Possible heir to DKS

[jl123]

http://forums.mtbr.com/showthread.php?t=155805&page=3&highlight=willits

Supposed to also get about 1/2 inch of travel, yet with no pivots. So maybe rebound and need for elastomers is reduced. Indeed I know it looks a lot like the Terraplane, not sure if theres a patent on those bends, but I don't think there is.
Designed by ex-Ibis and ex-serotta guys so maybe it works!

You never really know until someone rides one.

[RPS]

Which frame are you referring to? The seatstays of the one right below the DKS pivot at the top -- although I don’t quite follow the connection of whether it has pivots or not to the need for elastomers to control rebound.

[jl123]

RPS,
Yes thats the one. A friend of mine is considering buying one, and I'm sort of waiting to see; he was told
by Willits that in their testing this particular design
did not need the rebound control of the elastomers- due to
the bending of the stays being more spread out than the DKS
where the severe radius put most of the bending in one place.

RPS- Of course I know you too have a design so I'm sure you
know better than me- and probably most of us, just how hard
a trick it is to get 1/2 of rear travel that dosn't bob too much
during heavy pedaling, does not rebound on tricky downhills, and
keeps the rear wheel from moving too much horizontally.

As you said in your last post R&D time is certainly a must- and
that costs, so indeed this is a tough one. JL

[David Kirk]

One needs to keep in mind that the spring of the seat stays needs to be kept softer that the spring of the tire or when hitting a bump the tire will bottom out on the rim before the stays even move.

Add to that fact that if you make the stays soft enough to be softer that a MTB tire that the spring rate would have to be extremely progressive to limit the travel of the stays to 1/2".

The DKS set up (along with many others) was prototyped with fat tires and it was near impossible to get any benefit from it.......certainly not enough benefit to justify the cost and weight. In almost all cases going with a slightly fatter tire at a slightly lower pressure was the way to go.

Dave

[RPS]

JL, thanks for the clarification, and please let us know how your friend likes his bike.

I’d also like to clarify for those who may or may not know that my design is considerably different; not only in appearance, but also in function from a DKS or any other type of curved seatstay design. My intent is not to critique a DKS or any other, but merely to point out that since there are at least three major functional differences, that whatever is concluded about these may or may not apply to mine (for better or worse).

[jl123]

Dave,
Thanks for the insight on this! You make a lot of sense. I hope the Willits guys are thinking this through.

One thought/question on this, in a recent Vintage Bicycle Quarterly they tested a Trek Pilot with an elastomer (giving about 1/2 inch of travel) and they found that with 28mm tires they were able to feel the extra comfort- though still they felt the Trek still bobbed/pogo-ed too much. But indeed they did appreciate the extra comfort for certain rides. So to me I think the question remains goes something like this: Can a 1/2 inch of travel actually work (maybe not at the very very wide moutain bike tire width but at least in the 30-35 mm tire size? And if so how to do it- I suppose this part will remain open until its done.

"The DKS set up (along with many others) was prototyped with fat tires and it was near impossible to get any benefit from it"

What size tire did you guys use? Was the DKS originally built to be a
cross or mountain bike? Thanks, JL

[RPS]

The DKS set up (along with many others) was prototyped with fat tires and it was near impossible to get any benefit from it.......certainly not enough benefit to justify the cost and weight. In almost all cases going with a slightly fatter tire at a slightly lower pressure was the way to go.

DaveFor the most part as it affects the rider, the spring effect of the rear tire and bike frame are cumulative (there is the unsprung weight of the rear wheel but that’s low compared to the rider). When I hit a bump, both the tire and frame flex in unison to isolate me from the road. Dave, does the statement above suggest that the curved stays of a DKS are far stiffer than the compressibility of a racing tire?

[jl123]

Well since this conversation is done. I'll conclude it by saying that
I give props to RPS, the guys at Willits and even *some* of the people
over at trek and specialized for keeping the notion of rear suspension
for road bikes alive. I think it can work one day. JL

[dave thompson]

The notion of rear suspension on a road bike lives today in the Serotta ST and Kirk TerraPlane read ends. The DKS option from Serotta was a major attempt to bring it forward into todays bikes.

[David Kirk]

For the most part as it affects the rider, the spring effect of the rear tire and bike frame are cumulative (there is the unsprung weight of the rear wheel but that’s low compared to the rider). When I hit a bump, both the tire and frame flex in unison to isolate me from the road. Dave, does the statement above suggest that the curved stays of a DKS are far stiffer than the compressibility of a racing tire?

I agree in principal to much of the above. The devil is in the details.....both the stays and the tire act as a spring. Each of these springs have different spring rates (stiffness of spring). Now get silly about it and picture two coil springs one stacked on top of the other. The bottom spring is soft (say like 50 lbs/inch) and the upper spring is much stiffer (say 400 lbs/inch). Now take the whole assembly and compress it. What happens? The bottom spring will easily compress and actually bottom out (coil bind) before the upper spring moves at all the soft bottom spring can't exert enough resistance to call the upper spring into action.

