Topic: Dragbike rear shock angle

[Leino]

Has any of the drag racers on the forum played around with the rear shock angle on a twin shock bike?

I suppose it's common to just use struts but when the wheelbase is limited and bike makes some noticeable power a good rear suspension helps the launching a lot.

Any thoughts are welcome.

[dragracer]

Leino, I've played with shock angles on my bike a bit. I refuse to use struts as they don't allow the bike to squat which makes it easy to wheelie. I'll measure the angle and let you know. Of course my bike is lowered and longer than your rules allow. My bike is capable of low 1.40 sixty foot tines.

[Leino]

Leino, I've played with shock angles on my bike a bit. I refuse to use struts as they don't allow the bike to squat which makes it easy to wheelie. I'll measure the angle and let you know. Of course my bike is lowered and longer than your rules allow. My bike is capable of low 1.40 sixty foot tines.

Thank you Frank. My bike is as low as it can go without modifying the chassis, again limited by rules 
You summed up my point very well, exactly why I wouldn't use struts even if I could.
Made a huge difference when I ordered the short YSS shocks and even a greater difference after they were shimmed for more rebound.

 Juha

[TurboD]

Juha The more that you increase the angle of the shock (moving the lower mount back), in effect the softer the shock becomes. This can be helpful when using stock shocks, not only to lower but to soften and help promote squatting..

Today nearly everyone runs shocks on a no bar bike, they simply work better and are a lot safer.

[Leino]

Juha The more that you increase the angle of the shock (moving the lower mount back), in effect the softer the shock becomes. This can be helpful when using stock shocks, not only to lower but to soften and help promote squatting..

Today nearly everyone runs shocks on a no bar bike, they simply work better and are a lot safer.

TurboD. I've tried a few tricks over the past decade I've been racing my Suzuki but being basicly the only one left who still races a twin shock bike in Finland I was curious what other racers might have found out.

[Leino]

I asked the people who make racing rules in Finland am I allowed to modify the subframe and they said yes.
I can modify/replace the subframe and add bracing etc. as long as the engine cradle measurements, shape and angles don't change.

So, I can now play with two things, I can move the shocks back or forth from any end I like and play with the angle.

What effect has the swingarm pivot and rear shock upper mounting point distance?
I know it's all geometry and I should also play around with force vectors and such but I suppose I'm too lazy and stupid to do that.

Frank has experience in this and his bike launches very hard so he knows a thing or two about this (and has a great clutchhand).


We have talked about replacing the subframe with a CRMO one and this is a good opportunity to get the best of things available. I want to keep the twinshock setup, that's my style and I'll stick to it 

[dragracer]

Leino,

I haven't taken the time to measure my shock angle yet but will try to do so in the next few days. As TurboD mentioned, the greater the shock angle, the softer the shock will become. I set my bike up with 3 different lower shock mount holes. The hole furthest to the front of the bike provided the stiffest rate. The rearmost provide the softest rate that worked best for my dragbike.  However, I had to tighten up my stock shock springs to the maximum position to get the rate where i wanted for proper squat and rebound. The same applies to a bike with a longer wheelbase.  There are many factors at play here including rider weight, bike weight etc... That's why if ordering an aftermarket dragbike  shock, the first thing they ask is rider weight and wheelbase. If we're looking for exact science, it would take me back to my engineering classes in Statics and Dynamics. Of course I've completely forgotten how to calculate a formula to give specifics on how altering shock angle against a pivot point works out. Perhaps one of our professional engineers on this forum can step up.

In the meantime, i'll provide the length of my arm, the distance from pivot point to my shock mounting point and the angle of the shock.

[Leino]

Leino,

I haven't taken the time to measure my shock angle yet but will try to do so in the next few days. As TurboD mentioned, the greater the shock angle, the softer the shock will become. I set my bike up with 3 different lower shock mount holes. The hole furthest to the front of the bike provided the stiffest rate. The rearmost provide the softest rate that worked best for my dragbike.  However, I had to tighten up my stock shock springs to the maximum position to get the rate where i wanted for proper squat and rebound. The same applies to a bike with a longer wheelbase.  There are many factors at play here including rider weight, bike weight etc... That's why if ordering an aftermarket dragbike  shock, the first thing they ask is rider weight and wheelbase. If we're looking for exact science, it would take me back to my engineering classes in Statics and Dynamics. Of course I've completely forgotten how to calculate a formula to give specifics on how altering shock angle against a pivot point works out. Perhaps one of our professional engineers on this forum can step up.

In the meantime, i'll provide the length of my arm, the distance from pivot point to my shock mounting point and the angle of the shock.

Thank you Frank. I'll be pulling the engine out this week to adjust the squish and check everything out.
I've been looking at videos of 2013 and 2014 launches and the rear end of my bike sits too low now so I'll be doing something about it this winter. If I'm lucky a good friend of mine who is a well respected bike builder, would weld me a new subframe and add a little bracing too.

