
Denis J  Offline OP

BritBike Forum member

Joined: Nov 2012
Posts: 306
Sacramento, CA
Thank you for this insight

Here is what I have learned so far..I am unable to verify this is concrete truth. Any engineers are welcome to chime in

The shock is at it's stiffest in the vertical position
Moving the upper shock mount forward will increase the rear axle arc length (increase suspension travel) and soften the effect of the spring
Moving the upper and lower shock mounts to the middle point of the swinger and the mid point of the upper mount-to swing arm pivot point will double the rear axle arc length (double the suspension travel) and cut the spring rate in half (super soft)
If the upper shock mount is closer to the swingarm pivot than the lower the spring rate will become "digressive" meaning the shock will soften up through the range of travel

I feel like I am getting closer to a solution. I can get shock assemblies in the 100mm travel range...moving the BSA A65 upper mount 1" closer to the swing arm pivot should increase the overall travel .9" and soften teh rate of the spring/dampening. How much the spring softens I have yet to figure out a calculation


Inputs?


The formula for how many motorcycles to own is:
n+1=n
n = the number of motorcycles you currently own.

“You can wheelie any bike if you just try”

Broken Motorcycle Mechanic at The Vintage Monkey in Sacramento, CA ...Old Motorcycle Specialists
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #752981
10/16/18 3:07 am
10/16/18 3:07 am
Joined: Jun 2012
Posts: 141
California, USA
c caspary Offline

BritBike Forum member
c caspary  Offline

BritBike Forum member

Joined: Jun 2012
Posts: 141
California, USA
Denis

I agree with Pete S.
Get a hold of Matt at Speed and Sport - he is close to you - https://www.speedandsport.com/
He has the experience to advise you well.
I am using NJBs from Matt and they work well

Charlie







66 TR6 MkIII Rickman
66 TR6 Scrambler
68 T120 Flat Tracker
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #753002
10/16/18 12:30 pm
10/16/18 12:30 pm
Joined: Aug 2001
Posts: 4,251
argyll. scotland, uk
gavin eisler Online content
BritBike Forum member
gavin eisler  Online Content
BritBike Forum member

Joined: Aug 2001
Posts: 4,251
argyll. scotland, uk
Interesting article on rear shocks here.
http://www.eurospares.com/frame5.htm


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #758415
12/07/18 4:55 am
12/07/18 4:55 am
Joined: Feb 2007
Posts: 2,269
Atlanta, GA USA
S
Semper Gumby Offline
BritBike Forum member
Semper Gumby  Offline
BritBike Forum member
S

Joined: Feb 2007
Posts: 2,269
Atlanta, GA USA
Hi Dennis.

I bought my 14" Hagons for my 67 Hornet from Bruce Triplet. He spec'd the springs for me based on the weight of me and the bike.
I'm racing this in Flat track and hopefully Road Racing with ARHMA so I'm restricted to 4" travel. If you are not racing AHRMA then you can get more modern Shocks and NJB or Falcon above will also do just fine. I have a set of Falcons on a 1972 Montesa Cota Trials bike and they work just fine. Matt Hidebrand at Speed and Sport is a great source for lotsa bits: cables, ignitions, shocks etc...

http://www.brucessuspension.net/

Last edited by Semper Gumby; 12/07/18 5:00 am.

Secretly Mediocre....
Re: Choosing Rear Shocks for Offroad [Re: Denis J] #758460
12/07/18 4:35 pm
12/07/18 4:35 pm
Joined: Aug 2010
Posts: 325
New Hampshire, USA
David Dunfey Offline
BritBike Forum member
David Dunfey  Offline
BritBike Forum member

Joined: Aug 2010
Posts: 325
New Hampshire, USA
"Moving the upper shock mount forward will increase the rear axle arc length (increase suspension travel) and soften the effect of the spring."

Denis,

I don't know enough about suspension design to say that you are incorrect, but I think that you are confusing two things that are going on. It is a little counterintuitive, but if you rotate a spring on an angle and apply a load, you will need a stronger spring to support the load. This is about springs and angles, not axle arcs. For example, if you have a 200 lb/in spring that is vertical and supports your load the way you want and you put that spring at a 30 degree angle, you will need to replace that spring with a 230 lb/in spring to support exactly the same load. Saying this another way, as you angle the spring from vertical "it gets weaker" so to speak. Velocette used this on many of their bikes that allow you to move the top bolt of the shock on a slotted arc. This effect is called "the angle correction factor." The good news is it should answer your question about the spring rate at various angles. It turns out that the angle correction factor "acf" for 30 degrees is .87. When you divide 200 by .87 you get 230. You can use Google to find a chart of spring angle correction factors.

I have not done any work on changing the geometry of the arc of the rear axle, but it seems a little off to me. I would think that the attachment points for the shock would be better explained by the concept of a "motion ratio." The motion ratio is calculated by figuring out the lever ratios of the lower shock mount on the swing arm when compared to the upper mount on the bike in relation to the pivot. You are really measuring the levers that are working on the spring and rear wheel. All of this is reasonably eyeball glazing. Ultimately, it is important to measure the "wheel rate" of the rear wheel because that will tell you the "force" that is being generated on the wheel by the particular spring rate after it goes through all of the leverage ratios and angle adjustments. (The wheel rate is the motion ratio squared times the spring rate times the angle correction factor.)

Given the above, I tend to ask experts and copy what others have done. I try to have only the broadest view of the math.

David

Last edited by David Dunfey; 12/07/18 4:36 pm.

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