Very much the same thing happens when you put a fat MTB tire below a stiff set of stays. The tire does all the work bottoming out on the rim before it's spring rate gets high enough to call the stays into action. What happens if you make the stays much softer so that they will also move? Then things get funny and drive train rigidity becomes an issue.

The DKS was developed to work with a hard narrow tire. The harder the tire the more the stays will come into play....the fatter the tire the less they will move (because the tire does the moving instead being the weaker" link in the system). When tested on MTB size tires with their low pressures very little to no travel is achieved.

Dave

[Dekonick]

Thats interesting - I never thought about the DKS not benefiting from wide low pressure tires. What you have explained makes perfect sense!

Now that I have been thinking about it, I wonder what tire width and PSI combo would work best with my DKS - I suppose that the best benefit would be narrow (23mm or less?) high PSI tires - say 160psi. I have always enjoyed mine with conti GP's 25mm @ 100 psi.

Does this also play into the design of a terraplane? I suppose you make the response in the curved stays based on the customer's approximate weight, and tire type they will use.

What about Serotta's present stay's? No fine tuning there...(that I am aware of)

[Dekonick]

Thats interesting - I never thought about the DKS not benefiting from wide low pressure tires. What you have explained makes perfect sense!

Now that I have been thinking about it, I wonder what tire width and PSI combo would work best with my DKS - I suppose that the best benefit would be narrow (23mm or less?) high PSI tires - say 160psi. I have always enjoyed mine with conti GP's 25mm @ 100 psi.

Does this also play into the design of a terraplane? I suppose you make the response in the curved stays based on the customer's approximate weight, and tire type they will use.

What about Serotta's present ST stay's? No fine tuning there...(that I am aware of)

[jl123]

Mr. Thompson,

"The notion of rear suspension on a road bike lives today in the Serotta ST and Kirk TerraPlane read ends. The DKS option from Serotta was a major attempt to bring it forward into todays bikes."

Yes of course you are correct to a point. And of course the DKS and Terraplane are halmark designs that truly improve the breed.
However, I believe Mr. Kirk himself will tell
you that both the DKS and the Terraplane do not provide much suspension
with wider tires. (as was just written)

I was talking about rear-suspension (like I have tried for example on newer Trek's and on Moots Ybb) where you have a wide tire for most suspension duties but also a suspension (that works well- with good rebound) that can take up the bigger hits/bumps for rougher roads. The Trek especially works pretty good in this regard if still not quite ready for prime-tme. Though Trek did use these bikes with some sucess on portions of the TDf. Indeed I don't think that even a wide tire (say 30mm) has 1/2 inch of suspension to it. The question is, can extra suspension help on poorly surfaced roads, and I think the answer is yes, if the design isn't too inefficient. JL

[RPS]

David, I agree 100-percent that the devil is in the details; particularly when trying to balance weight, cost, performance, and comfort.

IMO, for a high-performance road bike it’s best to rely on the suspension rather than the tire to do most of the work. A major problem with using a tire as a spring is that its spring constant is not consistent – it’s impossible to predict in terms of suspension design in the real world. And also IMO to get equal results we’d end up with a slower and heavier bike.What happens if you make the stays much softer so that they will also move? Then things get funny and drive train rigidity becomes an issue.I also agree with the above statement, which is why I believe there is a limit to how much compliance can be achieved effectively with curved stays. A little is fine, but if we want enough to overshadow tires, wheels, etc... a different approach is needed. And depending on the design, the drivetrain may actually end up stiffer.

[jl123]

RPS,
"A major problem with using a tire as a spring is that its spring constant is not consistent – it’s impossible to predict in terms of suspension design in the real world. And also IMO to get equal results we’d end up with a slower and heavier bike."

Take a read of the issue before last of Bicycle Quarterly, Jan Heine
shows there is really very little difference between high quality thin and
wide tires. Indeed I think many on this forum will attest to the ride quality
and speed of quality 650b bikes/tires. Actually Jan has won quite a few long distance events using such (38mm) wide tires. JL

[jl123]

one last thing,

Here's yet another co getting into the fold:
http://www.tonicfab.com/bcxintro.htm note click on the right
to see their design which appartently does quite well on the
cross circuit. JL

[RPS]

JL, my comment you quoted above is about the spring constant of a tire not being linear, and it applies to skinny high-pressure tires as well as fat low-pressure tires. Tires don’t compress exactly like a spring – there are a lot of variables.

Also don't forget that over 100 years ago bikes ran much lower air pressure on fatter tires, and even then there were many attempts to add suspension. Most failed because they were costly, added a lot of weight, complexity and maintenance (and who needs that?). Please don't get me wrong, I have nothing against fat tires; but IMO they are not a replacement for suspension – each has its place.

[jl123]

RPS,
"I have nothing against fat tires; but IMO they are not a replacement for suspension – each has its place."

Agreed.