First version of my bike had stock length shocks and the mounts were as far to the rear as possible and this setup seemed like it wanted to bend the shocks rather than work in the direction it should. It worked OK for a couple of years but the shocks leaked easily, now I have the shock mounts in the same place as in stock swingarm but as the shocks are 50mm shorter, there is also slightly more angle.

[Leino]

Juha The more that you increase the angle of the shock (moving the lower mount back), in effect the softer the shock becomes. This can be helpful when using stock shocks, not only to lower but to soften and help promote squatting..

Today nearly everyone runs shocks on a no bar bike, they simply work better and are a lot safer.

TurboD, didn't mean to sound rude.
My dyno guy races a ZX14 Kawasaki turbo with over 600hp at the wheel and he could help me with setting up the suspension properly but as you know, things are a bit different between old bikes two shocks and modern bikes with all the fancy stuff 
Thanks for the input.

[scottly]

I played with shock angles on dirt bikes in the '70s, but my goals were to increase real wheel travel and provide a slightly more progressive rate; soft at first, then stiffer as the angle between the swingarm and the shock became closer to 90*. I generally relocated the upper shock mount more forward and lower than the stock mount.
For a drag racing situation, I think the static height is more critical than travel. If the center line of the axle is below the center line of the swing arm pivot, there will be more anti-squat when the thrust from the rear wheel is focused against the pivot, pushing the tire against the track as it tries to crawl under the bike. If, on the other hand, the axle is above the pivot, the forward thrust will try to lift the tire away from the track, creating more squat than weight transfer alone. If you want more "bite", raise the static height, and if you want less, lower it.

[dragracer]

You bring up an important point about "sag" Scottly. Lots of information about that.

[scottly]

Frank, for these purposes consider "static" height as sitting on the bike on the starting line.  

[Leino]

From the videos I watched yesterday my axle is now slightly above the pivot when I'm on the line and the problem is I seem to run out of suspension travel and the bike wheelies instantly when this happens. Worked pretty well with the smaller engine but not anymore.

If I add preload and compression damping it won't launch as well but still tends to wheelie. More wheelbase would be ideal but like Frank stated earlier, limited by the rules to what I have now.

I would like to use the shocks I have as they were made to specs for me and they weren't cheap either 

Thanks for the good points Scottly.

[Leino]

Looks like the shocks have slightly less angle compared to my bike. My initial thought was to move the bottom mounts forward. Have to calculate a little 

That's a good looking bike Frank.

[dragracer]

If you zoom in close to the shock mount on the bottom, you can see the extra holes I drilled. I repositioned the shocks in all 3 holes to figure out what worked best. I ended up at the rear most position to get the right amount of squat without completely bottoming out. Those are cycle ex shocks. The stock shocks wore out over time. Unlike mono shocks, I'm not sure we can just replace the springs with a different rate. Maybe someone else can chime in. A higher rate spring might work for you.

[dragracer]

Initially, I went through 3 sets of stock shocks until I found a pair with the right stiffness and rebound.

[Tintop]

If you zoom in close to the shock mount on the bottom, you can see the extra holes I drilled. I repositioned the shocks in all 3 holes to figure out what worked best. I ended up at the rear most position to get the right amount of squat without completely bottoming out. Those are cycle ex shocks. The stock shocks wore out over time. Unlike mono shocks, I'm not sure we can just replace the springs with a different rate. Maybe someone else can chime in. A higher rate spring might work for you.

Hagon will 'mix / match' shock lengths and/or spring rates.  They will do one change for free (British Cycle anyway ), if you haven't marked up the springs.  That's how I sorted out springs on the sidecar.

[bwaller]

I'm talking out of my ass because honestly I don't know, but the V-twins flat trackers always had an advantage in the early days because of axles, crank, & countershaft aligned as well as good swingarm angle under power. Franks not too far off that in his picture.

[Leino]

Thank you. I can get different rate springs for the YSS shocks, no problem. They were shimmed for more rebound a few years ago and they worked great after that but I think maintenance is now in order anyway. The guy who does the job told me last time he didn't believe drag racers were giving such a hard time on rear shocks. He usually works on motocross, enduro and road racing suspensions.

I'll pull the engine in a few days and start measuring, last time we replaced the swingarm it was two- or three days before the season started and for some reason I decided to copy the pivot-> shock mount distance from the stock piece but my shocks are 280mm and the stock stuff is 335mm from memory.

[scottly]

Changing the lower shock mount also has an effect on the geometry. If, for example, the lower mount is halfway between the axle and the arm pivot, with the shock 90* to the swing arm, the shock will be 1/2 the stiffness as if the shock is mounted directly in line with the axle, and still at 90*. Frank, by moving the lower shock mount closer to the axle, you have changed both the angle as well as the leverage, but in opposing directions. I'm hesitant to suggest changes given your 60' times, but you may want to consider adjustable upper shock mounts, such as a channel with a series of holes, similar to the lower mounts on the swing arm.