Eliminating Squat at Startup

Squat motion refers to the unweighting, sag and compression of the rear triangle of an FSR bike during startup or just accelerations. This unweighting feels like the rear tire is being lifted off the ground and such forces can lead to breaking traction at startup — a no, no. The position of the rear wheel chain in relation to the hinge pin of the lower stays is the main determining factor for the likely hood of having squat at startup. If the chain line at start up ( sitting on the bike) is above the hinge pin, you most likely will experience sag during any hard accelerations. The overall potential of squat action changes through out the motion of the triangle.

There are some simple changes to the rear suspension configuration that may reduce (or eliminate ?) sag at startup. More air pressure in the rear shock or cranking on more spring tension to the shock certainly can lessen sag. If you have a 2 chain reduction and the double sprockets are on the pedal axle, then a smaller sprocket for the cog connecting to the rear wheel can move the chain line down more from the lower hinge pin of the rear triangle.

Sometimes these two simple changes will not reduce squat sufficiently and the bike continues to break traction on hard accelerations or when the front tire hits a headwall. There is a fix that can be done a couple of ways. If your bike has a steel frame you can move the BB shell down — cutting and welding. If your bike has an alum frame, a new location for a BB shell can be added to your steel motor frame plates. This location must increase the distance the BB axle is below the hinge pin location compared to the the distance of the OEM position.

I had downhill Brodie Thumster bike that even at the mildest accelerations would squat and break traction. The simple changes I suggest made no significant change in traction breaking. The fix of lowering the BB shell location was an obvious success. In fact the bike has too much rise. Upon acceleration the rear of the bike seat rises noticeably at the rear, thus weighting the rear tire. I currently am testing larger sprockets on the BB axle sprockets to the rear. This trial and error testing is simply the tuning phase as these changes make an improvement towards lessening the rise.

Here are some pics. The OEM BB shell position was higher than a typical bike has, so this drop still leaves about 12” clearance from the ground. showing the new BB location:

It’s interesting that with so many factory mid drives, that very few have incorporated a Horst link into their rear suspension design. Santa Cruz just added them to one of their new e-bikes, and of course Specialized. One might think if it helps under just leg power, that it would be good to have with e-power too.

E-HP,

I have three Specialized Big Hit Downhill bikes converted to edirtbikes. They do have a tiny bit of sag while under motored acceleration, but they never seem to lose traction at startup. The most measurable feature of that bike design is that through out the full range of triangle travel the chain perimeter changes one link.

See video about predicting Squat:


On the Big Hit bikes the lower stay pivot is next to the BB shell while the chain line is very close. I would say the Horst Mechanism Bar is a way to place and centralize a single rear shock system and in itself has little effect on motor induced sag.

'It’s interesting that with so many factory mid drives, that very few have incorporated a Horst link into their rear suspension design."

Because those machines are designed by mechanical engineers who analyze the linkages and the dynamic loads which are exacerbated with the huge Dingus.

Every pivot point introduces some measureable lateral movement, the allowable tolerances of bicycle assembly incorporating a 'Horst' link add each of these tolerances for some initial allowable 'slop'. As you know the pivots then wear. 'Redesigning' the linkages as above will likely still produce the same lateral movement if the pivots are not replaced like the Specialized bikes.

Specialized purchased and owned the "Horst" patent until it ran out a few years ago, so the design could be used only by companies that licensed it from them. The result is the many other designs we see today.

2old said:

Specialized purchased and owned the "Horst" patent until it ran out a few years ago, so the design could be used only by companies that licensed it from them. The result is the many other designs we see today.

Click to expand...

Actually ran out 10 years ago now. The problem with them holding the patent for so long is that during that time, straight tubes and open triangles moved to hydro formed swoopy top and down tubes, that are more difficult to mount batteries and components to. So for the late 90s early 2000s bike, only specialized and a few companies they licensed to have a decent ebike conversion frames with the link.

You are incorrect in asserting that I have a ‘lateral movement’ problem due to worn out bearings. The suspension bearings are snug. This design, particularly the OEM location of the lower rear suspension pivot, is what contributes the most to the having significant squat.

Please watch this video to facilitate your understanding that the squat problem is about how frame geometry effects instantaneous force centers. No mention of bearing wear and lateral movement in this analysis.

https://www.pinkbike.com/news/definitions-what-is-anti-squat.html#:~:text=Anti-squat has become common,gets oversimplified to pedalling performance

Secondly the bike is a Canadian made Brodie Thumper, not a Specialized.

But I did have a wear problem on that bike. The location is where the chain could drag on the lower rear stay during full unweighting of the rear suspension. I solved that contact problem by clamping to the stay a sacrificial piece the chrome saw blade steel.

The Fix:

2-old and E-hP.

I had a late 90’s Specialized Stump Jumper that had the Horst Link Suspension. The Horst Link was an “H” shaped one-piece casted aluminum fabrication. Mine broke in about the middle of the horizontal part of the “H”, where the shock fastened. The part seemed impossible to get so I repaired it by drilleing a 3/8” hole through the connecting section of the H and bolted the sides together with an Allen Bolt. The fix worked fine. The bike actually has so little rear triangle travel that it was not very comfortable on serious downhill.

The Specialized Big Hit came to market a few years later. You may think Horst Link Suspension here but there is not a Horst Link employed, but merely 2 parallel bars with the shock bolted in the middle using a traverse bolt. Both the Brodie Thumper and the Specialized Big Hit employ 2 parallel bars for connecting the down tube to the upper rear stays with the shock fastened in between. The use of these two parallel bars is not the employment of a Horst Link. The Horst Link was a cast fabrication that could break up and as a casting was suitable for mild bumps but not big hits.

Specialized Big Hit — no Horst Link here
The Brodie Thumper




Is the one piece Horst Link still in use? Why bother with it when you can use 2 parallel bars, get a stronger design and have no patent fees? Ah! It must be simplicity?

I guess my original comment was more related to the issue of the drivetrain effect on suspension and how it can be solved or has been addressed through geometry. Bushing wear and slop and durability will always be issues for full suspension bikes, regardless of geometry. On the other hand, as long as you can predict the behavior, you can also leverage it. My second motorcycle was a shafty. It has the opposite effect of jacking up the reared under throttle. You had to be smooth on the throttle exiting turns, etc. but, off the line, it worked like old hot rods with traction bars lol.

E-HP,

Thanks for mentioning the jacking problem when turning. I will take your advice. too much snow here for any fast testing.

I seem to be able to lower the amount of jacking by employing bigger sprockets in the DSBB hub, yet keep the gearing the same.

The problem with some (all) bike systems is they were designed to isolate pedaling from braking and suspension movement for human power. Your use is way above that. My 2006 Trek with simple four bar & BBS02 works fine with 100mm travel and my much less severe trails, but you'd probably break the frame in one ride.

2old,

Agreed, but the Horst Link broke long before I had ever put a motor on that bicycle frame. My thoughts have been that if you want a lightweight edirtbike, make it from a downhill bike and it may holdup. My builds weight about 80+ lbs and have much better suspension with Fox components than the SurRon offers.